The Discovery of Global Warming Spencer Weart
June 2007     [ HOME ]    Table of Contents     for printer

Aerosols: Volcanoes, Dust, Clouds and Climate

Haze from small particles surely affected climate, but how? Old speculations about the effects of smoke from volcanoes were brought to mind in the 1960s, when urban smog became a major research topic. Some tentative evidence suggested that aerosols emitted by human industry and agriculture could change the weather. A few scientists exclaimed that smoke and dust from human activities would cause a dangerous global cooling. Or would pollution warm the atmosphere? Theory and data were far too feeble to answer the question, and few people even tried to address it. Among these few, the uncertainties fueled vigorous debates, in particular over how adding aerosols might change the planet's cloud cover. Finally, in the late 1970s, powerful computers got to work on the stupefyingly complex calculations, helped by data from volcanic eruptions. It became clear that overall, human production of aerosols was cooling the atmosphere. Pollution was significantly delaying, and concealing, the coming of greenhouse effect warming.
     climate change, global warming, computer models, haze, sulfates


Subsections: Aerosols as Global Pollution (1920s through 1960s), Warming or Cooling? (Early 1970s), Sulfates, Soot and Clouds (mid 1970s-1988), After 1988

In 1783 a volcanic fissure in Iceland erupted with enormous force, pouring out cubic kilometers of lava. Layers of poisonous ash snowed down upon the island. The grass died, and three-quarters of the livestock starved to death, followed by a quarter of the people. A peculiar haze shadowed western Europe for months. Benjamin Franklin, visiting France, noticed the unusual cold that summer and speculated that it might have been caused by the volcanic "fog" that visibly dimmed the sunlight.(1)        - LINKS -

Better evidence came from the titanic 1883 explosion of Krakatau (Krakatoa) in the East Indies, which sent up a veil of volcanic dust that measurably reduced sunlight around the world for months. The planet had so few weather stations that scientists were unable to learn for sure whether the eruption affected the average global temperature. But from then on, scientific reviews of climate change commonly listed volcanoes as a natural force that might affect large regions, perhaps the entire planet. Looking at temperatures after major volcanic eruptions between 1880 and 1910, a few scientists believed they could see a distinct temporary cooling. (The most impressive confirmation came much later, when examination of older records showed that the1815 eruption of Tambora, scarcely noted at the time outside Indonesia, had affected the world’s climate much more than Krakatau. Crops were frozen as far away as New England.)(2*)

Perhaps the smoky skies in an era of massive volcanic eruptions were responsible for the ice ages, or had even killed off the dinosaurs by cooling the Earth?(3) This image of climate change became familiar in popular as well as scientific thinking. In the 1950s, experts noted that the Northern Hemisphere had been getting warmer over the last several decades, a time when volcanoes had been relatively quiet, whereas the preceding century had experienced a number of huge eruptions and had been severely colder.(4)


=>Solar variation

=>Climate cycles




=>Public opinion

Full discussion in
<=Modern temp's


External input

climatologist J. Murray Mitchell
Murray Mitchell

Maybe aerosol science itself could solve the enigma. If it was plausible that volcanic emissions could alter the climate, what about particles from other sources? Meteorologists recognized that dust and other tiny airborne particles could have important influences. Simple physics theory suggested that such aerosols should scatter radiation from the Sun back into space, cooling the Earth. Through the first half of the 20th century, measurements and theory were inadequate to say anything about that, beyond vague speculation.(6*) Such speculation gradually came to focus on something that people were beginning to recognize as a major source of atmospheric particles: human activity.  
Aerosols as Global Pollution (1920s through 1960s)
Hints that human emissions made a difference in the atmosphere went back to measurements by the pioneering oceanographic vessel Carnegie and other ships on voyages between 1913 and 1929. Analysis of the sea air showed a long-term decrease in its conductivity, a decrease which seemed to be caused by smoke and gases from ships and perhaps from industry on land. "Thus we see," the researcher concluded, "that like a living thing, the conductivity of the lower atmosphere finds survival increasingly difficult in our modern industrial age."(7) Still, a 1953 review concluded that scientists simply did not know whether pollution had significantly affected the transmission of solar radiation.(8)  



Perhaps we were already doing something like that inadvertently. In the early 1960s, Walter Orr Roberts, a prominent astrophysicist at the University of Colorado, noticed that something was changing in the broad and sparkling skies above Boulder. Roberts had a long-standing interest in climate. One of the things that had driven his career in astrophysics was a hope of connecting climate with sunspot cycles. He had been especially impressed by the terrible drought of the 1930s, which he had seen firsthand when he drove through the Dust Bowl on his way to Colorado. Aerosols stayed on his mind. One morning as he was talking with a reporter from the New York Times, Roberts pointed out the jet airplane contrails overhead. He predicted that by mid afternoon they would spread and thin, until you couldn't tell the contrails from cirrus clouds. They did, and you couldn't.(10) The Times made it a front-page story (Sept. 23, 1963). "Until recently, Dr. Roberts explained, cirrus clouds were thought to be more of an effect than a cause of weather conditions. But data from balloon and satellite experiments now suggest... [clouds] may trap enough heat beneath them to affect the weather." Since jets evidently made cirrus clouds, they "might be altering the climate subtly along major air routes."  
Nevertheless, by the early 1960s the question of human influence on clouds was starting to attract at least some scientific attention. Roberts's observation of contrails was joined by other hints that various types of anthropogenic aerosols — microscopic solid particles or droplets of chemicals produced by human activity — could indeed increase the amount of cloud cover. That would increase the amount of sunlight reflected away from the Earth. A 1966 study of satellite photos of the oceans found linear clouds that might have been seeded by smoke from ships. Another study, tracking rainfall downwind from paper mills, concluded that humans were probably causing more precipitation inadvertently than by deliberate cloud seeding.(12)  
Another line of speculation about the effects of dust from human activities addressed pollution that settled on the polar icecaps. Could that lower their reflectivity enough to change the climate? The idea was inspired by one of those quirky speculations that both harassed and stimulated climate scientists — a suggestion that "dusting of the ice caps" by volcanoes and soil blown off of dry lands had caused the irregular changes in sea-level that had been recorded in historic times.(13) This was only one of countless theories of climate change, and did not get much credence or attention.  



<=>Other gases



=>Public opinion

Aerosol science was just emerging as a field standing on its own. Like many other fields it had gotten a strong impetus from warfare, where smokes, poison gases, and disease-carrying aerosols could be mortal concerns. The field first began to coalesce during the Second World War, and its first handbook was based on studies done under the Manhattan Project.(16) After the war, rainmaking jumped to the top of the list of practical interests. But it was too intractable and controversial for most aerosol specialists. Still less were they interested in tackling the physics of clouds or the physics of radiation high in the atmosphere, topics that were dauntingly intractable and remote from any useful application. Concern over fallout from nuclear weapons tests motivated some research (not always openly published) that was potentially relevant to climate. Most aerosol specialists, however, were far more interested in the practical problems of air at ground level, especially public and occupational health. And these problems, involving dust and pollution, "had no glamour to offer for young researchers," as one pioneer admitted. The field's first journal (named, naturally enough, the Journal of Aerosol Science) was not founded until 1970, and the editor remarked that even then "academic status has not been achieved."(17)  



<=Public opinion


Most of the aerosol scientists' attention went to "pollution" particles that fell out of the atmosphere (or were washed out by rain) within a few days. But there were also microscopic particles that could linger longer and travel farther. Entire regions were intermittently hazed over, raising questions about possible world-wide effects. Already in 1958, one expert had remarked that "there can no longer be any sharp division between polluted and unpolluted atmospheres."(20*) It was some time before many others recognized how far pollution spread beyond cities. Understanding came only after people studying smog set up a network of stations that regularly monitored the atmosphere's turbidity (haziness). In 1967, Robert McCormick and John Ludwig of the National Center for Air Pollution Control in Cincinnati reported a gradual increase in the general turbidity over regions spanning a thousand kilometers. Further checks of the record of turbidity turned up increases even in remote areas like Hawaii and the North and South Poles.(21) Could humanity's emissions be affecting the global climate, not in some abstract future but right now?





<=External input

These studies contributed to, and at the same time responded to, a broad change of thinking. This had started with observations that radioactive fallout and chemical pesticides could be found far from the places where they were emitted. The world's oceans and air could no longer be seen as a virtually infinite dumping-ground. Fewer and fewer people believed that the atmosphere could safely absorb (as one aerosol expert acidly put it) "any effluents which mankind might see fit to disgorge into it."(22) Through the early 1960s, ideas about human influence on climate had focused on greenhouse effect warming caused by industrial emissions of carbon dioxide gas (CO2). At the time the effect seemed no more than a fuzzy speculation. Adding to the uncertainty, weather experts were just now reporting that a world-wide cooling trend had been underway for a decade or so.McCormick McCormick and Ludwig suggested that the cooling trend itself might be due to human activities.


<=>Public opinion



<=Modern temp's

Reid Bryson, a University of Wisconsin meteorologist, joined the discussion. In 1962 he had flown across India en route to a conference. He was struck by the fact that he could not see the ground — his view blocked not by clouds but by dust. Later he saw similar hazes in Brazil and Africa. The murk was so pervasive that local meteorologists took it for granted and failed to study it. Bryson realized, however, that the haze was not some timeless natural feature of the tropics. He was seeing smoke from fields set on fire by the growing population of slash-and-burn farmers, and dust from overgrazed lands turning to desert. The effects of ever more widespread farming and grazing, together with pollution from industry, seemed large enough to alter the climate of the entire planet. Bryson suggested that a rapid and world-wide rise of atmospheric turbidity was counteracting the CO2 greenhouse effect. The consequence, he suspected, would be a drop in temperatures. Calling for more intense study, Bryson and a collaborator wrote that they "would be pleased to be proved wrong. It is too important a problem to entrust to a half-dozen part-time investigators."(23)  
Concern grew when studies showed that recent decades had seen a great increase in the amount of aerosols in the lower atmosphere ("troposphere"). The air over the North Atlantic was twice as dirty in the late 1960s as it had been in the 1910s, suggesting that the natural processes that washed aerosols out of the atmosphere could not keep up with human emissions.(24) As a back-page New York Times item (Oct. 18, 1970, p. 92) reported, "This is disturbing news for those weather experts who fear that air pollution, if it continues unchecked, will seriously affect the climate and perhaps bring a new ice age."



=>Public opinion
= Milestone

But how much of the haze was really caused by humans? In 1969 Murray Mitchell pushed ahead with his statistical studies of temperatures and volcanoes. He calculated that about two-thirds of the cooling seen in the Northern Hemisphere since 1940 was due to a few volcanic eruptions. He concluded that "man has been playing a very poor second fiddle to nature as a dust factory."(25) Other respected climatologists agreed that volcanic dust could account for a substantial part of the temperature variations in the last century or so. The most impressive work was done by the British meteorologist Hubert Lamb, who burrowed through many kinds of historical records to compile a "Dust Veil Index." His tables revealed a telling connection between dust and cooler temperatures. But if the experts now agreed that volcanic explosions could affect temperature, they disagreed on how strong the effect was.(26*)


=>Simple models

=>World winter

Warming or Cooling? (Early 1970s) TOP OF PAGE  
Enough scientists had their curiosity and concern aroused to the point where they pursued a modest number of studies in the early 1970s. They failed to find solid evidence for a global increase of turbidity. But the studies did turn up regional hazes — episodes of pollution spreading a thousand kilometers or so downwind from industrial centers.(27) Everyone now admitted that human pollution was growing headlong. While Mitchell continued to insist that humanity was "an innocent bystander" in the cooling of the past quarter-century, in 1971 he calculated that our emissions might begin to cause serious cooling after the end of the century.(28) Other scientists claimed that the increase of aerosols was important already, perhaps even more of a concern than CO2. But nobody trusted anyone else's calculations, which were in fact much too crude to give reliable answers. Adding to the uncertainty, Mitchell gave plausible arguments that aerosols could produce a warming effect instead of cooling. It depended on how much they absorbed radiation coming down from the Sun, and how much they trapped heat radiation rising up from the Earth's surface. And these numbers depended on the level in the atmosphere where the aerosols floated, and on whether they floated above bright regions like deserts (which reflect sunlight) or dark ones like the oceans (which absorb sunlight).(29*)




=>Other gases





=>Modern temp's

=>World winter


<=Radiation math

<=Simple models

=>Public opinion

<=Venus & Mars
Beyond the direct effects of aerosols absorbing or scattering radiation, an even tougher puzzle remained — how did particles help create particular types of clouds? And beyond that loomed the enigmatic question of how a given type of cloud might affect the temperature. Depending on whether clouds were thick or thin, and where they floated in the atmosphere, they might bring some amount of cooling, by reflecting sunlight, or they might even bring warming, through a sort of greenhouse effect. The one sure thing was that aerosols could make a difference to climate, and perhaps a big difference.


<=Simple models


<=Rapid change

climate scientist Reid Bryson
Reid Bryson

=>Public opinion

Newspapers and television in the early 1970s were regularly running stories on the appalling droughts in the Sahel and elsewhere, and the public was starting to worry about climate change. Would more dust and gases of human origin afflict us with even more deadly droughts or floods? That depended critically on the effects of aerosols. The scientists who had studied this recondite topic began to feel the public eye upon them, and they debated their technical questions with heightened intensity. They increasingly saw that it was theoretically possible for a small change of conditions to bring large changes of climate. But it would be another three decades before computer models of climate became good enough to confirm that pollution from sulfates probably had in fact contributed to the Sahel drought.(34a) Few experts were ever more than halfway convinced by Bryson’s argument that the "human volcano" brought a possibility of disastrous global cooling.  
<=Public opinion

The prominent meteorologist William W. Kellogg told a 1975 World Meteorological Organization symposium not to worry. He noted that industrial aerosols, and also the soot from burning debris where forests were cleared, absorbed sunlight strongly — after all, smog and smoke are visibly dark. They would thus retain heat. He calculated that the chief effect of human aerosols would be regional warming (although he admitted that the calculation relied on properties that were poorly known). Anyway, as Kellogg also pointed out, rains washed aerosols out of the lower atmosphere in a matter of weeks. Besides, many nations were vigorously working to lower air pollution. Eventually the warming due to the increase in CO2 — a gas which lingered in the atmosphere for generations — must necessarily dominate the climate.(35*)

Similarly, Stephen Schneider and a collaborator improved his rudimentary model, correcting his earlier overestimate of cooling (see above) by checking against the effects of dust from volcanoes. They got a decent match to temperatures over the past thousand years, after they added an estimate for changes of solar intensity. The model now predicted that "CO2 warming dominates the surface temperature patterns soon after 1980."(36) Other experts agreed that human production of aerosols, although it might alter weather slightly in industrial regions, had no noticeable global effect. Only a few people noted that pollution might cancel out some of the greenhouse warming, delaying the time when it would become obvious.(37)








=>Solar variation

=>Public opinion
=>CO2 greenhouse


In the early 1970s very few papers scientific were published on the question of climate change on a human time-scale, that is, faster than the thousands of years that most scientists thought glacial ages took to evolve. Very few of these projected cooling. During the second half of the 1970s, however, scientists published several dozen papers about century-scale global climate change. Some of these discussed cooling and warming factors without coming to a conclusion, but more than half projected that greenhouse warming would dominate. All recognized, however, that their understanding was still limited, and it would take a long effort to work out a reliable answer.(38a) (As described below, scientists in the 1990s with far better data and computer models concluded that industrial haze had indeed helped to depress Northern Hemisphere temperatures for a few decades in the mid 20th century, and warming from the accumulated greenhouse gases began to dominate only in the 1980s.)

Most of the debaters were not even addressing all the key problems. Ideas about human emissions focused on an image of dark smog and smoke obscuring the sky. Some scientists pointed out, however, that such direct effects of particles interfering with radiation could be outweighed by indirect effects. They emphasized new observations that nuclei for the condensation of water droplets into rain or snow were sparse under natural conditions. Thus "the most sensitive" leverage point for pollution particles might be their role as nuclei. "Although the changes are small," one scientist remarked, "the long-term effect on climate can be profound." Conceivably the clouds would dim sunlight so much that the whole climate system would flip into a new ice age.(39) An important 1975 review panel concluded that the impact of particles on global temperatures "cannot be reliably determined," for it depended on many factors that were scarcely known. Warning that the particle load in the atmosphere might rise another 60% by the end of the century, they called (in the usual fashion of study groups) for further study.(40)  
So it continued, as some scientists concluded that aerosols would cause warming, others expected cooling, and still others expected no significant global effect. One widely noted example was a survey of dusty days in Arizona by Sherwood Idso and Anthony Brazel, who concluded that additional aerosols from human activity would warm the Earth. They urged people to abandon any thought that industrial pollution would serve as a brake on CO2 greenhouse warming. Critics promptly tried to poke holes in the study's limited data.(41) Another group analyzed global weather statistics, found that the recent drop in temperatures was restricted to northern latitudes, and argued that this demonstrated a cooling effect of industrial particle emissions, which were far greater in the Northern Hemisphere. This approach too was quickly criticized, for lack of enough data on Southern Hemisphere temperatures.(42) Many other studies invoked physical models, data, and the history of volcanic eruptions and the ice ages as they debated the relation of particle size to albedo, clouds, and temperature. Like most aspects of climate studies, only even more so, progress on aerosol impacts would require help from many different fields.(43)





<=Modern temp's


=>World winter

The debates made one thing clear: climate change could not be properly understood without a better grasp of aerosol effects. As other scientists made theoretical calculations of scattering, the results were often difficult to reconcile with measurements. It was not clear whether the theories or the measurements were worse. Much more work would have to be done to get even the most basic data, such as how the various kinds of particles of various sizes scattered or absorbed light of various wavelengths. Several groups undertook these measurements in the 1970s, using instruments that, like so much in aerosol science and the rest of geophysics, could be traced back to a military application. (In fact one of the inventors of the original instrument that measured visibility through the atmosphere had died during a meteorological reconnaissance flight during the Second World War.)(46)  







<=Climate cycles



Solar variation

As scientists calculated the physics of aerosols more accurately, they realized they could not figure out any way that smoke and dust from an eruption could cause long-term effects on temperature. For these were solid particles which mostly dropped out of the air within a few weeks. The answer was hidden in something else thrown into the air.  
Sulfates, Soot and Clouds (mid 1970s-1988) TOP OF PAGE  
When thinking about aerosols, the public and most scientists had attended chiefly to the visible and obvious. That meant the fine carbon soot making up smoke (from factories, slash-and-burn forest clearing, and natural forest fires), mineral dust from dried-out soil (perhaps increased by human agriculture), and other solids such as salt crystals (from ocean foam). When scientists thought about climate change that volcanic eruptions might cause, they chiefly considered the minute glassy dust particles that snowed down thousands of miles downwind from an eruption.(50) Well into the 1970s, meteorologists concerned with aerosols mostly continued to assume they were dealing with such coarse mineral particles. However, anyone looking at city smog — or smelling it — might guess that chemicals could be a main component of a haze. The intense studies of urban smog that began in the 1950s focused the attention of a few scientists on the production and evolution of simple chemicals.  
One of the most important of these molecules was sulfur dioxide, SO2. Emitted profusely by volcanoes as well as by industries burning fossil fuels, SO2 rises in the atmosphere and combines with water vapor to form minuscule droplets and crystals of sulfuric acid and other sulfates. These can linger in the stratosphere for years. The particles reflect some of the radiation coming from the Sun and absorb some of the thermal radiation rising from the Earth's surface.  
To the considerable surprise of atmospheric scientists, studies in the early 1960s found that sulfuric acid and other sulfate particles were the most significant stratospheric aerosols. The sulfate haze was especially thick for a few years following a huge volcanic eruption in 1963, when Mount Agung in Indonesia blasted some three million tons of sulfur into the stratosphere. That was an order of magnitude more sulfur than human industry produced in a year, and most specialists thought human emissions of sulfates must be comparatively unimportant.(51) Flights in the stratosphere in the early 1970s (part of a huge government effort to study whether airplanes might harm the ozone layer) demonstrated conclusively that the principal aerosol there was droplets of sulfuric acid, presumably from volcanoes.(52)





Outside the smoggy cities, haze was commonly assumed to be a "natural background" from soil particles and the like, with occasional extra material from volcanoes. That was challenged in 1976 by two leading experts, Bert Bolin and Robert Charlson. Analyzing air purity data collected by government agencies, they showed that sulfate aerosols seriously affected wide regions. Sulfates measurably dimmed the sunlight not only in cities but across much of the eastern United States and Western Europe. This confirmed what McCormick and Ludwig had reported a decade earlier, a widespread haze somehow connected with urban smog.(53)  



Still, the effect seemed minor. Bolin and Charlson figured that human sulfate emissions noticeably affected scarcely one percent of the Earth’s surface. The sulfates were cooling the Northern Hemisphere by scarcely one-tenth of a degree. Most scientists thought that was negligible (even if the calculation were accurate, which seemed unlikely). They continued to assume that the problem of human aerosols was strictly local, or at worst regional. Bolin and Charlson themselves, however, noted that emissions were climbing steeply. They warned that "we are already approaching the time when the magnitude of the indirect effects of increasing use of fossil fuel may be comparable to the natural changes of the climate over decades and centuries."(54)  




In the mid 1970s, some groups studying greenhouse gases managed at last to develop computer models that plausibly connected climate to variations of radiation. A few groups tried to apply these models to aerosols. First they needed reasonably accurate information on the spectrum of aerosols normally in the atmosphere — the sulfuric acid droplets, salt crystals, rock dust, soot, and so forth. What were the sizes of the particles, their chemical composition, and their effects on radiation at various heights in the atmosphere? There were far fewer observations than the scientists needed, but some approximate numbers were laboriously worked out in a form usable for modeling studies.(56) The scientists also had to give up their preoccupation with the smog-ridden lower atmosphere, considering also the clear stratosphere. A few extra particles there, lingering for months, could make a big difference to the passing radiation. Despite daunting theoretical complexities and ignorance of many aerosol properties, the enterprise made progress. Different groups of modelers, using different techniques, converged on some tentative ideas.  
The first big idea was that the formation of clouds was not in fact already saturated by natural aerosols. Thus adding some particles to the atmosphere should noticeably affect climate. The second big idea was that the net effect of adding aerosols, an effect which could now be reliably calculated, was to increase the planet’s reflectivity and thus bring modest cooling.(57)  
<=>Models (GCMs)


<=>Simple models

<=Venus & Mars
The feeling that scientists were getting a handle on aerosols was strengthened in 1981 when Hansen's group fed their computer model a record of modern volcanic eruptions. They combined the temporary cooling effect of volcanoes with estimates of changes due to solar variations and, especially, to the rising level of CO2. The net result fitted pretty well with the actual 20th-century temperature curve, adding credibility to their model's prediction of future global warming.(61) (This result was robust: vastly more sophisticated computer models at the end of the 20th century continued to get a good match to modern temperature fluctuations if, and only if, they added together eruptions, solar activity, and the rise of greenhouse gases. Adding industrial aerosol pollution further improved the match.)(62)  
<=>Radiation math
=>Modern temp's
<=Solar variation
=>Public opinion
= Milestone
=>Models (GCMs)
Progress would depend upon more accurate knowledge of the intricate chemistry of the atmosphere. In the 1980s, aerosol physicists and atmospheric chemists finally established close contacts. It was becoming clear that the most important aerosols humanity produced were not dust and smoke particles, but products of chemical reactions of the gases we emitted. As in almost any field of geophysics, recognition of an important area of ignorance drove rapid improvements in measuring instruments and also in theory (which by now was done mostly through computer models).  


=>Other gases

The need to resolve the problem was driven home by undeniable evidence that dimming of sunlight by aerosols was increasing sharply all across the Northern Hemisphere. By one estimate, the reduction was as great as 18% per decade.(68) Even in the Arctic, where the immense empty landscapes promised only pristine air, scientists were startled to find a visible haze of pollutants drifting up from industrial regions. There was so much soot that it might alter the northern climate.(69*)  
(I have seen it myself. After backpacking in the Sierra Nevada and the Canyonlands, decades after my first treks in these sacred places, I am pained to confirm that the views of distant cliffs, and even of the stars, are never as sparkling clear as I was once used to seeing.)
global dimming haze picture
Grand Canyon haze
Thinking about cooling from aerosols took a spectacular turn in 1983, when a group of scientists, mostly people who had already been studying the question, went public with warnings of an even worse danger. If the blasts of a nuclear war injected smoke and dust into the atmosphere, a lethal "nuclear winter" might envelop the planet. The Russian meteorologist Kirill Kondratyev went on to point a finger at the nitrates (NOx) that had already been put into the atmosphere by weapons tests. These had produced aerosols which, he surmised, might have been responsible for the decreased transparency of the atmosphere, and thus the cooling, observed during the 1960s. Like others, Kondratyev warned that aerosols from human activity had temporarily masked the tendency toward global warming due to increased CO2.(70) Only think how much cooling might follow a thousand nuclear explosions!  
An even more horrendous effect of aerosols had been proposed back in 1980 by Walter and Luis Alvarez: the doom of the dinosaurs when a giant meteor struck the Earth 65 million years ago. Calculations showed that dust from an asteroid impact could have fatally cooled the planet.(71) All this was sharply contested by other scientists. The leading alternative that they developed to explain the doom of the dinosaurs was a series of gargantuan volcanic eruptions. That just showed another way that aerosols could change climate on an apocalyptic scale.(72)


<=World winter




=>Public opinion

Turning back to the way routine pollution might affect climate, scientists were slowly hacking a way through the jungle of complexities. A few meteorologists gradually worked out the implications of Twomey's studies, noticing how aerosols could create lingering misty clouds that might reflect enough sunlight to cancel the greenhouse warming.(73*) It was hard to know whether nature really acted according to these difficult calculations, and most experts paid little heed. After all, even massive direct cloud seeding had never been proven capable of doing much, despite decades of experiments. As Twomey admitted in 1980, "clear field verification has not been obtained" for various key predictions.(74)  
Finally in 1987 a dramatic visible demonstration convinced many scientists that the theory deserved respect. Satellite pictures of the oceans displayed persistent clouds reflecting sunlight above shipping lanes — a manifest response to ship-stack exhaust. Apparently aerosols could indeed create clouds, enough to outweigh the particles' direct interactions with radiation. (Later studies showed there were inconsistencies, as usual with aerosols. In some cases emissions from ships made for more cloudiness, in some cases less.)(75) It was also becoming clear that humans were the dominant source of the atmosphere's sulfate aerosols.(76) Nevertheless, many scientists continued to think of aerosols as "local" pollution and worried little about global implications.  
The closer scientists got to definite answers, the more they noticed additional factors that they ought to figure in. Especially troublesome was the fact that any climate change would alter the natural background emission of aerosols. For example, if deserts expanded (whether from direct human activity or climate change) there would be more airborne dust. Meanwhile pollution studies showed that altering the amount of one type of aerosol in the air would alter the distribution of sizes and other key characteristics of other aerosols (and these subtle calculations themselves, the author warned, "do not do justice to the complexities of the real atmosphere").(77) On top of this, there could be biological feedbacks. The most intriguing suggestion was that the nuclei for condensation of clouds in the pure air over the oceans might come primarily from dimethylsulfide (DMS) molecules, whose chief source is living plankton.(78) It was another feedback dependent on temperature which might stabilize the climate — or might not.






After 1988


By 1990, scientists understood that human activity produced somewhere between a quarter and a half of all the aerosol particles in the lower atmosphere, including industrial soot and sulfates, smoke from debris burned when forests were cleared, and dust from semi-arid lands turned to agriculture or overgrazed. The consequences, if any, were entirely uncertain — "at this stage neither the sign nor magnitude of the proposed climatic feedback can be quantitatively estimated."(80) Interest remained focused on greenhouse gases, which were expected to dominate climate change sooner or later.



=>Models (GCMs)

Nevertheless some scientists were starting to realize they might need to take into account what the rapid increase in aerosols would do to climate. Hansen called for better monitoring and more studies.(81) Experts increasingly admitted that global climate change was not a matter of CO2 alone. It came from a variety of effects ("forcings") on incoming and outgoing radiation due to a variety of gases and aerosols. A leader in the work remarked that it was this shift of viewpoint — looking at changes in the energy balance rather than attempting to calculate surface temperature changes — that made meaningful global calculations possible. He added that the calculations "would not have been possible without an enormous amount of work measuring the actual properties of atmospheric aerosols."(82) Workers in the various fields that dealt with aerosols increasingly exchanged information and ground out measurements and computations.


<=Other gases

In the early 1990s, Charlson and others worked to persuade aerosol experts that sulfates could cause significant cooling, simply by scattering back incoming solar radiation. The effects of sulfate particles through stimulating cloudiness were harder to estimate, but probably added still more cooling. In a pioneering 1991 calculation, Charlson and his allies concluded that the scattering of radiation by humanity’s sulfate emissions was roughly counterbalancing the CO2 greenhouse warming in the Northern Hemisphere. The calculation, however, was admittedly full of uncertainties.(83)  

<=>Modern temp's

= Milestone
<=>Models (GCMs)
<=Arakawa's math


Papers published in 1992 concluded that the smoke from slash-and-burn farming of tropical forests might have been enough all by itself to cancel a large share of the expected warming. Other scientists reported that the direct effect of sulfates blocking sunlight "completely offsets the greenhouse effect" in the most industrialized regions. Yet another team estimated that the indirect action of sulfates, making clouds darker, could have a still stronger cooling effect.(85*) As one expert remarked, "the fact that aerosols have been ignored means that projections may well be grossly in error."(86) Thus efforts to restrict sulfate emissions, however important that might be for reducing acid rain and other unhealthy pollution, might hasten global warming. (On the other hand, reducing the emission from smokestacks of another important aerosol, soot, would help the climate as well as human health, since the black particles made for warming.)

Pinatubo ash cloud
Computer modelers returned to their simulations of global temperature, and found they could get curves that matched the observations since the 1860s quite closely provided they included increases in sulfate aerosols as well as CO2.(87) Because aerosol pollution was greater in some regions than others, whereas CO2 levels were about the same everywhere, modelers could even try to disentangle the two influences.(88) To be sure, there was a risk that with aerosol effects poorly understood, the modelers might merely be adjusting their numbers until they reproduced the climate data, overlooking other possible factors. But the new results incorporating aerosols did give, for the first time ever, a plausible and consistent accounting of the main features of 20th-century climate. In particular, it seemed likely that industrial pollution had temporarily depressed Northern Hemisphere temperatures in mid century. As Bryson had speculated back in the 1970s, the effects of aerosol emissions from human industry were comparable to the effects of a large volcanic eruption. These results led directly to the announcement by a 1995 international report that human influence on climate had probably become discernible. Global warming might have become evident decades earlier, but for the overlooked cooling effect of aerosols.





<=>Models (GCMs)

The reprieve from warming was only temporary. Rains washed aerosols out of the sky within a week or two, so that the level in the atmosphere matched the level of emissions, whereas a large fraction of greenhouse gas emissions would linger in the atmosphere for centuries, steadily accumulating. The intergovernmental panel's next report, issued in 2001, also pointed out that industrialized nations were taking steps to reduce pollution, which would reduce the cooling influence. Considering various possibilities, the panel reported a high upper limit for where global temperatures might go during the 21st century. Computer models had not changed much since the previous report. But the panel offered an additional scenario, in which use of fossil fuels continued to expand at a breakneck pace while pollution controls strongly restricted aerosols. In that case greenhouse warming might shoot up nearly 6°C.(89)




<=>Models (GCMs)

A minority of experts dissented from the panel's confidence that the improved computer models gave solid information. The critics warned that "given the present uncertainties in aerosol forcing, such improvement may only be fortuitous."(90) To clear up the uncertainties, scientists needed better information not only on how aerosols interacted with weather, but also on just what kinds of aerosols human activity stirred up and just where the winds blew them.(91) None of that was measured well enough.  

The old discussion of whether pollution brought warming or cooling was still yielding surprises. In particular, evidence turned up that much more soot ("black carbon") was puffing into the air than had been suspected. For example, a team under Ramanathan deploying ships, aircraft and balloons in the Indian Ocean in 1999 detected a huge drifting "brown cloud" — a miasma caused by human activity, expanded from the haze that Bryson had noticed while flying over India a third of a century earlier. While the dark smokes shaded the surface and thus made for cooling there, higher in the atmosphere the soot absorbed radiation so thoroughly, according to new calculations, that overall it added seriously to global warming — perhaps more than methane and second only to CO2.(92*) Cutting this sort of pollution could probably prevent not only damage to local and global climates, but also hundreds of thousands of premature deaths from respiratory illnesses. Some scientists argued that before going all out to restrain greenhouse gases, the world should attack the rightly despised smokes, the most ancient form of technological pollution.(93)



= Milestone

<=>Other gases

Later, beginning around 2002, climatologists were surprised by evidence that hazes were having an even bigger effect than they had supposed. As far back as 1989, Atsumu Ohmura in Switzerland had published evidence that sunlight had been growing dimmer throughout the 20th century. Ohmura’s work had attracted scarcely any attention, even though some computer modellers had begun to worry that their models did not seem to include enough aerosol absorption. Now evidence turned up by other scientists convinced many experts that the Northern Hemisphere, at least, had seen a dimming of 10 percent or more — much more than the experts had thought, indeed probably great enough to affect agriculture. Aerosol pollution was the only plausible cause. "There could be a big gorilla sitting on the dining table, and we didn't know about it," Ramanathan admitted in 2004.

Many aerosol specialists now suspected that they had seriously underestimated how strongly greenhouse warming had been held back by the cooling effect of aerosols. If so, then temperatures would now rise more sharply. For the "global dimming" trend was not really global but regional, and during the 1980s it had reversed in many regions. Nobody could explain why this had happened. Perhaps one cause was the pollution controls imposed by some nations, which had certainly been reducing sulfates. The way these sulfates and other aerosols had previously kept some sunlight from reaching the surface had given the world "a false sense of security" about global warming, the respected atmospheric scientist Paul Crutzen warned in 2003. Whatever was happening, it was more obvious than ever that the world urgently needed better measurements of aerosols, and better models for how they blocked sunlight.(93a*)


= Milestone

Large uncertainties also remained in figuring how aerosols interacted with gases, and above all with water vapor (the main "indirect effect”"or "Twomey effect"). Questions were raised once again by detailed observations that confirmed the speculation that had first started scientists worrying back in the 1960s — cirrus clouds did grow from jet contrails, visibly influencing the climate in regions beneath heavily traveled air routes.(94*) Experts published widely divergent models for the formation of such clouds and their absorption of radiation. Controversial measurements published in 1995 claimed that clouds absorbed much more radiation than the conventional estimates said, raising a specter of "missing physics." As one researcher complained, "The complexity of this problem seems to grow with each new study." It was reasonable to expect that improvements in theoretical models and measuring techniques would eventually lead to a reconciliation (indeed within the next decade theory and observations would largely converge), but meanwhile, Ramanathan admitted, "If I wake up with a nightmare, it is the indirect aerosol effect." And this effect was only one of several areas where new studies kept showing that, as Ramanathan and a colleague remarked, people were still "in the early stages of understanding the effects" of aerosols.(95*)  






=>Models (GCMs)

General Circulation Models of the Atmosphere
Rapid Climate Change


1. Franklin (1784). First to suggest the connection was a French naturalist, Mourgue de Montredon, in a 1783 communication to the Académie royale de Montpelier. BACK

2. Krakatau's effects were seen only on subtracting supposed effects of the sunspot cycle. Abbot and Fowle (1913). The classic Krakatau study (then called Krakatoa) was Symons (1888). Tambora: Stothers (1984). BACK

3. A principal exponent of the view that volcanoes dominated climate change was W.J. Humphreys, see Humphreys (1913); Humphreys (1920), repeated in the 3rd (1940) edition, pp. 587-618. BACK

4. Wexler (1952), p. 78. BACK

5. Mitchell (1961). On fallout studies he cites a 1960 Defense Atomic Support Agency report by A.K. Stebbins; rhythm: Mitchell (1963), p. 180. BACK

6. It "would be necessary to bring [dust] into the scheme" of a complete calculation, but "that will not be attempted," in the most comprehensive effort at calculation, Richardson (1922), p. 45, see p. 59; discussed in Nebeker (1989), pp. 93-94; Ångström (1929) ; another speculation (first suggested by H. Shapley in 1921) was that long-term climate changes might come when the Earth passed through clouds of interstellar dust. Hoyle and Lyttleton (1939); Himpel (1947); Krook (1953). BACK

7. Wait (1946), p. 343. BACK

8. Wexler (1953), pp. 94-95. BACK

9. Twomey (1977), p. 290. BACK

10. On Roberts: Levenson (1989), p. 98. Reporter: John A. Osmundsen, personal communication; contrail studies are reviewed by Barrett and Landsberg (1975); one of the first observations was the brief report of Georgi (1960). BACK

11. "Plenty of nuclei," "air-quality": Twomey (1980), p. 1459. BACK

12. Conover (1966); mills: Hobbs et al. (1970), see p. 89. BACK

13. Bloch (1965); human impact was emphasized by Landsberg (1970). BACK

14. New York Times, May 1, 1965, p. 1. Aircraft were estimated to increase cirrus over as much as 5% of the worlds's skies, which "is not negligible," according to Bryson and Wendland (1970), p. 137; repeated in Bryson and Wendland (1975), p. 146. There were also concerns about exhaust from space shuttle flights. BACK

15. Wilson and Matthews (1971), p. 9, see Machta and Carpenter (1971); another major group effort found that while supersonic transports appeared to be harmless, the effect was close enough to the threshold of harm to merit concern. Pollack et al. (1976); in 1999 a scientific panel concluded that aircraft would contribute roughly 5% of the human influence on climate by the year 2050, IPCC (1999). BACK

16. United States (1950); see Benarie (2000). BACK

17. "glamour": Gerhard Kaspar in Preining and Davis (2000), p. 392; "academic": Davies (1970). BACK

18. Davies (1970); Othmar Preining (personal communication) writes that an aerosol scientific community began forming in the mid 1960s, following the publication of Fuchs (1964); see Preining and Davis (2000), pp. 9, 148-49, 393. The American Association of Aerosol Science was formed only in 1981. A journal Atmospheric Environment, founded 1967, dealt only with pollution. BACK

19. For a review of aerosol history, see Charlson (1998). BACK

20. Junge (1958), p. 95. He asserted that "unpolluted areas... no longer exist" in Western Europe and the northeastern United States (p. 101), but was not thinking of pollution great enough to alter climate. BACK

21. McCormick and Ludwig (1967); Bryson (1967); for establishment of network in 1960-61, see Flowers et al. (1969). BACK

22. Twomey (1977), p. 1. BACK

23. Bryson and Wendland (1970), quote p. 137; see also Bryson (1968); India: Bryson (1967), p. 53. BACK

24. Cobb and Wells (1970); see also Hodge (1971). BACK

25. Mitchell at meeting of AAAS, Boston, Dec. 1969, as cited by Landsberg (1970); quote: Mitchell (1970), p. 153, from a 1968 symposium. BACK

26. "It seems probable that present changes of the Earth's temperature are determined mainly by... the level of volcanic activity," concluded Budyko (1969), p. 613; on the other hand, Lamb concluded that "volcanic dust is not the only, and probably not the main, influence," Lamb (1970); skepticism held up publication of this paper for five years, see Lamb (1997), p. 189. BACK

27. Barrett and Landsberg (1975), pp. 42-43, 48. BACK

28. Mitchell (1971), quote p. 713. BACK

29. If exponential growth continued, Mitchell foresaw a 1°C greenhouse effect temperature rise by 2000, followed by accelerating cooling as aerosols accumulated faster than CO2. Mitchell (1970); "In my opinion, man-made aerosols... constitute a more acute problem than CO2," Landsberg (1970). BACK

30. Kaiser (2000). BACK

31. This globally averaged model didn't allow for changes in convection or clouds, and got only 2°C of warming for an eightfold rise of CO2, an error soon corrected by other calculations. Rasool and Schneider (1971), quote p. 138, with references to work of Budyko and Sellers; they calculated the doubling of dust from data reported by Hodge (1971); see confirmation and priority claim by Barrett (1971); criticism: Charlson et al. (1972); Weare et al. (1974); Chylek and Coakley (1974); possible warming was calculated by Wang and Domoto (1974). BACK

32. Also, it was suggested that such dust storms might initiate a radical warming by darkening the polar ice caps. Sagan et al. (1973). BACK

33. Bryson (1973). BACK

34. Bryson (1974), quote p. 756; for earlier mention, see Bryson (1973), p. 9. BACK

34a. Rotstayn and Lohmann (2002). BACK

35. Kellogg made a distinction between effects of aerosols over land (cooling) or sea (not necessarily cooling), but held that the pollutants were mostly over land. Kellogg et al. (1975); Bryson's theory of cooling was "almost the opposite of the true situation," Kellogg said at a 1980 international workshop, Kellogg (1980), p. 282. BACK

36. Schneider and Mass (1975). BACK

37. E.g., Barrett and Landsberg (1975), pp. 53, p. 77. BACK

38. Bryson and Dittberner (1976); see the challenge by Woronko (1977) and reply, Bryson and Murray (1977).

38a. Peterson and Connolley (2007). BACK

39. "Perhaps the most sensitive," Hobbs et al. (1974); "profound," referring to Budyko's 1969 paper, Twomey (1974). BACK

40. GARP (1975), quote p. 44; they cite Mitchell (1973); aerosol effects were "lost in the noise": Barrett and Landsberg (1975), p. 72. BACK

41. Idso and Brazel (1977); Herman et al. (1978). BACK

42. Damon and Kunen (1976); Damon and Kunen (1978). BACK

43. For example: Baldwin et al. (1976); Pollack et al. (1976); Shaw (1976); Ninkovich and Donn (1976); Herman et al. (1978); aerosols "can hardly have a significant effect" except regionally: Kellogg (1980). BACK

44. Twomey (1977); Twomey (1977); Twomey (1977); see also Twomey (1974) (which showed that while very few nuclei would inhibit precipitation, so would very many, multiplying droplets too small to fall as rain); for brief review and further references on aerosols and precipitation, see Rosenfeld (2000), p. 1793. BACK

45. Toon (2000), p. 1763. BACK

46. Heintzenberg and Charlson (1996), p. 987. BACK

47. Kondratyev (1981) ; Ginsburg and Feigelyson (1980) . BACK

48. E.g., "A significant dip in temperature can be found within a few years after the major eruption dates..." according to Taylor et al. (1980), p. 175. BACK

49. Hamilton and Seliga (1972). Recent studies: Bauer et al. (2004. BACK

50. Humphreys (1940), p. 595; Junge (1952) . BACK

51. Wilson and Matthews (1971), pp. 279-80, 283-84. BACK

52. Barrett and Landsberg (1975), pp. 44-45. BACK

53. Bolin and Charlson (1976); for other studies of regional haze, see Husar and Patterson (1980). BACK

54. Bolin and Charlson (1976), p. 50. BACK

55. I have not seen the original Russian language publications, including Budyko (1974); Budyko (1974); see Budyko and Korol (1975); Budyko (1977), pp. 239-41; quote from Geophysical Abstracts B (1977), p. 63, an English summary of Budyko and Drozdov (1976). BACK

56. E.g., Toon and Pollack (1976). BACK

57. Harshvardhan and Cess (1976); Harshvardhan (1979); Charlock and Sellers (1980); for an overview, see Hansen et al. (1980). BACK

58. A one-dimensional model. Hansen et al. (1978); see also the approximate calculation by Pollack et al. (1976); Charlock and Sellers (1980); recent opinions: e.g., B.J. Mason, see Gribbin (1976). BACK

59. The Venus greenhouse was invoked regarding the importance of sulfuric acid in Hansen et al. (1978). BACK

60. Hammer et al. (1980). BACK

61. Hansen et al. (1981); see also Bryson and Goodman (1980) (eyeball comparison going back to the 1880s); Gilliland (1982). BACK

62. Stott et al. (2000). BACK

63. Hofmann (1988), quote p. 196. The paper includes a historical review of 1980s work. BACK

64. Coakley et al. (1983). BACK

65. Freeman and Liou (1979), p. 283. BACK

66. Hansen et al. (1981), p. 960, "inevitable" p. 966. BACK

67. E.g., Husar and Patterson (1980) (listing 1970s studies); Ball and Robinson (1982); for useful reviews, see Charlson and Wigley (1994); Charlson (1998). BACK

68. Peterson et al. (1981). BACK

69. "Arctic Haze, an aerosol showing a strong anthropogenic chemical fingerprint," Shaw (1982); scientists "startled": Kerr (1981). Already in the 1950s, J. Murray Mitchell had guessed the haze was caused by distant industries. BACK

70. Kondratyev (1988), pp. 179-95. BACK

71. Alvarez et al. (1984); Wolbach et al. (1985). BACK

72. McLean (1985). BACK

73. More pollution divided the water among more and hence smaller droplets, which not only made clouds linger (by inhibiting precipitation) but would also raise the reflectivity of the clouds and lower their absorption of solar radiation, keeping them cool and further lengthening their lifetime. Twomey (1980); the effect of aerosols in increasing cloud lifetimes and reflection, especially over the oceans where nuclei are rare, was worked out particularly by Albrecht (1989); "...the climatic effect is quite comparable to that of increased carbon dioxide, and acts in the opposite direction." Twomey et al. (1984). BACK

74. Twomey (1980), p. 1461; he went on to report a verification at a single site, Twomey et al. (1984). BACK

75. Coakley et al. (1987); Radke et al. (1989); the cloudiness is probably due to nitrates, see Lawrence and Crutzen (1999). Inconsistencies: e.g., Ackerman et al. (2000). BACK

76. Schwartz (1988). BACK

77. White (1986), quote p. 1671. BACK

78. Charlson et al. (1987). BACK

79. Joseph (1984); Charlson et al. (1992), p. 425. BACK

80. Quote from chapter on "Greenhouse gases and aerosols" by R.T. Watson et al., IPCC (1990), p. 32. BACK

81. Hansen and Lacis (1990). BACK

82. R. Charlson, personal communication, 2002. BACK

83. "Comparable to but opposite in sign to the current greenhouse forcing by increased CO2 to date," Charlson et al. (1991); the first, primitive version was Charlson et al. (1990). BACK

84. Hansen et al. (1992). BACK

85. Smoke: Penner et al. (1992); similarly, "It is becoming apparent that anthropogenic aerosols exert a radiative influence on climate that is globally comparable to that of greenhouse gases but opposite in sign," Charlson et al. (1992), p. 423; see Kerr (1992). Direct effect calculation: Kiehl and Briegleb (1993), quote from abstract; indirect effect calculation: Jones et al. (1994). BACK

86. Wigley (1994). BACK

87. Mitchell et al. (1995); IPCC (1996), chap. 8. BACK

88. Taylor and Penner (1994). BACK

89. IPCC (2001). BACK

90. Ledley et al. (1999), p. 458; Singer (1999) also notes uncertainty about aerosol effects. BACK

91. E.g., on increased dust, see Andreae (1996). BACK

92. Satheesh and Ramanathan (2000), discussed in Wall Street Journal, May 6, 2003, p. 1; Hansen et al. (2000). "The magnitude of the direct radiative forcing from black carbon itself exceeds that due to CH4, suggesting that black carbon may be the second most important component of global warming after CO2 in terms of direct forcing," Jacobson (2001). Subsequently Hansen and Nazarenko (2004) argued that decreased reflection of sunlight from snow and ice dirtied by soot gives another significant contribution to global warming. BACK

93. Hansen et al. (2000); Andreae (2001). BACK

93a. Ohmura and Wild (2002) and Roderick and Farquhar (2002) drew attention to the summary of evidence in Stanhill and Cohen (2001); Ohmura and Lang (1989). For "gorilla" and more see Kenneth Chang, "Globe Grows Darker as Sunshine Diminishes 10% to 37%," New York Times, May 13, 2004. Reversal: Wild et al. (2005); Pinker et al. (2005), Stanhill (2007). Underestimates: Anderson et al. (2003); Crutzen quoted Pearce (2003). A 2005 analysis of satellite measurements indicated a disturbingly strong aerosol effect, Bellouin et al. (2005). Work by others including Farquhar, Liepert, Ramanathan and Roderick was described on the BBC "Horizon" broadcast of April 3, 2005, transcript on the BBC Website listed as 20 January 2006. BACK

94. Boucher (1999). This and later studies showed the effect was significant but not as large as the greenhouse effect and other sources of climate change. BACK

95. Cess et al. (1995); Pilewskie and Valero (1995); Ramanathan et al. (1995); Li et al.(1995); see Kerr (1995); "complexity:" Kiehl (1999), p. 1273; "nightmare:" Ramanathan quoted in Schrope (2000), p. 10; "early stages," Satheesh and Ramanathan (2000), p. 62; for an argument that there was nothing serious missing, see Hansen et al. (2000), pp. 147-54. "Because nearly all recent studies show good agreement between observations and models, the dust of the CAA [cloud absorption anomaly] debate appears to be settling down," Li et al. (2003). BACK

copyright © 2003-2007 Spencer Weart & American Institute of Physics