# Hans von Storch

# From Decoding Turbulence to Unveiling the Fingerprint of Climate Change

Klaus Hasselmann—Nobel Prize Winner in Physics 2021

From Decoding Turbulence to Unveiling the Fingerprint of Climate Change

Hans von Storch From Decoding Turbulence to Unveiling the Fingerprint of Climate Change

Klaus Hasselmann—Nobel Prize Winner in Physics 2021

Hans von Storch Max Planck Institute for Meteorology Hamburg, Germany

ISBN 978-3-030-91715-9 ISBN 978-3-030-91716-6 (eBook) https://doi.org/10.1007/978-3-030-91716-6

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# **Preface**

This book brings together how we, friends, colleagues, and co-workers, see the scientist Klaus Hasselmann. We have tried to assess the legacy of his work, oceanography, climate science, economy, and physics.

The original momentum for this project came from Ola M. Johannessen. Hans von Storch designed the concept of the book, supervised the composition, and holds the overall-reponsibility of the book.

One group of authors has contributed to various aspects of the book, in particular to Chaps. 1 and 3; others also to the interviews in Chap. 4, and the library of MPI to the hopefully eventually complete publication list in Chap. 5: Ola M. Johannessen, Susanne Hasselmann, Gerbrand Komen, Peter Lemke, Dirk Olbers, Carola Kauhs, Martin Heimann, Hans von Storch, Dmitry Kovalevsky, and Lennart Bengtsson.

Another group has provided "personal accounts" in Chap. 3, additional to the first group, these are (in random order) Hans Graf, Jin-Song von Storch, Jürgen Willebrand, Achim Stössel, Mojib Latif, Jörg Wolff, Christoph Heinze, Ulrich Cubasch, Ben Santer, Patrick Heimbach, Robert Sausen, Gabriele Hegerl, Luigi Cavaleri, Kristina Katsaros, Peter Janssen, Jürgen Sündermann, Klaus Fraedrich, Hartmut Graßl, and Udo Simonis. These accounts were provided "upon invitation", and there are certainly many more companions of Klaus, whose account would have been interesting and even entertaining. But obviously, the book must have a limited size; thus, we owe an apology to those, whom we had not invited.

There was some funding needed for this project; a little for technical support, but in particular for having the book printed by Springer Publisher. These costs were generously covered by the Max Planck Institute of Meteorology in Hamburg.

Cover-drawing: Victor Ocana. Two significant work contributions need to be recognized—the final check of the various references by Carola Kauhs, and the final reading and detecting of glitches of all kinds by Dirk Olbers; quite possibly Hans von Storch added new glitches when trying to do the final corrections. After completion, we were informed that Klaus Hasselmann was awarded the Nobel prize in Physics of 2021.

Hamburg, Germany Hans von Storch

# **Contents**

# **List of Facsimiles**

# **1**

# **Klaus Hasselmann—His Scientific Footprints and Achievements**

# **1.1 Overview**

Klaus Hasselmann was born in Hamburg in 1931. His family fled to England in 1934 because of the Nazis, so he grew up in an English-speaking environment, and returned to Hamburg after the war, where he studied physics, started a family, and became an innovative researcher. Later, he spent several years in the United States of America, but always returned to Hamburg, where he became the founding director of the Max-Planck-Institut für Meteorologie in 1975. His Institute soon became one of the world's leading research facilities in the field of climate science. He retired in 2000, but continued his work in climate science as a "grey eminence" in the background, whilst his heart and mind turned to particle physics. He recently turned 90, and we—a group of former co-workers, scientific friends and colleagues—decided that we had to tell the story of this remarkable man.

One of the challenges we were faced with was the sheer breadth of interest and commitment of Klaus's career. In this section, we shall attempt to evaluate his achievements as a Naturwissenschaftler1 (Sect. 1.2), an enabler (Sect. 1.3), and as a public figure (Sect. 1.4). Klaus himself will have his say in Chap. 2 in which we reproduce an interview from 2007 as well as a recent

1 We have chosen to use the German word "Naturwissenschaft" (and its derivatives) rather than "science", because of an important difference in meaning—the latter refers to "a branch of knowledge or study dealing with a body of facts or truths systematically arranged and showing the operation of general laws", and therefore to the product of scientific endeavor, whilst the former describes the process of creating knowledge about the character and dynamics of natural systems, i.e., the endeavor itself.

brief addition to it, and a chat between two Hasselmann acolytes, Dirk Olbers and Hans von Storch in May 2021. His key scientific achievements, which are summarised in Sect. 1.2, are discussed and evaluated in some detail in Chap. 3 in which a number of his original texts, some in German, are reproduced. In Chap. 4, some of Klaus' former co-workers and colleagues present their personal accounts and memoirs of what it was like to work with him: the reader will see a certain amount of overlap between the different accounts recurrent themes include his personal friendliness but also his sometimes-rude insistence on scientific rigour. But the various accounts differ in terms of content, and the scientific issues addressed. Reading these accounts will give the reader an insight into the variety of endeavours, interests and successes of this man. The final Chap. 5 includes an overview of his major publications, awards and a CV.

# **1.2 The Naturwissenschaftler**

When considering how to survey Klaus Hasselmann's scientific achievements, we were reminded of the old Indian parable2 of the wise blind people, who want to understand what an elephant is. One examines a leg, another the trunk, the third an ear and so forth. They all understand exactly what they have in front of then, but none of them sees the whole animal. The authors of this book are the blind people, and Klaus Hasselmann is the elephant.

2 E.g., https://en.wikipedia.org/wiki/Blind\_men\_and\_an\_elephant.

#### **1 Klaus Hasselmann—His Scientific Footprints and Achievements 3**

We have identified seven major fields, to which Klaus made significant contributions; there may well be more, which we, the blind, have not yet classified and fully evaluated, one of which could be "internal' waves"—a subject with which Klaus probably became involved during his stay at La Jolla and his collaboration with Walter Munk, and which has much in common with the surface wave problem (generation, wave-wave interactions and dissipation), to which he devoted a great deal of his time. At that time, Walter Munk was working with Chris Garrett to establish the scientific basics of internal wave research in the form of a unified wave spectrum of the three-dimensional oceanic wave field. Klaus himself never published any substantial work on the subject (except for [27]) but he did encourage several PhD students (Kern Kenyon at La Jolla and Peter Müller and Dirk Olbers later at WHOI and Hamburg) to work on the question and provided a number of far-reaching new concepts.

Klaus has always been interested in ocean waves, which gave him his entry point into mainstream science (see Sect. 3.1). His PhD thesis from 1957 was titled "Über eine Methode zur Bestimmung der Reflexion und Brechung von Stoßfronten und von beliebigen Wellen kleiner Wellenlänge an der Trennungsfläche zweier Medien "(A method for determining the reflection and refraction of shock fronts and of arbitrary short wavelength at the interface between two media.) [191]. The first seminal paper he published on the subject was "Grundgleichungen der Seegangsvorhersage" (Basic sea state prediction equations) in 1960 ([3], see facsimile in Sect. 3.1). This paper provided a basic foundation for a reliable, generally applicable method of sea state prediction based on the basic energy balance equation of the ocean wave spectrum. He later published 3 papers entitled "On the nonlinear energy transfer in gravity-wave spectrum" in 1962–1963 [6, 8, 9], and "Propagation of ocean swell across the Pacific" [18] with Walter Munk in 1966.3

Klaus' work on remote sensing and the satellite ERS-1 (see Sect. 3.2) was also related to the field of ocean wave dynamics. He and Manfred Schieler published a paper on "Radar backscatter from the sea surface" [26] in 1970. Later papers, beginning with [45] from 1978, addressed aeroplane or satellitebased ocean wave spectra measuring methods. The MARSEN experiment (see below) led to another breakthrough when Klaus and his co-authors published the "Theory of SAR ocean wave imaging: A MARSEN view" [75] based on an imaging model, which was fundamental for SAR imaging of the ocean

3 There was also a movie produced, mostly with Walter Munk, but with Klaus showing up every now and then–https://youtu.be/MX5cKoOm6Pk.

surface from future satellites SARs. Another fundamental paper, which he co-authored with his wife Susanne, was "On the nonlinear mapping of an ocean wave spectrum into an SAR image spectrum and its inversion" [102], a reproduction of which is included in Sect. 4.2. This paper serves as an example of one of the major contributions the Hasselmann couple made to the future of the retrieval of the ocean wave spectrum from the ERS-1 C band SAR on the global scale. Klaus' latest, and perhaps final contribution to this topic was the extensive review entitled "The ERS SAR wave mode: A breakthrough in global wave observations" [176].

When Klaus Hasselmann took on the responsibility and challenge of running the Max Planck Institute for Meteorology (MPI-M) in Hamburg, most of his time and attention was taken up by the subject of climate change. His initial thoughts on the subject were set out in his seminal "stochastic climate model", which was published in 1976 ([38], see facsimile in Sect. 3.3), which provided an insight into the formation of long-term internal variations excited by short term random fluctuations. Although this approach was not particularly surprising for a theoretical physicist, the concept did change the way climate scientists thought about the problem. The stochastic climate model firmly established the concept of a stochastic climate system, which included the separation of externally provoked variations ("signal") and unprovoked internal" variability ("noise"). This led to the emergence of the general concept of "Principal Interactions Patterns" ([86], see facsimile in Sect. 3.4), which included the key idea that the full infinite state space may be split into a low-dimensional "signal" space in which deterministic dynamics hold sway, and an infinite higher-dimensional "noise"-space, which is well approximated by stochastic dynamics. The "Principal Oscillation Patterns" [89] represented a special case. But the most important aspect of this approach was the question of detection and attribution [54, 110], i.e., of detecting the footprint of anthropogenic climate change in the empirical record of climate variation. This approach emerged as a key argument in the Intergovernmental Panel on Climate Change's (IPCC) assessment that anthropogenic climate change is real and will intensify if greenhouse gas emissions continue unabated.

When the science of the mechanisms of climate variability and change had matured [118] in the early 1990s, Klaus Hasselmann became interested in the interaction between climate and society and how mankind could deal with human-induced climate change. He understood quite early that the anthropogenic climate change problem goes well beyond the domain of climate science. It is not nearly enough to frame climate change research solely within the limits of what is sometimes referred to as "curiosity-driven science". Instead, research into anthropogenic climate change should support policymaking and coordinated climate action. These ideas prompted Klaus to create the Potsdam Institute for Climate Impact Research as a matter of urgency to deal with the economic dimension of climate change. In collaboration with Dmitry Kovalevsky and others, such as Michael Weber and Volker Barth, Klaus attempted to construct optimal policies to balance the expected costs of damages with the expected costs of mitigation. Section 3.5 provides more information about this field.

Following his retirement, Klaus Hasselmann became less interested in climate science, probably because he thought that he had already contributed everything that he could to the field and that the remaining challenges, such as the economic dimension, would be taken care of by others. Instead, he returned to a topic, which he had been thinking about in his spare time throughout his career. On his 60th birthday, he surprised his guests with the announcement that he would present something new—which would also explain to his family that he had really been thinking about particle physics (Sect. 3.6), when they falsely believed that he was just trying to get out of mowing the lawn. His talk took about two or three hours: "You can ask me, but you cannot stop me!" he said, and most of the audience did not understand a thing but enjoyed the show. Of course, Klaus was serious about the topic and his Metron concept. His wife, Susanne, volunteered to manage his schedule and he began to present his ideas to the physics community, the majority of whom were unfortunately not inclined to listen to him. The full concept has now been documented in a series of articles and in an unfinished book whose introduction we have reproduced in Sect. 3.6. All we can do at present is to wait to see if Klaus ideas' will eventually rule the waves, as they often did in the past.

## **1.3 The Enabler**

When we talk about Klaus Hasselmann as an "enabler", we are referring to his ability to set things in motion, to create a scientific environment, which enables individuals to realise their full potential. Enabling activities may not leave a scientific footprint, but they do make an indirect contribution to the scientific process whether by creating a well-functioning working environment or by providing access to crucial empirical evidence. Klaus was an enabler in multiple ways.

His most important achievement was certainly the Max-Planck-Institut für Meteorologie, which provided many (at that time) young scientists with an environment in which to develop their skills. The various personal accounts included in Chap. 4 illustrate this breeding ground convincingly. "Do something that you consider interesting"—this Klausian request sounds like a recipe for disaster, an invitation to a hoard of intelligent young scholars to develop without coordination, without reference to a programme, and in various directions. But that did not happen. The young researchers all converged on the same problems but from their own unique angles. Of course, an Ernst Maier-Reimer would not take orders from anybody, but he would always have a suitable FORTRAN code for most problems in one of his desk drawers. In short, the Institute was a marvellous incubator, which was sometimes compared to an aquatic ecosystem populated with bottom feeders, primary producers, and gracious predatory fish. At the top of the chain there was just the one big fish, Klaus himself, who somehow managed to steer the dynamics within the incubator, with unconditional scientific rigour, personal friendliness and an endless reservoir of ideas.

One may well ask whether the MPI was organised in any way? There were a number of "Zwischenkapazitäten" (something like "lieutenants"), such as Dirk Olbers, Jürgen Willebrand, Mojib Latif, Martin Heimann, Peter Lemke, Ernst MaIer-Reimer and Hans von Storch, who acted as mentors to the younger researchers, but ultimately it was Klaus who guided and managed ideas, often by rejecting, replacing, or rectifying them. Even the management functioned smoothly. Klaus served as Managing Director throughout most of 1975–2000; only once did he wish to take a short break, and someone else took the helm and attempted to install a certain amount of administrative order, counting pens and the like. One of the then unhappy Zwischenkapazitäten went to the elder statesman, Reimar Lüst, at the end of the hallway to ask if we had misunderstood something? Nope, said Lüst, you've understood perfectly well; don't give in, it'll soon be over. And, really, Klaus was back after just a few days, and there was no more counting pens. Scientific paradise was re-established. Some of his co-workers, who later became Institute directors themselves, tried to copy his approach with some success.

As Institute Director, Klaus was also responsible for the financial side of things and had a special way of looking at this challenge, possibly guided by his colleague Hans Hinzpeter's famous dictum "a number is not a number", which played upon the empirical fact that any assertion about the financial situation would be preliminary, and prone to significant changes at short notice. Klaus spoke of "flying in fog", to suggest that knowing whether money would be available for hiring someone or making a capital purchase was largely based on a *feeling*. In a sense it was also a signal-to-noise problem. It worked out well and broke the potential spell of financial details involved in running a scientific Institution.

As outlined in his interview (Sect. 2.1) Klaus spent most of the first ten years trying to clarify the basic dynamic aspects of climate variability and change. Although the Institute has been founded to conduct research into climate change, most people there failed to notice this link. He also decided against purchasing a big computer, as most people had expected him to do but he worked instead with a relatively small group and a modest suite of hardware. But when the need for quasi-realistic climate models had become obvious by the early 1980s, Klaus established a close link to Günter Fischer's group at the Meteorological department of the University of Hamburg, which was located in the so-called in the Geomatikum some 10 m below the MPI. One member of that group, Erich Roeckner, was the then leading expert on atmospheric modelling. The first step was to replace the dynamical core that had been developed in-house with the model of the European Center for Medium Range Forecast, ECMWF, with parametrizations of the Hamburg model. The model was dubbed ECHAM—EC + HAMburg.

At about that time, Klaus had decided to set up a separate large computing centre, the Deutsches Klimarechenzentrum (DKRZ), which was to be led by Wolfgang Sell. This move made it possible to carry out gigantic simulations using a climate model based on Erich Roeckner's atmospheric ECHAM and Ernst Maier-Reimer's ocean LSG models. The computer system was updated regularly, with generous funding from the German Federal Ministry of Education and Research, whilst the costs for running the DKRZ were shared between a the MPI (the majority shareholder), the University of Hamburg, the Alfred Wegener Institute in Bremerhaven, and the GKSS in nearby Geesthacht. This system continues to run smoothly in 2021.

This process was eventually completed when Lennart Bengtsson was persuaded to become co-director of the MPI, where he would focus on atmospheric modelling (see Sect. 3.7). The ECHAM has become one of just a few leading quasi-realistic climate models, which are used in various institutions all over the world.

The incorporation of the DKRZ and the Bengtsson-department within the MPI were completed in the 1990s when the Potsdam Institute of Climate Impact Research, headed by Hans-Joachim Schellnhuber, was set up. This completed Klaus' original vision. He went on to think about metrons (Sect. 3.6).

### **Gathering data**

Although Klaus Hasselmann was really a theoretical scientist, he often got involved in the practical challenge of gathering the relevant data, whether in preparation for satellite missions, or for setting-up and managing various campaigns (in atmospheric and oceanographic science jargon: experiments).

The first experiment he ran as the lead scientist was the JONSWAP project in 1969 (see Sect. 3.1). Wind stress, atmospheric turbulence and swell attenuation were monitored in the German Bight, and eventually the JONSWAP spectrum was derived.

Following the success of JONSWAP, Klaus apparently felt secure enough to initiate the IWEX (Internal Wave Experiment) mooring campaign during his stay at the Woods Hole Oceanographic Institution (WHOI) which he visited alongside Mel Briscoe, Terry Joyce, Claude Frankignoul, Peter Müller and Dirk Olbers. To our knowledge, the IWEX tripod was the first mooring capable of measuring current cross-spectra with sensors at horizontal and slanted separations. The experiment was carried out in the Sargasso Sea in 1973.

This culminated in the international Marine Remote Sensing Experiment, MARSEN which Klaus coordinated. It was carried out in the North Sea between the 16th of July and the 15th of October 1979 and was designed to achieve the following two objectives: (1) to investigate the use of remote sensing technology for oceanographic applications and (2) to utilise remote sensing technology in concert with in-situ oceanographic measurements to investigate oceanic processes in finite-depth water in the near-shore zone. MARSEN made use of 6 remote sensing aircraft including the NASA CV-990 with the JPL SAR. 60 scientists from 6 countries took part in the experiment, which spawned a plethora of papers, 14 of which were published in a special issue of the Journal of Geophysical Research in 1983.

In his later career, Klaus no longer participated in long-term empirical observational campaigns, perhaps because his attention was increasingly focused on the climate issue. Experimental work in ocean science is more about understanding and parametrizing various processes in ocean models and the JONSWAP spectrum is an excellent example of this—whilst climate science mostly depends upon ongoing monitoring efforts. So, Klaus became involved in remote sea state monitoring technologies, which also, of course, had to do with his interest in the predictive potential of ocean waves. By the 1980s he had already become a key member of the ESA High Level Advisory Committee (EOAS), which had been set up by the ESA DG (Sect. 3.2). His commitment to the preparation of the ERS-1 satellite among other things was honoured by ESA, who invited him to join the launch of ERS-1 on the 17th of July at the Guiana Space Centre in French Guiana.

When Klaus became aware of the societal urgency of the climate problem in the 1990s, he founded the European Climate Forum (ECF), which was later expanded to cover a broader geographical range and renamed as the Global Climate Forum (GCF).4 Klaus made an active contribution to various ECF/GCF operations, where was vice-chairman and a member of the board for many years.

### **The WAM group**

In the spring of 1984 Klaus invited a number of ocean wave researchers to a meeting in Hamburg. There he proposed to jointly work on the development of an advanced numerical ocean wave prediction model. He wrote a new acronym (WAM, for Wave-Modelling group) on the blackboard and opened the discussion. The meeting supported the idea, because previous collaborations had created a shared feeling of urgency: model improvement was needed. Klaus recruited the participating Gerbrand Komen as chairman of the new group. He and Klaus would collaborate closely to "create a scientific environment, which enables individuals to realise their full potential", but which also enabled Klaus to achieve his goals.

One of the first challenges was to develop a scientific strategy. Some members focused on model development, but others simply wanted to collaborate on ocean wave research. This led to a number of subprojects, one of which was aimed at the development and implementation of global and regional versions of the model. Other subprojects focused on growth curve reanalysis, directional effects, shallow water effects and data assimilation.

Annual meetings were hosted at participating institutes on a rotational basis. They all opened with a review of the group objectives and what had been achieved in the past year. This was then followed by one or more formal presentations by guest speakers and a *tour de table* where each participant could say what he or she had done and was planning to do. After discussions, the tasks and commitments for the coming year would be listed. Klaus usually played a rather passive but very inspiring role in these meeting. Of course, some stricter co-ordination was later agreed upon in smaller groups, especially when the model was implemented at ECMWF.

Initially, there was no special funding. Most people contributed because the objectives of the WAM group aligned with the objectives of their home institutes. Once the research got underway the group became successful

4 https://globalclimateforum.org/.

in acquiring additional funding. Outreach and enlargement of the group played an important role in generating support. Outreach took the form of many (invited) lectures at participating institutes and specialist conferences, sometimes as a showcase of a successful collaboration. The group was made truly international when it merged with a newly established working group (Working group 83, 'Wave Modelling') of the Scientific Committee on Oceanic Research. This brought in participants from China, the Soviet Union and elsewhere, to everyone's mutual benefit. The group ultimately included about 70 people from 15 countries.

One characteristic of the group was the spirit of collaboration. Group identity was reinforced by a newsletter in addition to informal contacts during working visits and the annual meetings. One of the participants wrote some alternative lyrics to the well-known Beatle song "Those were the days, my friend", which became "Those were the waves, my friend", which included the unforgettable line "Comparisons have shown // The physics are unknown". It was sung loudly after dinner during a meeting in Canada, which created a warm feeling of solidarity. In hindsight one may well wonder whether there was too much of a warm feeling, as it may have blocked some healthy dissidence. But so it goes.

Understandably, given his many other activities, Klaus was strongly focused on model development, which culminated in the implementation of the WAM model at the ECMWF. The other subprojects were also successful and resulted in several publications. One of the outreach highlights was a five-week course on ocean waves and tides at the International Centre for Theoretical Physics with support from the World Meteorological Organization and about 100 participants, mainly from developing countries. Another one was the successful completion of a jointly written monograph (Dynamics and Modelling of Ocean Waves [244]) which was published by Cambridge University Press in 1994. After that the group was dissolved. Mission completed.

# **1.4 The Public Figure**

Klaus Hasselmann's work on ocean waves, which included remote monitoring and predictions was relevant and represents elegant science, which, however, hardly attracted public interest. The climate issue was completely different: his two major achievements, namely the detection of the signal of anthropogenic change against the background noise of internal variability, and the attribution of this signal to human greenhouse gas emissions, as well as the provision of a scientifically first-class climate simulation platform, had an enormous impact on the relevant public discourse, not only in Germany but around the world. Nevertheless, he remained mostly unknown to the general public. This was not a matter of bad luck but of his own deliberate intent—he was simply not "interested in informing the public, [he was] always interested in basic research", as his wife Susanne put it in Sect. 2.3. Thus, he was happy when others offered to do the job for him. These others were Hartmut Graßl and Mojib Latif, who both became very well known to the German public. Both excelled in explaining complex dynamics and perspectives in a way that lay persons could readily understand. Interestingly, they did so without coordination between themselves or with Klaus, which obviously caused no problem.

Asked for his opinion on the public perception that the most important climate researchers in Germany were Graßl and Latif, whilst he himself was barely known, he responded (Sect. 3.3): "I was very pleased about that."

Thus, not surprisingly, Klaus left few traces in the media. Among the few examples he did leave were:

The last piece is reprinted in the next Sect. 1.5. It has not been translated, as it is a wonderful example of living German whose special charm would hardly survive translation.

*Klaus and two of his Zwischenkapazitäten, Ernst-Maier Reimer (left) and Mojib Latif (right).*

*(c) Günther Menn, Lea La Greca; mit freundlicher Genehmigung*

# **1.5 In 1997, A Visitor Told His Perceptions When Visiting the MPI**

Freitag/ Samstag/Sonntag, 31 Oktober/1./2. November 1997.

Süddeutsche Zeitung Nr 251 / Seite 3.

*© Süddeutsche Zeitung GmbH, München. Mit freundlicher Genehmigung von Süddeutsche Zeitung Content (* www.sz-content.de*).*

Das Foto auf der vorhergehenden Seite wurde mit dem Text zusammen veröffentlicht. Urheber: Günther Menn; Genehmigung des Nachdrucks durch die Nachlassverwalterin Lea De Greca.

### Peter Sartorius

### **ln Sandalen die Welt von morgen suchen**

Klimaforschung: Welche Auswirkung hat die vom Menschen beschleunigte Aufheizung der Erde?lm Hamburger Max-Planck-Institut arbeiten 150 Wissenschaftler an Formeln, mit denen sich der Zustand des Planeten in 100 Jahren errechnen läßt

#### Hamburg,im Oktober

Bakan, der Bayer, studiert Wolken, manchmal von unten miels eines Laser-Geräts aus der Abteilung des Kollegen Bösenberg, manchmal an Bord eines Spezialflugzeugs von oben oder, was noch spannender ist von innen, etwa über Spitzbergen. Meistens aber macht Bakan Beobachtungen von seinem Hamburger Arbeitsplatz aus, wenn sich die Wolken in physikalische und chemische Formeln aufgelöst haben. Dann stellt er Fragen an sich selbst - die Teilchengröße im Wasserdampf betreffend oder den Umstand, daß sich Wolken über dem Nordatlank in Hunderte von Kilometern langen, parallel laufenden Wolkenbändern organisieren. Welche Bedeutung könnte dies für den Wärmeaustausch zwischen Atmosphäre und Ozean haben? Eminent wichtig, sagt Bakan, ein Arbeitsschwerpunkt

Aber was eigentlich ist nicht wichg? Bakans Kollege Graf, der Sachse, verfolgt bei allem Mitgefühl für die Betroffenen begierig Vulkanausbrüche, egal in welcher Weltecke.

 Aber zu berichten ist zunächst von einer anderen Keenreakon, die ganz persönlich Hasselmann betri, einen in Ehren und mit silbernem Bart ergrauten Mann. Als Physikstudent hae er vor fast einem halben Jahrhundert nach einer Systemak im Zusammenwirken von kurzen und langen Wellen im Meer gesucht, woraus sich folgerichg ergab, daß er Ozeanologe, dann Klimaforscher, dann Professor und am Ende - indes auch schon vor mehr als 20 Jahren - Begründer und Direktor des Hamburger Max-Planck-Instuts für Meteorologie wurde.

Und dort haben wir uns eingefunden, um zu studieren, was das eigentlich ist, Klima. Und wie es erforscht wird in der doppelstöckigen Baracke, in der das Instut zwischen einem Uni-Hochhaus namens Geomakum und einem zerfransten Biotop mit dazugehörendem Tümpel ein Hinterhofdasein führt, freilich eines, dem immer größere Bedeutung zukommt, jetzt, da eine vom Menschen mitverursachte allmähliche Klimaaufheizung registriert und die erhöhte Temperatur als ein ernsthaes Krankheitssymptom der Welt erkannt worden ist. Wer sich professionell damit beschäigt, muß nicht unbedingt

Meteorologe sein.  Mit der mangelhaen Klimasierung der Baracke könnte das zusammenhängen.   Und was das zur Folge haben kann, wird jedem schlagarg klar, der Bengtsson zuhört, dem Schweden und Co-Direktor im Instut, der bei Tee und Keksen darauf zu sprechen kommt, daß das eigentlich Dramasche der Umstand sei, daß die Erwärmung, soweit der Mensch sie bewirkt, vermutlich nicht mehr rückgängig zu machen ist. Die Keenreakon, wie bei der Atomkra. Aber Bengtsson hält sich bei der Analogie nicht auf, referiert jetzt über El Niño, das pazifische Klimaphänomen, das die Passatwinde erlahmen läßt und dadurch weltweit das Weer durcheinanderbringt. Dürre in Australien, Waldbrände in Indonesien, Fischsterben vor Peru, winterliche

Kältewellen in Regionen, wo es erst Herbst sein sollte. Vielleicht kommt zum Jahresende noch Schlimmeres, wenn El Niño seinen Kulminaonspunkt erreicht haben wird. Man wird dann, soviel ist jetzt schon sicher, den stärksten El Niño aller Zeiten messen. Nicht, daß Bengtsson dies unbedingt der allgemeinen Erwärmung der Welt und menschlichen Einfluß zuschreiben würde. Dafür gibt es noch zu wenig Erkenntnisse, außerdem ist es, ein paar Türen weiter in der Baracke. der Forschungsgegenstand des Kollegen Laf Aber Gedanken macht sich Bengtsson doch darüber, ob sich da nicht im Zusammenwirken von langfrisger Klimaerwärmung und kurzfrisger Weeränderung Tiefdruckgebiete verschieben und dann eines Tages, zum Beispiel, in China der Gelbe Fluß überläu, der bereits jetzt nur mühsam in seinen Dämmen gehalten werden kann. Das Oder-Hochwasser in fürchterlicher Potenz Millionen Opfer ist denkbar, sagt Bengtsson leidenschaslos, Todesopfer.

Aber es ist nicht die Stunde von Horror-Visionen. Ums Prinzipielle geht es, darum, dass sich der Planet eben nur eben in beschleunigtem Tempo erwärmt.  Großen Eindruck hat dies nicht gemacht, und auch auf der bevorstehenden Weltklimakonferenz im japanischen Kyoto wird eine wirksame Reduzierung des Kohlendioxidaustoßes der Industriestaaten kaum zu erreichen sein.

#### **Angst vor dem Irrtum**

Und ist es nicht tatsächlich beruhigend fürs Gewissen, dass man so ganz genau nun doch nicht weiß in welchem Ausmaß der Mensch der Übeltäter ist?  Hat sie nicht den Kühleffekt durch vulkanische und industrielle Aerosole unterschätzt und deshalb vor einem Jahrzehnt eine zu starke Erwärmung vorausgesagt, worauin ein Hamburger Nachrichtenmagazin bereits aufgeregt die Ozeane wegen des Abschmelzens der Polkappen überschwappen und den Kölner Dom untergehen sah?

   Aber weil das Max-Planck-Instut der hauptsächliche Nutzer ist, amert Hasselmann im

Nebenjob auch als Co-Direktor des Rechenzentrums und kann alle paar Jahre für rund 20 Millionen Mark einen dieser Rechenkolosse anschaffen, der dann mit Kränen von außen bis hinauf ins 15. Stockwerk des Geomakums gewuchtet werden muß - vermutlich, weil ein Gehirn immer irgendwie oben zu sitzen hat.

Prakscher wäre, es befände sich auf ebener Erde. Aber Hauptsache, es liefert Ergebnisse, und zwar schnell. Bö nger, der wissenschaliche Betreuer des Computers, spricht zungeschnalzend von mehreren Milliarden Gleitkomma-Operaonen in der Sekunde, was Hasselmann indes nicht zu beeindrucken vermag.    Aber mit mehr power sagt Hasselmann, könnten mehr Gierpunkte – Stop. Haben wir da nicht noch ein Wort von Graf im Ohr. Graf hae bis zur polischen Klimawende 1989 an der Humboldt-Universität in Berlin/DDR geforscht. Jedesmal, wenn er rechnen lassen wollte, mußte er mit seinen Unterlagen unterm Arm Tagesreisen um Westberlin herum zum DDR-Weerdienst unternehmen. Man habe, hae Graf

gesagt, angesichts der Verhältnisse den eigenen Kopf zu gebrauchen gelernt.  Bei Graf, bei Bakan, bei Bengtsson, bei Laf, bei Hasselmann und all den anderen geht es darum, gedanklich die Voraussetzung für eine Hochrechnung zu schaffen, für ein Modell, eine Welormel, in der alle Erscheinungen der Natur enthalten und in Relaonen gebracht worden sind. Man kann das als eine Denksportaufgabe nennen, bei der ist eine komplee Lösung nie geben wird, sondern immer nur verbesserte Annäherungen. Man kann es auch die Hersteller von Soware nennen, deren Computer-Ausdruck Meter hohe Papierberge ergibt.

 Testbilder sind es o nur, dazu dienlich, herauszufinden, wie fehlerha möglicherweise einzelne Vorgehensweisen beim Lösen der großen Aufgabe gewesen sind Gleichwohl, wie exosche Pflanzen wachsen die Bilder aus Aktenschränken, setzen sich an Bürowänden fest, wuchern hinaus auf Flure und von dort in andere Büros hinein - von Arpe zu Roeckner, von

Roeckner zu Laf. Von Laf zu Maier-Reimer.

  Der Wind aus Engels Mund. Die Wolken als wehende Teppich. Der Ozean als gewellte Muschel. Maier-Reimers Welt ist profaner. Die Lösbarkeit von Kohlendioxid im Ozean. Marime Biologie im Strömungssysteme. Die Konzentraon von Phosphat, Nitrat, Silikat, Sauerstoff in unterschiedlichen Tiefen. Wie wird es sich aus auf das ozeanische Kohlendioxid aus, wenn sich der atmosphärische Kohlendioxidgehalt erhöht? Welchen Einfluss hat es auf Algen, wenn sie mehr Kohlendioxid aufnehmen?  Experimentell schwer zugänglich, sagt Maler-Reimer mehr zu sich selbst als zum Besucher.

#### **Das Gehirn der Natur**

Antworten führen zu immer neuen Fragen in diesem efen, rätselvollen Forschungsgebiet namens Ozean. Und wieder muß man von Hasselmann reden, den das Meer nie losgelassen hat Das Phänomen der kurzen und langen Wellen in an anderer, größerer Dimension.  Laf - ein waschechter Hanseat trotz des Vornamens Mojib - kann in einer schönen Metapher ausdrücken, was da dahintersteckt. Die kurzfrisgen Wierungsschwankungen in der Atmosphäre, so seine Erklärung, müsse man als Tennisballe sehen, die unentwegt gegen einen Medizinball geschleudert werden: gegen den Ozean, der zunächst nicht reagiert, am Ende aber ins Rollen kommt, nein, etwas ins Rollen bringt eben: diese langfrisge Klimaänderung.

Will sagen: Der Ozean ist das Gedächtnis des Klimas. Wenden wir uns also an Roeckner, der in einem Zahlenwerk zu verrechnen versucht, was das Gehirn der Natur für die Zukun gespeichert haben könnte. Umgehend wir Roeckner eine Formel aufs Papier, dabei erläuternd, es handele sich um eine parelle Differenalgleichung mit lokaler Ableitung von Modellgröße nach Zeit, und er wolle jetzt Delta hinschreiben und F. Nicht, daß wir Roeckner gern unterbrechen, aber unsere Wißbegierde geht eher ins Grundsätzliche.  Jahrelang feilte man an dem Modell. Und heute ist es der Stolz des Instuts. MP/ECHam heißt es. Bei einem

internaonalen Leistungsvergleich hae es sich gegenüber anderen Modellen unlängst vorzüglich behauptet.

Also was alles ist in der Formel enthalten? Die Frage ist doch kaum ausgesprochen schon ergießt sich eine Sturzflut von Fakten und Faktoren auf den Besucher. Atmosphären-Temperaturen, Wasserdampfgehalt, Winde in alle Himmelsrichtungen und in der Verkalen, Ludruck an der Erdoberfläche, Wolkenbedeckung mit Niederschlägen in flüssiger und gefrorener Form. Bakan sehen wir über Spitzbergen kreisen. Druck in allen Luschichten. Prozesse im Erdboden. Wärme und Wassergehalt dort. Schneehöhen. Ozeantemperaturen an der Oberfläche und in allen Tiefen. Eisdicke. Eisverteilung. Salzgehalt. Dreidimensionale Strömung. Maier-Reimer kennen wir hinter rutschenden Bergen von Papier, nachdenklich dem Bart streichend.

Nicht enthalten, sagt Roeckner, seien Gletscher und Plaentektonik. Aber ferg ist er noch lange nicht. Die chemischen Komponenten. Grafs Vulkane und Aerosole Das troposphärische Ozon. Autoabgase. Kohlensto reislauf in der Atmosphäre und im Ozean.   Aber plausibles dynamisches Vegetaonsmodell, sagt Roeckner, als ob er sich entschuldigen müsse, ein solches Modell gebe es noch nicht, also vom Klima keine Rückkoppelung zur Vegeta-onsänderung und von dort zurück zur Klimaänderung.

 Und alles wird in Hasselmanns Großrechnern mit Milliarden Gleitkommaoperaonen pro Sekunde verarbeitet zu diesen vo Computer ausgespuckten Kunstwerken mit ihren schillernden Schlieren, ein errechneter Zustandsbericht der Welt, wie geklaut, nein, geklont von der Wirklichkeit der Natur mit ihren über den Globus verstreuten Trocken- und Regengebieten?

    Aber Arpes geübte Augen entdecken feine Unterschiede. Ein pedanscher, unnachsichger Prüfer ist er, der festzustellen hat, wo die Theorie mit der Wirklichkeit kollidiert. Eine Schprobe hat er gemacht für eine Qualitätskontrolle des hauseigenen Klimamodells.  Nun hat er das Resultat vor sich, und sein Blick bleibt an Spanien hängen, dem winzigen Appendix eins kleinen Europas. Es ist das Gelb dort,

#### **1 Klaus Hasselmann—His Scientific Footprints and Achievements 19**

das ihn stört. Ein winterliches Dauerhoch zeigt es an. Aber dieses Hoch kann nicht sein, darf nicht sein, weder im Interesse des Hamburger Max- Planck-Instuts noch in dem der Iberer.

Es grünt nämlich nur deshalb so grün, wenn Spaniens Blüten blühen, weil es im Winter in Spanien regnet.  Keine Ahnung, sagt Arpe, was da schiefgelaufen ist, warum sich das Bild verschoben hat.   Hae man es ursprünglich mit zwei Fehlern zu tun, die unbemerkt blieben, weil sie sich gegenseig auoben, und von denen dann einer ausgemerzt wurde, so daß der andere in Erscheinung treten konnte?

#### **Ein Joker bleibt**

Es kann lange dauern, bis man Antworten finden wird.     Klimaschwankungen, präzisiert nun Hasselmann, entstünden in Analogie zu den langsamen Zufallsbewegungen schwerer Moleküle, die von leichten angestoßen werden. Das definiert einen Mechanismus. Aber erklären läßt sich damit natürlich nicht alles. Der variierende und sich in Sonnenflecken äußernde Energieausstoß der Sonne, der in seinen Auswirkungen nicht richtig abschätzbar ist.    Der große Entwicklungssprung also steht erst noch bevor.

Wie im Fall der vulkanischen Aerosole. Aber nicht bei Graf sind wir, sondern noch immer bei Arpe, dem Wahrheitssucher. Spanien, sagt Arpe, denken Sie an Spanien! Und so begreifen wir am Ende Spanien als Synonym für verbleibende Unwägbarkeiten und als Metapher für den zusätzlichen Faktor, der sich zu den unzähligen physikalischen, chemischen und sonsgen festen und variablen Größen der großen Klimagleichung gesellt, den Unsicherheitsfaktor.

*A critical inspection of the older sister Almut. Hamburg, shortly before leaving for England in 1934 (left), and In Welwyn Garden City, England, shortly before leaving for Hamburg, 1949 (right).*

*With Karl Wieghardt, diplom thesis advisor and later post-doc employer in Institute for Naval Architecture, at inauguration ceremony, 1975.*

*With Susanne and two oldest children, Meike and Knut, in La Jolla, 1963.*

*At Woods Hole Oceanographic Institution, in front Research Vessel Knorr, 1972.*

*With Bob Stewart, Brian Tucker and Australian sheep during break of the Joint Organizing Committee meeting of the Global Atmospheric Research Programme in Melbourne, 1974.*

*With Reimar Lüst, President of the Max Planck Society, at inauguration ceremony of the Max Planck Institute, 1975.*

*With Peter Fischer Appelt, President of the University of Hamburg, Senator Biallas of the City of Hamburg and Reimar Lüst during the inauguration ceremony, 1975.*

*Explaining ocean wave prediction, 1982.*

*In the new prefab building ("pavillon") behind the Geomatikum, after creation of the DKRZ, 1989.*

*Making a point, 1988.*

*Robertson Memorial Lecture Award, US National Academy of Sciences, 1990 (proposed by Carl Wunsch, second row, first left).*

*With Hartmut Graßl, 1996.*

#### **1 Klaus Hasselmann—His Scientific Footprints and Achievements 27**

*Explaining the multi-agent aspects of a coupled climate-economy model, 2002.*

*Explaining the detection of an anthropogenic climate signal at 95% statistical confidence level, with the Federal Minister of Research and Technology, Jürgen Rüttgers, 1992.*

*60th birthday, Rissen 1991*

*With Walter Munk, during Hasselmann's 60'th birthday symposium, 1991.*

*With Wolfgang Sell, Lennart Bengtsson and wife Susanne during emeritus dinner, November 1999.*

*Ola M. Johannessen, Walter Munk and Klaus Hasselmann sailing the fjords near Bergen. Ola comments: "Walter and I are discussing my CO2-Ice paper from 2008, too simple for Klaus, who took a nap"*

*Rissen, 2011.*

*with Hans von Storch*

*Susanne, Klaus and Dirk Olbers 2012 in Fischerhude, discussing ocean physics*

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# **2**

# **Klaus Hasselmann—His Own Account**

In 2006, Hans von Storch and Dirk Olbers ran an interview with Klaus Hasselmann. This interview is here reprinted without alterations. The numbered references in the interview refer to the publication list at the end of this book. The interview was published as von Storch H., and D. Olbers, 2007: Interview with Klaus Hasselmann, *GKSS Report 2007/5*; 67 pp.1

# **2.1 The 2006 Interview**

### *Question: How did you become interested in physics?*

**Hasselmann**: One of my early experiences which kindled my interest in physics was buying a crystal detector from a school friend for two shillings and six pence—half a crown—or about the price of a movie ticket. I must have been about 13 years old. I was quite impressed that even without plugging the device into a socket, I could listen to wonderful music through the earphones. I wanted to better understand the puzzling phenomenon that you could get something from nothing. I went to the town library in order to find out in books on physics for beginners how electricity and radios work. That was my introduction to physics. At that time, it was an exciting experience for

1 See https://www.hereon.de/imperia/md/content/hzg/zentrale\_einrichtungen/bibliothek/berichte/ gkss\_berichte\_2007/gkss\_2007\_5.pdf.

In 2013 it was also published online by the Niels-Bohr Library and Archives of the Center for History of Physics, https://www.aip.org/history-programs/niels-bohr-library/oral-histories/33645.

The original interview featured the photos shown before plus a few more, and additional to a foreword an introduction by Reimar Lüst and a concluding comment by Walter Munk.

me, completely independent of the fact that I was taught physics in school. I did not see any connection between our physics lessons in school and my personal learning from the books in the library—I think this experience of personal learning and discovery was very important for me.

*We have just heard that the detector had cost half a crown—so you did not attend school in Germany but in England. How did that come about?*

**Hasselmann**: When I was close to three years old my family—my parents and older sister—emigrated to England. My father was a social democrat and did not want to stay in Germany in 1934. Our family moved into a so-called community, consisting mostly of Jewish emigrants from Germany. The English Quakers helped us a lot in those days. Until we returned to Hamburg in 1949, we lived in a very nice small town, Welwyn Garden City, 30 km north of London. I passed my A-levels there (then called Higher School Certificate). I felt very happy in England. So, English is in effect my first language.

# *Nevertheless you studied in Germany*.

**Hasselmann**: I studied in Hamburg. I did a half year practical training in a machine factory first, because I was not sure whether I wanted to study engineering or physics. In addition, I was not yet at home living in Germany—neither were my parents, in fact, because Germany had changed. So I had to find my feet first. When I started studying, the idea of having to work hard for my studies was also a new experience. So I fell back a little during the first year. I had doubts whether I really was talented enough to continue with my studies, so—as a test—I took a study exam (Fleißprüfung), which I passed, and so I continued. I did not regret that period of adaptation, but it was a drastic change between my English school days spent in a healthy, suburban garden town north of London and living in Hamburg, where everything was bombed to ruins. However, I had always wanted to go back to Germany to explore my roots. My parents were always patriotic, in a natural, pre-nazi sense. But I was always very happy in England and did not really experience any difficulties due to my German origin, not even during the war. Still, I wanted to find out where I belonged. In spite of the difficult period of adaptation during the first one or two years, I did not regret returning to Germany.

### *Did you study only in Hamburg?*

**Hasselmann**: I studied in Hamburg for eleven semesters until I obtained my diploma in physics, in the summer of 1955, with mathematics as a second subject. Then I obtained my Ph.D. at the Max Planck Insitute of Fluid Dynamics and Göttingen University from 1955 until 1957. Afterwards, I returned to Hamburg, where I spent three years as a post-doc working with my former diploma supervisor, Prof. Karl Wieghardt, at the Institute for Naval Architecture, before going to America in 1961.

### *Would you like to recount the theme of your diploma thesis?*

**Hasselmann**: In my diploma thesis I worked on isotropic turbulence and found an—in my opinion—slightly more elegant derivation for the basic dynamic equations for isotropic turbulence [1]. For my doctoral thesis I changed subject to study the propagation of so-called von Schmidt head waves, elastic waves at the boundaries between two solid objects. In Hamburg I returned again to fluid dynamics research, mostly to experimental work on turbulence in ship wakes, using hot-wire instruments in a wind tunnel and a towing tank. But I also continued working on turbulence theory.

*This did not correspond to the mainstream of education in physics. Were not atomic theory and nuclear research considered the normal case in physics already in those days?*

**Hasselmann**: Yes, that was the mainstream, but I wanted to work in an area in which I thought I would be able to contribute something. I always had a practical bent, I wanted to work on problems which I thought I would be able to solve. I did not want to work on abstract, theoretical problems, and I did not have enough self-confidence to think I could make significant contributions to such difficult fields as general relativity or quantum field theory. So I went into fluid dynamics. I was always interested in the way planes and rockets worked. I liked my field of work, and I only gradually drifted into oceanography, meteorology and climate research. Later, I did then become interested in quantum field theory, elementary particle physics and general relativity, through my work on nonlinear interactions in geophysical wave fields, starting from ocean waves. I pursued these investigations for many years in parallel to my regular research, so to speak as a private hobby. However, all this developed in the course of the years. First I had wanted to work on a practical, solvable task as a physicist.

*Then there actually was a practical task resolved by you?*

**Hasselmann**: This is an embarrassing question.

*The turbulence theory has surely not been resolved.*

**Hasselmann**: Exactly, but then I was young and naive, and I hoped to make some progress in this problem, despite the fact that several generations before had failed. Nevertheless, my struggles with turbulence theory taught me a lot on stochastic processes and interactions in nonlinear systems. This enabled me to solve other problems later on. The first problem I solved theoretically was the question of the nonlinear coupling of ocean wave components. I would not have been able to solve this problem if I had not worked on turbulence before.

*Which mark did you get in your doctorate thesis? This question may provide moral support for millions of others.*

**Hasselmann**: Another embarrassing question. I received a 2 (corresponds to B). The reason was presumably that I solved the problem I was posed (propagation of von Schmidt head waves) in a different way than suggested by Prof. Tollmien's assistant. I found out quite early, after a few months, that the way suggested by my supervisor would not work. So I chose another path, which led to the goal, but my supervisor was not enthusiastic. Nevertheless he accepted my thesis and gave me a 2, because I had produced some very nice computational results obtained with Germany's first electronic computer, the G1, which had been developed in Göttingen. It is now in the German Science Museum in Munich. It had a total memory of–believe it or not—25. It was quite a challenge to use it to solve a system of several equations with many different parameters. I had access to the machine at night, and played table tennis with another student until the alarm bell of the G1 informed me that there was an error, which I would fix by cutting out and replacing part of a holerith paper tape, which was glued together in a closed loop. Different computational loops were realized by different holerith paper tape loops on different readers. One could follow the course of the computation as different readers were switched on and off. I presented my results very nicely in numerous graphs, which apparently impressed my supervisor. So I obtained my Ph.D. in less than two years [2, 191], in spite of the forbidden approach I had used to solve the problem.

*Your family did not discuss physics at breakfast. How did you head towards science?*

**Hasselmann**: I was always interested in understanding physical processes. As I already said, one trigger was the crystal detector. But I also constructed electrical motors and such things, and was continually producing short circuits at home. I got good grades in physics in my final school examinations, but without any relation to what I was taught in school. My physics teacher did not inspire me at all; for him I was an unruly trouble maker whom he often kept in after school. "Hasselmann, detention at four!" is still ringing in my ears.

Later at the university I was strongly motivated by my fellow students, particularly Wolfgang Kundt, Gerd Wibberenz and Ewald Richter. with whom I solved exercises together and had many discussions. That was a very intense period, forming lifelong friendships. Wolfgang Kundt and Gerd Wibberenz became Professors of physics in Bonn and Kiel, and we worked together occasionally also later. Ewald Richter became a professor of philosophy in Hamburg, and we had many interesting discussions with him too. I was also inspired as a student by Pascual Jordan, who taught theoretical physics in Hamburg. I was not in personal contact with him, but I really enjoyed his lectures. After the diploma I mainly instructed myself. I read interesting books and familiarized myself with the literature related to my research—as I suppose all young scientists do. But I never really had a proper mentor, neither at school, nor during my studies. In 1961, when I was already 29, I got to know Walter Munk,2 who invited me to his institute in La Jolla. I have had a close relationship with him ever since. His open, generous personality as well as his enthusiastic approach to science have always impressed me. Nonetheless, although I wrote one or two joint publications with him, I regard Walter more as a personal than a scientific role model.

### *Would you say that you had a factual supervisor?*

**Hasselmann**: For my Ph.D.? No, I did not have a real supervisor. Prof. Tollmien, then Director of the Max Planck Institute for Fluid Dynamics, was no longer active. As I explained, his assistant had a different idea on how I had to solve the problem posed for my thesis. I could not really discuss the

2 After recording this interview, Klaus Hasselmann and Hans von Storch prepared an interview with Walter Munk, see: von Storch, H., and K. Hasselmann, 2010: *Seventy Years of Exploration in Oceanography. A prolonged weekend discussion with Walter Munk*. Springer Publisher, 137 pp, https://doi.org/ 10.1007/978-3-642-12087-9 (http://www.hvonstorch.de/klima/books/munk-springer-final.pdf).

problem with him. I worked and learnt independently and read the necessary literature. In the following three years in Hamburg I had very good relations with my former diploma supervisor, Prof. Wiegandt, but scientifically, we did not interact very strongly, as he was oriented more towards experimental work. Although I was also involved in experimental turbulence measurements at that time, using hot-wire instruments, I worked more or less on my own—with limited success experimentally, I have to admit. But it was still fun finding out how to build the equipment, learning about feedback systems and the havoc that they can create in trying to construct high level amplifiers to measure weak turbulence signals.

### *Then you went to America.*

**Hasselmann**: Yes, this was through Prof. Roll, the former president of the German Hydrographical Institute, today called BSH. Parallel to the development of hot-wire measuring instruments, I had become interested in ocean waves. At the Institute for Naval Architecture there was considerable interest in the wave resistance of ships and ship motions in waves, motivated by the director of the institute, Prof. Georg Weinblum, a very kind and supporting person, who was an international expert in the field. The behaviour of vessels in rough seas in particular was a central topic at the institute. In this context, I read some very interesting papers by Owen Phillips and John Miles on the wind generation of ocean waves, which further stimulated my interest in the subject. My own first contribution to the subject was simply the introduction of the spectral energy balance equation for the prediction of ocean wave spectra, which, strangely, nobody had used before. Then it became clear to me that to understand the spectral energy balance of ocean waves, one had to solve the problem of the nonlinear interactions between wave components. I realized that the problem could be solved by the methods I had learnt in struggling with turbulence theory. Although the relevant closure methods were inadequate to solve the strongly nonlinear turbulence problem, they were directly applicable to the problem of weak interactions between ocean wave components. So I was able to derive a closed expression for the nonlinear energy transfer between ocean waves. It was represented by a relatively complicated five-dimensional so-called Boltzmann integral. Basically, I solved this problem to relieve my frustration at not being able to solve the turbulence problem.

I presented my results on the spectral energy balance and the nonlinear energy transfer in a seminar at the Institute for Naval Architecture [4]. Although most of the naval architects were somewhat confused by the mathematics, Prof. Weinblum was enthusiastic and encouraged me to continue with theoretical research. Prof. Wieghardt also concluded that I was probably more effective working theoretically than making painstaking experiments with hot-wire instruments, that had a troubling inclination to oscillate. Prof. Roll, who had been working in air-sea interaction for many years, was also there and was apparently favourably impressed. He proposed that I should attend the coming Ocean Wave Conference in Easton/USA in April 1961, to which he had been invited, but could not go. That is how I came to America, where I again presented my results. At that time—although I had not known this–the problem of the nonlinear interaction between ocean waves was seen as one of the central problems of ocean waves. I immediately received invitations to the Ocean Research Institutions in La Jolla, California, and Woods Hole, Cape Cod, as well as to the University of Illinois. I accepted the position of Assistant Professor in La Jolla offered by Walter Munk, whom I met for the first time at the Easton Conference. I found the atmosphere at the Institute for Geophysics and Planetary Physics that he had just founded at Scripps Institution of Oceanography very stimulating. So half a year later, at the end of 1961, I went to La Jolla, and enjoyed more than three very fruitful and stimulating years there.

 It is known through your publications that the triple interaction of surface waves does not function and that, one must extend interaction theory to higher perturbation order to get reasonable results.*

**Hasselmann**: Actually, independently of my papers [5, 6, 8–10], Owen Phillips had already shown that the necessary conditions for the resonant energy transfer between different wave components could not be satisfied by three wave components, but only by four. However, Phillips had not derived the Boltzmann equation. Before Phillips published his paper, I had already independently derived the complete Boltzmann equation for the lowest-order triple-wave coupling. When I wanted to calculate the integral, however, I found to my dismay that the resonance condition could not be satisfied. That was a shock. I had calculated the complete theory up to the third order, and understood all the details about the energy transfer through resonant interactions in a continuous ocean wave spectrum, only to discover that the third-order resonance conditions could not be satisfied due to the special dispersion relation of ocean waves. That meant that the calculations had to be extended to fifth order.

I went for a three-hour long walk in the town park in Hamburg and debated within myself whether I could muster the energy to carry through two further orders of these quite complicated calculations. I decided to go through with it and spent another two or three months working on the algebra. It proved not as bad as I had first feared, although I had to derive formulas extending over one or two pages. By the time I received the invitation to present my results at the Easton Conference, I had already found a very talented young student of applied mathematics, Herr Krause (students in those days were addressed rather formally in Germany), who programmed the numerical calculation of the Boltzmann integral for me. He used the highest possible resolution available on the computer of the University, which by now was more than the G1, but still quite limited. I was very impressed that within two or three months he came up with the first numerical results. Although we later obtained more accurate results with improved computers, his results were qualitatively correct. However, they did not agree in all aspects with what I had anticipated intuitively, and so when I gave my talk in Easton [10], I pointed out that they were probably incorrect in some details. Later it became clear, however, that his calculations had in fact been qualitatively quite correct. He had even correctly computed the most important process which I had questioned intuitively–namely the transfer of energy from waves near the peak of the spectrum to still longer waves. Ten years later we were able to show–through the JONSWAP experiment–that this is the dominant process responsible for the continual growth of wind generated waves from shorter to longer and longer waves. I am still grateful for this impressive contribution by Herr Krause. It enabled me to present not only the theory, but also first numerical results in Easton.

*Was it customary these days that you did not program yourself? I am slightly astonished that as a relatively young man, as a postdoc, you got someone to program for you. Were there special technical obstacles to be overcome?*

**Hasselmann**: No, you only had to have some experience in programming. Of course, I cooperated with the student. I explained to him which numerical algorithms should be applied, but he implemented that knowledge into the program, carried out the computations, made the usual tests and searched for errors, etc.. He fully understood what he was doing. I simply hired him as a student assistant.

# *We are talking about 1960/61. Did FORTRAN already exist?*

**Hasselmann**: I can't actually remember. FORTRAN may already have existed, but I cannot recall in which language Krause wrote the program. I know that the first programs I wrote for my Dr. thesis were in machine code, and my later programs were all in FORTRAN, but I am not sure whether Krause was alredy using FORTRAN.

### *Starting from 1960, can you please tell us when which persons entered your life?*

**Hasselmann**: During the first period in Germany it was Professors Karl Wieghardt, Georg Weinblum and Hans Roll, and Pascual Jordan as a physics teacher and the usual mathemathics professors, but I was not in personal contact with them. In America, as I said, Walter Munk left—and still leaves– a lasting impression on me. I had already known his name from the first classic publication by Sverdrup & Munk (1947) on the prediction of ocean waves, from which I had concluded, however, that his knowledge of physics was rather limited. At first, I underestimated him as a scientist, but when I got to know him personally, I was very impressed not only by his clear scientific thinking but also by his open-minded, positive and supportive generosity. He had a Viennese charm. He was an Austrian, who had emigrated to America already in the twenties, but still spoke with a strong Austrian accent. I gladly accepted his invitation to his new IGPP in La Jolla. I had an office in the beautiful new redwood building of his institute, that his wife Judy had designed, overlooking the Pacific on a cliff. I felt very happy in La Jolla from the beginning, especially with the open American way of welcoming new visitors. Coming from the somewhat, well, perhaps not stuffy, but not particularly creative atmosphere of German science in the fifties and early sixties, to America, where everyone was really enthusiastic, was a great experience for me.

Walter Munk was the central figure, but there were also other very stimulating people in La Jolla, such as Michael Longuet-Higgins, a wellknown applied mathematician and fluid dynamicist from Cambridge, who had contributed many basic papers on ocean waves, microseisms and other geophysical phenomena. He had a guest professorship in La Jolla while I was there. Other guests were Norman Barber from New Zealand, a pioneer in ocean wave research who had studied the propagation of ocean swell, and David Cartwright, a co-developer of the pitch-and-roll buoy for measuring directional ocean wave spectra, and also a leading expert on tides. At Scripps there were also John Miles, who had developed an important theory on wind-wave generation, and Hugh Bradner, an interesting former high-energy physicist, who measured pressure variations in the deep ocean. I further enjoyed the interaction with George Backus and Freeman Gilbert, two young geophysicists of more or less my age, who had done some very nice work on inverse methods in geophysics and whose basic mathematical knowledge was very impressive. Klaus Wyrtki3 who later became one of the leading figures in El Nino research, and Carl Eckart, who had written an impressive book on theoretical oceanography, were also two well known figures in Scripps at that time, although myself had little direct contact with them. Another person who came to Scripps while I was there was David Keeling (he signs his papers Charles Keeling), who was making measurements of CO2 on Mauna Loa in Hawaii. He had just started the measurements four years earlier. I didn't know at the time that I would later be continually referring to the now famous Keeling curve as the most important observational basis of the climate change debate. Our main contact at that time was through the madrigal choir that a few of us started. It later blossomed into quite a large university choir led by David until he died last year.

So I was immersed in a highly stimulating scientific environment. The discussions continued also in the weekly wine and spaghetti parties in Walter Munk's home—a beautiful spacious redwood bungalow overlooking the Pacific, which his wife Judy had also designed.

There were also many stimulating students. The first student I supervised was Russ Snyder, who worked later also in ocean waves. I kept in contact with him, and several years later we wrote a joint paper, together with my wife and two other colleagues [114]. My wife and I also joined Russ's family on a twoweek sail in the Eastern Mediterranean along the beautiful Turkish coast. It was on their way back to America after a three-year sail around the world in a ketch Russ had built himself. My second student was Kern Kenyon, who visited me later in Hamburg and is still at Scripps today. Then there was Brent Gallagher, who also was very talented and did some nice work on nonlinear barotropic waves. He is now somewhere in Hawaii. Finally, there was Tim Barnett, who in his Ph.D. thesis developed the first model for ocean wave prediction based on a realistic representation of the spectral energy balance, including the nonlinear energy transfer. Some years later we worked together in the JONSWAP experiment, and still later, after the Max Planck Institute was created, we cooperated in several papers on climate. Today he is a wellknown climate researcher. So, these were my first students. I am glad they all did well.

3 Klaus Wyrtki has been interviewed in English earlier, see von Storch, H., J. Sündermann, and L. Magaard, 2000: Interview mit Klaus Wyrtki. http://www.hvonstorch.de/klima/Media/interviews/Wyr tki.pdf (GKSS Report 1999/E/74, 41 pp.).

*I know that you were not always seated at your desk, interpreting integrals. You also did experimental research, e.g. on Hawaii.*

**Hasselmann**: This was the first large, ocean-wide wave experiment organised by Walter Munk and coordinated by Frank Snodgrass, a technician and Walter's right hand man in all experimental matters. Similar to Norman Barber, Walter Munk had carried out continuous measurements of the spectral properties of swell arriving at a single coastal station, in his case near La Jolla. He had inferred from the gradual change in the observed swell spectra–the arrival first of very long waves, followed by waves with gradually decreasing wavelengths—that the swell must have originated in storms very far away in the South Pacific and Antarctic. Munk now wanted to find out how the energy of the swell changed as it propagated from its source somewhere south of Australia, in the high-wind region of the "fighting fifties", across the entire Pacific up to Alaska, over a distance of about two thirds of the earth's circumference. Some waves even originated in the Indian Ocean, propagating into the Pacific along a great circle between New Zealand and Australia. So Munk set up a series of wave measuring stations along a great circle extending across the entire Pacific, starting in New Zealand and ending in Alaska. In between there were stations at Samoa, Palmyra, an uninhabited atoll between Samoa and Hawaii, Hawaii, and "Flip". Flip was a special ship anchored between Hawaii and Alaska that could be flipped so that it stood vertically like a float in the water, the bows up high and the stern down below. The idea was that this way the boat stayed almost still in the waves and could be used as a wave measuring station.

Walter Munk, with Judy and his two daughters, stayed in Samoa, a scientist, Gordon Groves, and radio operator were flown to Palmyra, Frank Snodgrass and I myself, with my wife Susanne and three children, were in Hawaii. Frank Snodgrasss took care of the logistic organisation, and I had to tend a wave instrument and check the data from the entire experiment, which was flown to the computer center in La Jolla and then back to Hawaii for a first analysis. The experiment ran for the three summer months of 1963.

We had a wonderful time in Hawaii. One of the first things Frank Snodgrass did was to install a telephone connection from the swell measurement station off Honolulu to our house in Kailua, which was situated on the other (northern) side of the island. My measurement task was to turn on the tape recorder for an hour at 06:00 a.m. and again for an hour at 06:00 p.m, check for a couple of minutes whether the data on the paper tape looked OK and airmail the tapes to Scripps for spectral analysis. And occasionally I would plot up the analyzed spectra from all the stations that were sent back to Hawaii from La Jolla.

Unfortunately, this wonderful time was occasionally interrupted by the electric generators on Palmyra breaking down. They had five generators, of World War II vintage, which one would have thought was sufficiently redundant, but four were usually broken down. I had to drive around Oahu to find replacement parts. Palmyra had served as an airbase during World War II, but was now deserted except for our scientist and the radio operator. Frank Snodgrass felt rather uneasy about leaving two people alone on a deserted island for three months. So he had arranged that if Gordon Groves should inform him via the radio operator that "the second amplifier had failed", this was code for "urgent problem, come immediately". After two weeks we received the message. I went there by plane to find out what was wrong. In the meantime, however, the two had already patched up. Two weeks later the radio went silent and we did not hear anything from the two. Then I received a radio message that Gordon Groves had hurt his hand, which was bleeding strongly. This was followed by another week of total silence. We became quite worried and decided to go there by plane.

The first time I flew there it was in an old B25, a twin-engined bomber from World War II, used by former marine aviators to spray fields. A short time earlier, they had already tried unsuccessfully to fly to Palmyra. They did not have any modern navigational aids. They flew by Dead reckoning, i.e. like a sailor without navigational marks. You fly in a certain direction at a certain speed for a certain time and calculate your position accordingly. In addition, you must know the winds. They arrived at the calculated position, but Palmyra was nowhere to be seen. So they flew on to Tahiti. But there a thunderstorm prevented their landing. So they flew back, again over Palmyra without finding the atoll. With their last drop of fuel they just managed to land in Honolulu. The whole airport had been closed down. No other plane was permitted to land before they had landed. Directly after landing, the two pilots were taken off by the police.

That was the crew I flew to Palmyra with. If my wife had seen those bearded and dirty characters, sparsely clad in shorts, with or without T-shirts, she never would have let me fly. They again had problems finding the atoll. I was seated behind the navigator who was busy with his square search, and I could see pearls of sweat developing on his neck. But suddenly he cried: "There's the island!"

After that first time, Frank Snodgrass decided not to repeat the experience. He was able to obtain a transport aircraft of the US Coastal Survey, a large four-engined machine with a crew of eight, modern navigational aids etc. When we arrived and wanted to rescue our assumedly seriously ill scientist we were met by our two friends, both extremely cheerful, and with Gordon Groves sporting a small band-aid around one finger.

It was a time full of fun and adventure. Walter Munk, however, was a little disappointed by the outcome of the experiment [18] because he had hoped to observe the attenuation of swell by interactions with the local windsea, when the swell crossed the trade wind areas. However, no significant loss of swell wave energy could be found over the entire distance travelled by the waves, from Antarctica to Alaska. This was nevertheless an important result, which was used in the wave prediction models that were developed later. We did infer some energy loss immediately after the wind-generated waves left the area of high winds and started on their long journey as swell, that is, as long waves that are no longer forced by the wind. We were able to explain this by the nonlinear energy transfer. This was perhaps the first observational evidence of the significance of this process for the energy balance of the wave spectrum.

The Pacific swell experiment supplied also the idea for JONSWAP, the Joint North Sea Wave Project, which we carried out in the summer months of 1968 and 1969. JONSWAP was complementary to the Pacific swell experiment. Instead of studying the propagation of swell after the waves had left the wind-generating area, we investigated the growth of wind-generated waves themselves within the wind generating area. To understand the dynamics of waves, this question was clearly fundamental. We used the same strategy as in the Pacific wave experiment, but on a much smaller spatial scale: we observed the change in the wave spectrum under off-shore wind conditions at ten wave stations spaced over a distance of 160 kms off the West coast of Germany, off the island of Sylt near the Danish border, in the North Sea.

### *Nevertheless, many things were still to happen before the JONSWAP experiment. Your time in the USA ended, and you returned to Germany. Why?*

**Hasselmann**: As I explained, the scientific working conditions in the USA were excellent. However, my wife was less happy, although this improved after we made friends, sang in the San Diego chorale and in the madrigal group that we had founded with Dave Keeling. Susanne had also made friends with a very stimulating piano teacher. But our children were also not as happy as they had been in Germany, especially our oldest, Meike, who had always been a beaming sunshine. At that time California was going through a phase of laissez faire, in which children grew up without any restrictions. They never knew any rules, what was permitted or forbidden, and they always seemed illtempered. At least in the kindergardens we knew the children did not seem to be really happy. Meike had become rather unstable. She had a pseudo croup, and we nearly lost her. In the end, we finally decided to return to Germany and bring up the children there.

But the decision was difficult and we did not make it immediately. Before going back I first tried a joint appointment, with six months in Hamburg and then six months again in La Jolla. But then we finally decided to return to Hamburg. It was not an easy decision.

*How did you go on? Assistant at the Institute for Shipbuilding. Returning to the much more authoritatively organised German university must have been quite a difference from the more liberal structures in California? And to be taken up only as an assistant.*

**Hasselmann**: No, I really had no problems. I had to give relatively few lectures, and this suited me, because I always felt that I could not explain things better than they were explained already in good text books. I was never a motivated lecturer on basic courses. I liked talking about research in seminars, but I was not motivated to repeat the basics that people could better study in text books that had been prepared with much greater care than I ever devoted to my lectures. I myself also preferred learning from books, at a pace set by myself, rather than being told things by someone else. Presumably, this influenced my attitude. So I was left in relative peace regarding lecture activities. And I tended to choose subjects which attracted only a small number of students, so that contact could be more personal.

Also, although I was in an Institute for Naval Architecture, I was able to follow up on my ocean wave research, in which I was still interested, and prepare the next JONSWAP experiment, which I mentioned earlier. So I was not really hemmed in by Germany's relatively conservative system, because I was in a rather unconventional position.

*Concerning this back and forth between Germany and America. The Center for Fluid Mechanics in that time was in England. Had you any time, opportunities or desire to go to England and work there?*

**Hasselmann**: I was in fact invited as a Visiting Fellow for half a year, in 1967, and visited the Department of Applied Mathematics and Theoretical Physics. But I did not have a strong desire to visit Cambridge while I was working in La Jolla because I was more interested at that time in oceanography. There, in Scripps, were the leading scientists in oceanography, in ocean waves, currents and so forth. In England, in Cambridge, the effort was more on pure fluid dynamics and turbulence theory, and my interests had already switched from turbulence theory to wave dynamics in the ocean. I enjoyed my later visit to Cambridge and the relaxed style there, but La Jolla was more stimulating.

*So, you came back to Hamburg and to the Institut für Schiffbau and then something interesting happened, something what could not happen nowadays, namely people took very swiftly decisions of what to do.*

**Hasselmann**: I was gradually becoming an embarrassment for the Institute for Naval Architecture, because their main interest was in ship resistance, ship stability in waves—and, of course, in the design and construction of ships themselves–but not in the dynamics of ocean waves as such, or in oceanography in general. And I had started a large international experiment to measure the growth of waves under off-shore wind conditions in the North Sea. It evolved into quite an extensive affair, involving several institutions from different countries: Scripps from America, the National Institute of Oceanography from England, the Dutch Weather and Oceanographic Service KNMI, and the German Hydrographic Institute. There were four or five research vessels and other ships, a lot of activity installing wave measurement masts and wind measurement stations etc. All this created a lot of logistic overhead, and so I was tying up the secretaries, technical people, the workshop and so on in the institute for a project that had nothing to do with naval architecture.

So my former diploma thesis advisor, Prof. Wieghardt, in whose department I was working when I came back from America, came in one day and said quietly: Herr Hasselmann, don't you think you should find some other position somewhere, because it is actually not the main task of the Institute of Naval Architecture to measure waves in the North Sea. I wondered what to do, and so I asked Prof. Roll, President of the Deutsches Hydrographisches Institut, whether he could give me a job. He thought about it for a minute and probably decided that it would be a nuisance to have me in his institute as well. So he called the Federal Ministry for Science and Technology and inquired whether they could not provide a position for me in some form or another.

What then happened was that, at very short notice, the Ministry provided the funds to create a Department (Abteilung) of Theoretical Geophysics at the University of Hamburg, of which I was to become the director. An Abteilung had to be part of some institute, so Professor Menzel, the director of the Institute for Geophysics, was asked whether the new Department for Theoretical Geophysics could become part of the Institute of Geophysics. Professor Menzel, a very kind man, agreed. And so I became a member of the Institute of Geophysics. I received some research funds from the Ministry for Science and Technology, as well as a secretary, and a small apartment, of about six rooms, I think, next to the Institute for Geophysics, in the Schlüterstraße. I worked there until the Max Planck Institute for Meteorology was founded in 1975—apart from a two year stay in America between 1970 and 1972. So the department was created, basically, through an informal discussion between the Ministry for Science and Technology and the director of the Deutsches Hydrographisches Institut, with the good-willing cooperation of everyone involved.

# *"Short notice"—how short was that notice?*

**Hasselmann**: I cannot remember exactly how short it was, but it was really fast, because I was in the Abteilung when JONSWAP started, already in 1968, and I had just come back from Cambridge in 1967 and was already strongly involved in the planning of JONSWAP when this development began. It must have been less than half a year or so.

# *This would not be possible nowadays.*

**Hasselmann**: Well, that was in a period of rapid scientific expansion everywhere. The same atmosphere prevailed in America, where a position was offered to me more or less spontaneously and was formalized within a few months. That was a time when one was looking for good young people everywhere, trying to build up a good research environment in response to the challenge of sputnik. Everyone was trying to be in the forefront of science. This was particularly true in Germany, where in the wake of the Wirtschaftswunder one wanted to catch up also in science.

### *Other people known to work with you entered the stage at that time.*

**Hasselmann**: That's right. When the Department of Theoretical Geophysics was created I took on some Ph.D. physics students who were interested in working in geophysics, in particular in ocean wave theory and in the general theory of nonlinear interactions in geophysical wave fields, such as internal waves. At that time I had a number of good young students, for example, Dirk Olbers, Peter Müller and Jörn Kunstmann.

*Kunstmann did not do any oceanography, he was working on plasma physics.*

**Hasselmann**: That's true, I remember. At that time I was interested also in plasma physics. I had written a couple of papers with my former student friend Gerd Wibberenz on the scattering of protons in the solar wind by irregularities of the solar wind magnetic field. As lecturer in physics in Kiel, Wibberenz was working on problems of interplanetary space. I found the problem intriguing because it could be treated by exactly the same formalism that I had applied to determine the nonlinear energy transfer in an ocean wave spectrum. I also found working on this problem was useful because I gained some practice in the notation of relativistic electrodynamics, which was helpful for my recent excursions into particle physics—another of my interests that we can discuss later. Actually, the solar wind community was also not used to the relativistic notation, so that they had some problems reinterpreting our results in their language, but our papers were well received nonetheless [23, 28, 29].

Anyway, to better understand plasma physics, I decided to hold a seminar course on plasma physics together with Gerd Wibberenz and my other student friend Wolfgang Kundt, who at that time was a physics lecturer at Hamburg University. That's how Jörn Kunstmann came to me. His Ph.D. thesis was on interactions in the solar wind.

*You said, you took some students. What you really did was to ensnare a whole seminar group from your friend Wolfgang Kundt. You gave a half of them new topics to work on their diploma, because we did not know what to do at that time.*

**Hasselmann**: Yes, I seem to have hijacked Peter Müller and Dirk Olbers and maybe some others. Arne Richter and Hajo Leschke were also in that group, I think, but they did their diploma and Ph.D.s. with someone else, probably with Wolfgang Kundt. The people that came to me seemed to be quite content just learning methods, physics and mathematics, but had no clear idea of what they should do for their diploma or Ph.D. thesis. So they were quite happy when I suggested some topics to them.

*There was an IUGG Conference in Bern in 1966. There you suddenly became the coordinator of the JONSWAP effort.*

**Hasselmann**: I became coordinator to my big surprise, by default, probably because I initiated the idea that we should do a joint experiment. I invited some colleagues I knew—David Cartwright from the National Institute of Oceanography in England, Tim Barnett from Scripps, Karl Richter from the Deutsches Hydrographisches Institut, and some colleagues from the Netherlands, to discuss the idea of a joint experiment on wave growth in the North Sea. We met at the IUGG in Bern. We wanted to measure wave growth under off-shore wind conditions. I remember I had the crazy idea—as a physicist and theoretician—that in case of an east wind, we could measure the waves off the west coast of Germany, and when we had a west wind, we could measure waves off the east coast of England. But then some experimental colleague pointed out that it would be impracticable to install wave measurement stations on both sides of the North Sea, and that ships can not steam fast enough to go from one place to the other when the wind changes. So we decided to have the experiment on the east side of the North Sea, off the island of Sylt.

All this was agreed upon in principle, and then we went off home again. And then we suddenly realized that we have not discussed at all how to organize the experiment, and who should be the coordinator. Everybody assumed that because I had proposed the experiment, I should be the coordinator. I thought this was not a very good idea at all, as I had absolutely no experience in seagoing oceanography, and my past experience with experimental work with hot-wire turbulence measurements had convinced me that I was better employed doing theoretical work. But anyway, I was landed with this task and had to organize it.

The experiment was planned for the three summer months of 1968. A few months before the experiment was due to start, and everybody was geared up to install their equipment, I received a telephone call from the German Ministry of Defence saying that we would have to cancel our experiment. NATO was planning a large sea-to-air missile test in the North Sea at the same time. They would be testing radar methods of tracking missiles, and the ships and wave masts that we were planning to deploy would interfere with their radar signals. I said that it is impossible to cancel our experiment at this late hour, as we had already spent at least two million Deutsch Mark preparing for the experiment. The Ministry of Defense said that this might be true, but that they already spent fifty million on their exercise, so we have to cancel ours. I said, well, we cannot cancel it this way. The only solution I can suggest is that we reduce our experiment this year, without the wave masts and some of the ships, on the condition that you fund us to carry out the full experiment as originally planned next year. The Ministry of Defence agreed, and so we carried out two experiments, a reduced trial experiment in 1968 and the full experiment in 1969.

In retrospect, we were very fortunate that this happened, because it turned out that, from the point of view of logistics, the first experiment was a complete disaster. I had worked out precisely when every wave-measurement station should start recording, and for how long and how often, based on the wind conditions and the speed of propagation of the waves from one measurement station to the next. So on one particular day a particular station, a wave mast, say, should start recording at 7:30, measuring for half an hour every three hours. Further out a ship, say, should start recording at 11:45, and so on. But the communication system we had installed turned out to be completely inadequate to transmit this information reliably. This was not helped by the Russians jamming our radio stations everytime we went on the air because they thought we were part of the NATO exercise. We did get some nice data in the end, more or less by chance, but much less than we had hoped for. The coordination of the experiment was a continual stream of improvisations.

But we gained a lot of experience, and the next year, when we carried out the full-blown experiment, everything went very smoothly. We had a functioning communication system, a reliable predetermined schedule of measurements, and well organized logistics. All the equipment worked fine, and we obtained a very good dataset. The analysis of the data laid the foundation for the modern wave models that we later developed. So we were very fortunate that the Ministry of Defence interfered with our original plans and gave us a free trial experiment, so that we could carry out a good experiment one year later.

### *Would you mind assessing the impact of this experiment on your personal career, standing and satisfaction?*

**Hasselmann**: JONSWAP was certainly the most successful experiment I have been involved in. We were extremely lucky, not only because of the free trial experiment, but—still more important—because we were able to explain the principal results of the experiment by the one single process governing the dynamics of wave growth that we were also able to compute theoretically from first principles, without any empirical parameters—namely the nonlinear energy transfer I had derived earlier.

The idea of the experiment was that we would determine the processes governing the dynamics of ocean waves by measuring the change in the wave spectrum as the waves develop under an off-shore wind from small, short waves close to shore, to longer, higher waves further off-shore, out to still larger distances off shore where the waves had reached a fully-developed equilibrium state—assuming such a state exists. The spectral energy balance of the waves is controlled by three main processes: the generation of waves by the wind, the dissipation of wave energy by white capping, and the redistribution of energy across the wave spectrum by the nonlinear energy transfer. Prior to JONSWAP, we had assumed that the nonlinear transfer had only a minor impact on the evolution of the spectrum. This was based on the results I had presented at Easton, which were computed for a fully developed spectrum. But we discovered in JONSWAP that the spectrum of a growing wind sea has a much higher, sharper peak. This greatly enhances the strength of the nonlinear transfer. And it is this feature, the sharply peaked spectral shape, that is the origin of the transfer of energy from the peak to still longer waves—that is, for the continual increase in the wavelengths of a growing windsea. I still remember the excitement when we repeated the nonlinear energy transfer computations for the new JONSWAP spectra and the points came out, one by one, directly on top of the observed spectral growth.

Based on these results the wave community was then able—several years later—to develop the wave model WAM that is used today by more than 200 centres world wide, including operational global weather forecasting centers such as ECMWF, the European Centre for Medium Range Weather Forecasting, that produces daily global forecasts of the two dimensional ocean wave spectrum. The forecasts are supported today by wind and wave data from modern satellites, that the wave community also helped to develop in follow-up experiments of JONSWAP, and for which they developed the necessary retrieval algorithms and assimilation methods. But ultimately, the success of much of this development really hinged on luck: the fact that the one process that we could really compute rigorously, the nonlinear energy transfer, turned out to be the dominant process governing the form and rate of growth of the ocean wave spectrum.

Regarding my own personal career, I was recognized as the lucky person who happened to have developed the relevant theory, initiated the experiment and coordinated the analysis. We carried out the initial analysis first in our various home institutes and completed the analysis in a workshop at the Woods Hole Oceanographic Institution—which I was visiting at that time in the spring of 1971. The results [35] were presented the same year at the IUGG Conference in Moscow.

For me it was also a great experience that you can carry out an experiment which was a complete fiasco in 1968 and still be respected by your colleagues. In the business world I would have been fired. But the scientific community is extremely tolerant and understanding. I had the same experience later with other experiments, some of which also turned out to be a flop. I was always encouraged by my colleagues, who stood by me and accepted the fact that not everything that you try to do in science works. I personally very much enjoyed the experience of JONSWAP and the follow-up experiments JONSWAP2—although this was a flop—and MARSEN—this time a full success—in which we tested various remote sensing techniques relevant for the new wave-measuring satellites SEASAT and ERS-1. I also enjoyed the work later in the WAM group, in which we jointly developed the global wave model WAM that I mentioned [90].

All in all, JONSWAP clearly had a positive influence on the way my life developed. Probably, the fact that I was able to combine a field experiment with theory, both of which I had been involved in, also helped when I was later asked to become the director of the Max Planck Institute. It was presumably assumed that this indicated that I had enough flexibility to develop a new research program in climate. But that is only my guess. Anyway, JONSWAP was a lot of fun. It was a period in which we generated many lasting friendships. We had many parties and get-togethers with everybody involved, from the technicians to the radio operators to the ship people to the scientists. There was a great team spirit.

### *Could you speak about the role of Wolfgang Sell?*

**Hasselmann**: The success of the experiment was due to the team work of many people, but two people in particular deserve mention. One was Addi Hederich, a technician from the Deutsche Hydrographische Institut. He coordinated the entire logistics, the ship schedules, the installation of the wave masts and wave buoys, including the main tower PISA for meteorological and wave measurements, as well as the complex operations for servicing the equipment at sea. He worked tirelessly in 1968–1969 to bring everything together.

The other person was Wolfgang Sell. We had collected an enormous amount of data—for those days—nowadays it would be peanuts. But, for that time, we were immersed in an intimidating array of data from instruments of many different types, with different data formats, obtained at different times and different places. Nobody had really thought seriously about how to bring all these data together into a coherent dataset. Nowadays this is routine. But for us it was quite new. I personally did not think about it at all and simply assumed that we would muddle through somehow. Fortunately, there was Wolfgang Sell in the team who realized that we had a problem. So he immediately sat down and worked out a data analysis scheme of how to store the data, how to process them, bring them together and manipulate them with a single data processing software. Without that input from him we would never have been able to complete the analysis of the JONSWAP data within only two months in Woods Hole—in time to present the results at the IUGG conference later that year in Moscow. Wolfgang Sell and a few other stalwarts, Peter Müller and Dirk Olbers, stayed on after the main workshop and helped clean up the results for the IUGG meeting.

*At that time also a number of new persons came on the stage. One was Elsa Radmann.*

**Hasselmann**: That was my secretary, a very reliable person. She came in 1968 when the Department of Theoretical Geophysics was founded and stayed with me until her retirement some thirty years later. She helped first in the organization of JONSWAP. When I went to Woods Hole for two years, in the autumn of 1970, she took care of the institute while I was away, kept up the communication, and so forth. She was an extremely reliable, conscientious person that I owe very much to. If I had to travel somewhere, I never checked where I was staying until I arrived, she had always arranged everything perfectly. She also had various likes und dislikes. If you were unfortunate enough to belong to her few dislikes you had a hard time, but for all others she was very helpful and friendly.

*You mentioned the data analysis. I remember that you were doing the energy transfer calculations on many different computers. We were in DESY, in Darmstadt, we were here in Hamburg, on the Hamburg computing center and we were also in Woods Hole. Why did you go to Woods Hole? As far as I can see, Woods Hole is not a classical research centre for surface waves, for ocean waves.*

**Hasselmann**: That was basically independent of JONSWAP. I received the offer of a professorship in the Woods Hole Oceanographic institution, on a chair that had just been donated by the Doherty foundation, to develop a joint program on oceanography between Woods Hole and MIT. I said that I would be happy to accept the professorship for two years, but could not decide yet whether I would to stay longer or go back to Germany. However, one of the reasons I accepted was that Ferris Webster, who had made the invitation, said that Woods Hole had just obtained a new computer that would be ideal for the JONSWAP analysis. So when I arrived, I talked to Art Maxwell, the director responsible for research at WHOI, and explained that we had this experiment, and that we somehow had to get together to analyze the data. He immediately offered not only the use of the computer, but also all other needed facilities, as well as some funds so that we could carry out the workshop there. That is the reason we had the JONSWAP workshop in Woods Hole.

*There must have been a little bridge nearby.*

**Hasselmann**: I believe you are referring to my memorable encounter on a bridge with Peter Müller. Peter Müller was one of the members of the JONSWAP working group.  We had a tremendous amount of work to do, a lot of computations, reorganizing and reanalyzing the data from different aspects, and so forth. I was running back and forth under enormous stress to get all this done, between the computer center and the operations room, where we were all working together. And while I was running back and forth and completely out of breath and stressed, I saw one of the members of the group, namely Peter Müller, leaning over this bridge looking calmly down onto the water. I said: "Hello Peter". And he answered dreamily, after a long pause: "Yes, life is good … but one needs time for contemplation."

*Peter Müller and Dirk Olbers were responsible for designing the particular parameter representation of the JONSWAP spectrum.*

**Hasselmann**: Yes, that's right. Peter and Dirk were the creators of the so called JONSWAP spectrum, which has since been widely used. They proposed a very simple three-parameter representation which reproduced the spectral shape very well for the different stages of wind-wave growth.

*From your publication list I can see that there were other issues you were interested in, besides the solar wind problem that you mentioned, for example sound waves in the ocean with Hans-Hermann Essen.*

**Hasselmann**: Yes, I wrote a set of papers, mostly with other colleagues or Ph.D. students—although usually the Ph.D. students would carry out the work and publish on their own—looking at different interactions between different types of wave fields in the ocean, the atmosphere and the solid earth. One paper was with Heinz-Hermann Essen [25], on the generation and scattering of sound waves in the ocean by surface waves, one was on surface gravity waves scattering off the ocean bottom, one or two papers were on interactions between internal gravity waves in the ocean and atmosphere, although this subject was mostly well covered by several nice papers by Dirk Olbers and Peter Mueller. One of my early papers was on microseisms [12], the generation of random seismic waves through resonant interactions between surface gravity waves, and between surface gravity waves and the ocean bottom.

In most of these papers we applied the interaction-diagram formalism that Feynman had developed to summarize the interactions between particles. I had slightly modified the Feynman diagram rules in a 1966 paper [16] to adapt the formalism to classical random wave fields.

This brings me to a rather interesting comment on the communication between different scientific disciplines. My standing in the ocean science community was originally founded on my papers on nonlinear interactions between ocean waves. Shortly after coming to America I gave a talk on this work at the Californian Institute of Technology. After the talk my colleague Gerry Whitham came to me and said "That is an interesting talk you gave, but did you ever notice that the plasma physicists appear to be doing similar things to what you are doing?". I replied, no, this was new to me, could he give me some references? So I looked up the references and discovered that the plasma physicists had indeed been doing exactly the same things that I had been doing, except that they were looking at plasma waves instead of ocean waves. This was a bit easier because they did not have to go to fifth order, the resonances occurring already at third order. But to my surprise they never actually presented the nonlinear computations. They simply took the analysis for granted. Sometimes they quoted a paper by Peierls back in 1929, in which he showed that the diffusion of heat in solids could be explained by the nonlinear interactions between phonons. I looked up the paper and discovered that Peierls had carried out exactly the same analysis as I had, using a different notation, but based on exactly the same approach. At that point I realized that my reputation in oceanography was based on very old results in physics that were simply not known in oceanography. I then started reading other physics papers and discovered that exactly the same formalism was used everywhere in quantum field theory, in describing the interactions between different particles, which are represented in quantum field theory by wave fields. Feynman had developed a well-known set of diagrams and rules summarizing the algebra involved. So I wrote my 1966 paper in which I showed how Feynman diagrams could be applied to geophysical wave fields, with a few simplifications appropriate for classical rather than quantum theoretical fields. We applied this formalism subsequently to the various wave interaction problems we investigated.

It was really an eye-opener to realize how specialized we are in our fields, and that we need to know much more about what was going on in other fields. Through this experience I became interested in particle physics and quantum field theory. So I entered quantum field theory through the back door, through working with real wave fields rather than with particles. From this other vantage point I became convinced—and remain convinced today that Einstein was right in his criticism of the conceptual foundations of quantum theory, and that there was more to the concept of a particle than can be captured by wave dynamics. So since 1966 I have been exploring other approaches to elementary particle physics, parallel to my official research work. But I did not publish my first results, on the metron theory, until thirty years later [121, 122, 131, 132].

*You mentioned already that you carried out the JONSWAP workshop in Woods Hole. And after the workshop we all became engaged in internal waves and a large internal wave experiment, IWEX. WHOI was an institute of oceanography. They did completely different things. What was this about? Did they ask you to do this?*

**Hasselmann**: No, I was already interested on internal waves before I came to Woods Hole. Not experimentally, but with respect to wave dynamics. At Woods Hole they were more interested in ocean currents and water masses in the ocean than in surface waves or internal waves. But they had also developed current meters and thermistor instruments, and had considerable experience in deploying current-meter and thermistor-chain moorings. So I thought that WHOI would find it a challenge to deploy a large triangular array of current meters and thermistors to measure the internal wave spectrum in the main thermocline. This they did, very enthusiastically and professionally. Dirk Olbers and Peter Müller, together with Mel Briscoe, analyzed the data and wrote up the results in some very nice papers.

*You finally came back to get a professorship for theoretical geophysics in Hamburg in 1972.*

**Hasselmann**: Yes, Professor Brocks, the director of the Meteorological Institute of the University and the Fraunhofer Institute of Maritime Meteorology and Radio Meteorology, had succeeded, with the support of other colleagues, to create a new chair for me in Theoretical Geophysics, which I accepted.

*Also, at that time you became a member of the Joint Organizing Committee of the Global Atmospheric Research Program GARP. You were one of the two oceanographers in that committee. In this way you became acquainted with the issues of climate, climate variability, climate change and problems of that sort. How was that?*

**Hasselmann**: I had become a member of the Joint Organization Committee of GARP already in 1971 or 72, before I returned to Hamburg. They were looking for some young scientist who could contribute to the strengthening of the Global Atmosphere Research Program with respect to climate, the second GARP objective. The first was improving weather prediction. They wanted an oceanographer, because of the importance of the oceans for climate, but also an oceanographer who had some experience in air-sea interaction. There was already one oceanographer with this background on the committee, Bob Stewart, and he probably proposed my name. The work in the JOC of GARP was quite fascinating, as we were laying the foundations of what was later to become the World Climate Research Program.

*Then you participated in a number of historically important meetings, namely the first climate conference in Stockholm 1974, then another one which focused on ocean problems, in Helsinki. You did not present your own work there, but you were part of the overall brainstorming which took place at that time.*

**Hasselmann**: That's right. The Stockholm Conference was on climate in general, with a number of different working groups looking at different aspects of climate. The working groups were introduced by a few general talks, but the purpose of the conference was to work out recommendations on which research should be done in which areas. I was chairing one of the working groups involved in oceans and climate. I had a similar coordinating role in the following Helsinki Conference on Oceans and Climate, which I convened together with Alan Robinson of Harvard University. The two conferences provided the basis for the creation of the World Climate Research Program a year or two later at a conference in Geneva.

*There was something else in about 1971/1972, namely the formation of the Sonderforschungsbereich 94 in Hamburg, of which you became the speaker. That was then when you really became responsible for bigger organization of science, for coordinated and interdisciplinary science. How was that?*

**Hasselmann**: The discussions for the Sonderforschungsbereich 94 began before I went to America—around 1968–69. The proposal was written and accepted in about 1971. The first speaker of the SFB 94 was Karl Brocks, who had been the driving person in the formulation of the proposal. I had very good relations with Brocks. His institutes participated in the meteorological measurements and telemetry in JONSWAP. And he gave me much fatherly advice on how to run big projects, of which he had considerable experience. Unfortunately, he died in 1972 just before I returned from Woods Hole, and I was elected as his successor as speaker of the SFB 94.

That was a very interesting time, because the SFB 94 was the biggest Sonderforschungsbereich at that time—in fact, later, too. It was extremely broad in its ambitions, encompassing oceanography and meteorology, air-sea interaction, ocean chemistry and ocean biology, with many different participating institutions. The challenge was to bring all these research activities together into a joint program. Many of these groups had never cooperated before and had quite different research cultures.

My first task was to start a series of seminars to define the joint projects that we wanted to carry through. We had written down some general objectives in our proposal, but we really had no clear idea of how these objectives were to be achieved. In these seminars we first had to understand how the different groups thought, and had to learn to communicate between these different cultures. Out of these discussions then came some very interesting ideas, for example, the first Fladen Ground experiment FLEX. The experiment took place in 1976 in the so-called Fladen Ground area of the northern North Sea. It was designed to investigate the coupling between the thermocline and mixed layer and the biological productivity and phytoplankton distribution during the main phytoplankton bloom in the spring. It was carried out in corporation with British groups and I believe some Dutch groups. It was quite a successful experiment. I understand the data is still an important reference data set today.

*This is just thirty years ago. Could you say something about how difficult you found it—this first time when you truly became interdisciplinary. So far you were just in the realm of physics and as a physicist you should feel confident. But now you suddenly met very different people, very different scientific cultures.*

**Hasselmann**: That was indeed a very interesting period. I remember our first discussions with the biologists. As physicists, we would ask: what happens during a spring-time phytoplankton bloom in the mixed layer? The biologists would answer with a highly detailed description of the various interacting processes that produce the exponential growth and subsequent decay of the bloom. We would reply: that's great, you seem to understand what happens, so let's put that into a model and test the ideas against some measurements. They would reply: but that's impossible, its much too complicated. And we would say: but if its so complicated that you cannot express it in a model, you cannot say you understand it. And so we would talk around each other.

But once the biologists realized that they were not simply slaves making measurements to test the models of high-brow mathematical physicists, and the physicists realized they were not simply slaves producing computer models to test the ideas developed by better educated biologists, a fruitful cooperation developed. In fact, the phytoplankton model that came out of this cooperation with the biologists formed the core of the global carbon cycle model that later became part of the Max Planck climate model.

# *You mention the modelers. Maybe you can drop some names?*

**Hasselmann**: The two main people involved in the biological modeling were Ernst Maier-Reimer and Günter Radach. Radach developed the details of the phytoplankton model, but Maier-Reimer was the driver. In fact, he was the driver in all areas of modeling. If you tell him any idea about any process, he immediately produces a model. Actually, I have the same mentality: I like to produce models. But I am not as efficient as Maier-Reimer. In one of our first SFB seminars we were listening to what the biologists were telling us about phytoplankton growth in the mixed layer, how the phytoplankton gets mixed down, and how its growth or decay depends on the depths of the mixed layer and the euphotic layer, the layer penetrated by light. I thought that this would be a nice example to demonstrate how such ideas can be expressed in a simple model. So I coded a simple conceptual model on our small computer in the Institute for Geophysics. At the next seminar I was just going to present my simple computations when Ernst Maier-Reimer produced the model he had developed independently. His model was much better than my simple model. It was a detailed one-dimensional mixed layer model including temperature, phytoplankton and the penetration of the light. And he had produced some very nice plots demonstrating how the phytoplankton distribution depended on the various mixed layer parameters. I was quite impressed, and so were the biologists.

### *The only thing I am surprised about is that Ernst Maier-Reimer came forward with his model.*

**Hasselmann**: You are referring to the many drawers in which Maier-Reimer has stacked away models that he has not yet shown to others, let alone published. Anyway, in this case—and many others—Ernst had a strong positive influence on the cooperative programs we developed in the SFB 94.

*So you became engaged in networking, in bringing large groups of different sorts of scientists together to tackle questions of a system—in this case the system of the* *North Sea. You were also confronted with questions about climate and then, some day, Reimar Lüst* 4 *came into your office.*

**Hasselmann**: I did not find out the background of why he came into my office until later. Apparently, the Max Planck Society had decided to accept the proposal of the Fraunhofer Society to take over the former Fraunhofer Institute for Maritime Meteorology and Radio Meteorology of Professor Brocks in exchange for an institute of the Max Planck Society. The Fraunhofer Society was dedicated to applied research, but Brocks' Fraunhofer Institute was engaged in basic research on air-sea interaction and radio meteorology. At that time the Max Planck Society had an institute in Würzburg that was engaged very strongly in applied research in solid-state physics. Thus the proposal was that the two societies should simply exchange institutes. It seems that the Max Planck Society had agreed. So the President of the Max Planck Society, Reimar Lüst, came into my office in 1974, apparently looking for a director of this new institute.

The concept was that the institute should not simply continue Brocks' work on air-sea interaction, but should focus primarily on climate research. The principal advisors of the Max Planck Society in this decision appear to have been Hermann Flohn in Bonn and Bert Bolin in Stockholm, the chairman of JOC. The Max Planck Society probably thought that, as a physicist, with experience in various areas of research in the past, I would have enough flexibility to develop an effective program in the new area of climate research. As member of the Joint Organization Committee of GARP, I had been involved in preparing what was later to become the World Climate Research Program, which was probably also one of the reasons they chose me.

The embarrassing thing was that when Lüst came into my office I had only met him once before—he was present at the most disastrous talk I had ever given in my life.

I was supposed to give a formal presentation about oceanography to a lot of high ranking people that were responsible for funding research in Germany. I had intended to work on my talk in the plane on my way over from Woods Hole, but I was tired and I could not concentrate. The next day I was still more tired with jet lag, and felt very uncomfortable when I entered the large lecture room full of people in suits and ties. So I thought that I would break the ice at the beginning by telling a little joke. But the microphone was not

4 Reimar Lüst has been interviewed in German earlier, see von Storch, H., and K. Hasselmann, 2003: Interview mit Reimar Lüst. http://www.hvonstorch.de/klima/Media/interviews/luest.interview. pdf (GKSS Report 2003/16, 39 pp.).

working properly, and somebody in the front row said "could you please repeat what you said?" I did not see much point in repeating my feeble joke, and started off on my poorly prepared talk.

So I went off rambling about all sorts of vague things about ocean research in general. I finally tried to escape from this floundering by giving an example of research. I wanted to explain how the random spectrum of ocean waves is generated by superimposing many different sinsusoidal waves. This part I had prepared back in Woods Hole with a set of transparencies which I superimposed one after another. The result was impressively realistic and quite convincing. This time, however, when I began overlaying the different transparencies, I noticed that the audience was getting uneasy, then it started tittering, and finally it broke down in uncontrolled laughing. So I looked back onto the screen and saw that it had become completely black. The projector was too weak to shine through more than one or two transparencies, and my harmonic superposition, instead of producing a random wave field, had gradually transformed my sinusoidal waves into pitch black darkness. I somehow stumbled through to the end of the talk, but it was the worst talk I have ever given in my life and long haunted my dreams.

This was in the hotel Atlantic in Hamburg. My colleagues were very mad at me because they thought that this was hardly the way to convince the people that held the purse strings that investment in ocean research was a good idea.

So I was very surprised that, despite having witnessed this disaster, Reimer Lüst was offering this position to me.

*So you were suddenly confronted with this Max Planck Society. Have you met with people in that group before? There was no Max Planck Institute, there was just the Max-Planck Society President who came in your office offering the position of the director of a new institute. What were the constraints of this offer? Did he provide you up front with a generous budget?*

**Hasselmann**: When he made this offer, I had of course a discussion with him over the level of support the institute would have. I said that I would need one director for the group from the former Fraunhofer Institute for air-sea interaction.5 Lüst accepted. I added that I probably would need two more directors, one for climate data, one for the atmospheric part of the climate system. Lüst replied that that would be very difficult, because the

5 This position was later taken over by Hans Hinzpeter, wo was also earlier interviewed in this series, see: von Storch, H. and K. Fraedrich 1996: Interview mit Prof. Hans Hinzpeter, Eigenverlag MPI für Meteorologie, Hamburg, 16 pp, http://www.hvonstorch.de/klima/Media/interviews/hinzpeter.pdf.

Max-Planck Society did not have the budget for this now. But if it turned out to be necessary later on, the Max-Planck Society would consider a third person, at least. This was a gentleman's agreement. We did not have it written down anywhere.

Reimar Lüst then asked whether we needed a computer. I said that I did not need a large computer straitaway, but would want one later. First, we would need to develop our research program. It was clear to me that we had to solve many fundamental issues first. Once they were clarified, we would come back to the issue of a large computer. That we would need a supercomputer sooner or later was clear to me from the beginning. Lüst accepted this too.

So, essentially, I started the institute on the commitment of one additional professor to take over the former group of Professor Brocks and the gentleman's agreement of a possible third director and a supercomputer at a later time. The staff for the climate group consisted of five scientists and some additional technical and administrative staff. The group was not large, but this complied with the general Max Planck Society policy of not assigning more than about five scientists to a director, otherwise the director would turn into a manager rather than remaining a creative scientist.

It took three or four years before I had gradually filled the five scientist positions and the climate research program began to take shape. So this was the starting basis of the institute. Later on, as the institute developed, the other elements of the gentleman's agreement with Reimar Lüst were also eventually realized.

The budget—I forgot what the actual value was—was more or less fixed. It was agreed that it would not be changed significantly from one year to the next. This is also general Max Planck policy. A constant, dependable funding level is clearly a necessary requirement for the development of a long-term research program. If we needed additional funds we could apply for these from third sources, which we did later when it became necessary. The Max Planck Society also had additional funds for special projects, but we normally received supplementary funds later through the climate programs of the Federal Ministry of Science and Technology (BMFT) and the European Commission. I was very grateful that the basic funding through the Max Planck Society was reliable and did not require a fight each year to become renewed.

### *Concerning models—here was a running atmospheric model in the group of Günter Fischer in Hamburg.*