Wednesday, August 02, 2006

Is the science of climate change settled?

In the early 1990s, I worked on stratospheric photochemistry, i.e., ozone depletion. I even wrote a book about it (makes a great gift for that person who has everything!). By the mid-1990s, it was clear to me that the science of stratospheric ozone was truly settled. We understood just about everything. You know what I did? I decided to switch fields and study the climate system.

A constant argument in the policy debate over climate change is whether "the science of climate change is settled." It is clearly not. If it were, then I would have switched to another field, just as I did in the mid-1990s. No respectable scientist wants to do research on a system that's well understood.

However, everyone needs to recognize that we don't need perfect knowledge in order to take action. We make important decisions in the face of uncertainty all the time (e.g., should we invade Iraq?). In the case of climate change, we know enough to know that climate change carries a very real risk of severe, even catastrophic impacts over the next century. We can argue about the exact value of the risk: is it 20%, 50%, or 80%? I don't know the exact value, but it's not zero.

The decision about what to do about the uncertain risk of climate change is a policy decision and not a scientific result. How risk averse are we? Are we willing to trade some economic growth to reduce a risk of catastrophic climate change? This is something we have to decide as a society, and this is where the public debate needs to be.


joe-6-pack said...

Dr. Dessler;

Welcome to the blogosphere. I blog on a number of issues including my viewpoints on the current Climate Change paradigm.

In my humble scientific opinion, based on my BS & MS degrees in Geology, the carbon dioxide component of the Troposphere is too small to significantly control the Greenhouse Effect, rather humidity and clouds "run the show". And when you consider the small anthropogenic contributions vs. natural contributions, it is really difficult to assess our effects on warming. Is it 0.001%, 0.01%, 0.1%, 1.0%, or 10%? How can we conclusively say?

Again, in my opinion, deforestation and the growth of Urban Heat Islands may contribute a significant amount of warming.

I don't know if you have read the internet article on the origins of the current paradigm. If not, please see:

Thanks for your time and contributions.

Roger Pielke, Jr. said...


From a political standpoint the debate is indeed "settled." Consider this excerpt from a UK report on the political dynamics of climate change:



Where science is invoked, it now needs to be as ‘lay science’ – offering lay explanations for what is being treated as a simple established scientific fact, just as the earth’s rotation or the water cycle are considered."

This is a tricky time for the climate science community.

On the one hand, some might say, if climate science is "settled" enough for policy action, then why the need for a lot of additional research in this area (much to the dismay of those interested in support for climate research)?

On the other hand, some might say that if policy action depends upon further "settling" the science, then maybe those skeptics have a point (much to the dismay of those wanting political action on climate policy)?

Gook luck shooting that gap ;-)

Andrew Dessler said...


Thanks for your comments. The estimates of the radiative forcing of CO2 on the surface/troposphere system have been worked out in great detail using radiative transfer calculations. These suggest that doubling CO2 will increase radiative forcing by 4 W/m^2, a large increase. Unless you think that either quantum mechanics, CO2 spectroscopy, or the radiative transfer calcs are wrong, then there's little wiggle room there on this issue. [note: I have read that web site in the past and not found it very credible]

As far as attribution of present-day warming goes, there's a lot of peer-reviewed detection and attribution studies out there that conclude that humans are contributing a significant part of the present day warming. And just as importantly, there's no peer-reviewed evidence that today's warming is predominantly natural.


Andrew Dessler said...


I agree that the "case for action" is settled even though uncertainties remain in the science. However, this is a normative judgment based on my values, and I don't think everyone would agree --- e.g., Lomborg.

My point here was that the "science is settled" is simply not true. There's a lot of uncertainty in our knowledge. That uncertainty does not preclude action --- in fact, I judge it compels action. But saying the science is settled invites the other side to argue ("no, there's lots we don't know") and that argument induces gridlock in the policy debate. It's much harder for those opposed to GHG reductions to argue that climate change is not a risk.


Dano said...



Thinking, instead, about win-win solutions helps us avoid binary constructs or assumptions that actions to reduce risk are inherently bad for the economy.



EliRabett said...

Given the views of a number of committee chairs in the US congress, the president of the United States, various coal oil and gas industries, various utilities that burn coal and raise money for Pat Michaels, the blogging state climatologists, etc. for Roger Pielke Jr to claim that politically the issue is settled is about at the same level of veracity as Don Rumsfelds claim that he was never overly optimistic about the invasion of Iraq and its outcomes.

A mildly better statement would be that outside of the US and Australia (additions possible?), the issue is settled from a political standpoint, but that there are significant political and economic forces opposing such a consensus in the US. Which is unfortunate as meaningful action would require the US to sign on..

Roger, you gotta be kidding

Mark H. said...


If it were only that simple. Any policy that reallocates a major portion of the world GDP from providing food, shelter, medical care, etc. TO the mitigation of global warming needs a number of questions resolved. Among them:

1) What are the chances that the GCM models will correctly predict (a range of) significant global temperatures 50 to 100 years from present?

2) What are the chances that such tempature prediction ranges will also confidently predict the the climate impacts and its global distribution?

3) What are the chances that such climate changes will seriously harm human well-being, rather than being "a wash" or an improvement in well being?

4) What are the chances we can confidently find an effective policy of prevention, mitigation, or adaptation?

5) Given the above uncertainties, what will be the economic cost vs. the economic benefits (i.e. a calculation of "risk, uncertainty, and profit" (F. Knight).).

Climate science is answering questions 1 & 2, but 3 through 5 are questions are even more unsettled and need to be resolved by agronomists, economists, ecologists, and applied technology scientists.

In other words, we need to resolve the large uncertainties over the actual risks to human well-being (vs. the uncertainty over warming).

Anonymous said...

I simply must agree with Eli:

Those who say that "From a political standpoint the debate is indeed "settled" ' can't be talking about the US.

Andrew Dessler said...

Mark H.-

What you're saying, in a nutshell, is that "We don't know enough to take action now. More research is required." This sounds like a scientific statement, but it is a moral judgment on your part, and one that I disagree with. I agree that there are uncertainties in general in the areas you've identified, but my moral judgment is that the chance of catastrophic harms from climate change is sufficient that it compels us to begin to take action now, even though uncertainty exists.

Of course, any policy we adopt must recognize the uncerainty and be able to flexibly adjust as new knowledge arises. We discuss this in mind-numbing detail in our book in the second half of chapter five.

As George W. Bush said in regards to Iraq:
On climate change, we also cannot wait for final proof. It might be too late to act by the time it becomes available. As I said in my post, this moral question is where the public policy debate needs to be.


Anonymous said...

Ah, very smart Dr. Dessler, I agree that comparing Iraq and AGW is appropriate, yes it was so smart to rush to drastic action in Iraq with "enough" evidence and a "consensus" of experts. Surely you can't seriously want to use that analogy, I am a scientist who is a skeptic of AGW after 35 years of research, lecturing and teaching, but I thought you were a believer, it surely damages your cause to compare Iraq and AGW so forcefully, but who am I to tell you what to do, I just wish all the AGW believers I know were as foolish and politically awkward.

Andrew Dessler said...

Thanks for your comment. First, I see our point on the disaster that the Iraq war has become. In addition, I really really really don't want this blog to turn into a discussion of the Iraq war, so let me restate my argument more generally.

We make many important policy decisions in the face of uncertainty. We are rarely certain how things will turn out --- sometimes our decisions are right, sometimes they are not. Intelligent policy can minimize the harm when our decisions turn out to be wrong.

The specific question here is whether there is too much uncertainty for us to take action on climate change. This is a moral, not a scientific, judgment. We can agree completely on the level of scientific uncertainty, but disagree on whether that precludes action. You obviously think we should wait for more information, while I disagree with that point. This is where the public policy debate needs to be. Arguments over the reality of climate change, or whether the "science is settled," miss the point.


Andrew Dessler said...

One more thought for everyone:

Roger Pielke, Jr. often argues that a lack of policy options is one of the barriers to a productive climate change debate. I think he makes a good point here. If I could convince you that my proposed policy has lots of benefits and that, if we're wrong about climate change, minimizes costs to society, then I suspect that this might significantly move the debate forward. In that sense, arguing whether there is too much uncertainty or not would be greatly aided by a menu of concrete policy options.


Anonymous said...

OK, well said Dr. Dessler. I agree that we disagree on whether there is a sufficient scientific case to take policy actions, but then again my expertise is paleo-climatology, not politics (seems the same as "policy" to me, in my univ., they have taken to calling politics "public policy", for obvious reasons). Therefore I don't know what specifically you mean by policy actions, and I do agree with Dr. Pielke Jr. on that point. I think drastic, draconian actions are what I envision, as do many of my associates and why we usually cringe at the word "policy". Dr. North, Dr. Karl, the Pew Center, and the other pro-AGW forces that testified at Barton and Davis' committee hearings didn't do that impression any good either, as they seemed almost frantic to do something immediately before the sky falls. I am sure many of us on both sides of the issue could agree on some actions that are useful and politically neutral, but the left side tends to want to see huge taxes, new subsidies and tariffs, dismantled fossil fuel industries, and other onerous government regulations imposed, that is just not warranted and will continue to stall any debate or progress, scientific or political.

Mark H. said...


The "chance of catastrophic harms from climate change" is not an informed moral judgement, unless you know the econmic and enviromental impact (a postivist judgement) and JUDGE it to be undesired (a moral judgement).

In the absence of knowing what is/will be (the five postitivist questions) VS. what ought to be (the normaive question) it seems that you, as a Climate scientist, are merely electing to answer most of question 1, a little of question 2, and then suddenly subjectively concluding (my perception) that: 'we need no more science, forget world economic development, I fear something might happen that's really bad, no lets try to prevent warming no matter how costly or problmatical'.

It's natural that one has a certain hubris towards one's own speciality and climate scientists might overlook other (economic) sciences for relevant postivist answers - for my part, my speciality was in economics and international development so I am more concerned about the impact of policy on human well being (while an ecologist might be more concerned about wildlife).

None the less, even you would agree there must be limits to what policy makes sense in view of all these factors. Don't you?

Mark H. said...

An afterthought,

Your quotation of Bush in regards to Iraq is instructive:


Bush's policy choice was a normative judgement (what ought to be) grounded in a postitivist (mis)understanding of what was, and uncertainties (fears) of what may be.

It was not necessarily an illogical choice. If, for example, there was a 10% chance that Saddam would acquire a nuclear arsenal AND a 50% chance he would use it resulting in a war and the deaths of 25,000,000 people then one might measure that risk against the 100% certainity that 250,000 would die in a war of prevention. A simple comparison would suggest Bush correct: (.1x.50x.25,000,000) / 250,000 ... mind you I am merely illustrating).

Although the above probabilities are subjective numbers, they illustrate the rational and necessary framework for one's normative choice within what is or can be known.

The risk of a catastrophic event in 100 years caused by global warming is also the result of the probability of several propostions about warming 'reality' ; i.e.

Probability of significant warming
Prob. of serious negative impact
Prob. of " the government policy" working...divided by social/economic cost.

Compared to cost of doing nothing by government (adjusted by probability of a positive benefit).

You get my drift.

Anonymous said...


Actually, if approached in a certain way, it's both.

The insurance industry approaches risk very "scientificially". Namely, they approach it from the standpoint of "expectation".

They figure the cost of possible outcomes and assign probabilities to each and from this figure an "expected cost". This is then balanced against the cost of possible mitigating measures.

If one were to approach the problem of possible future global warming "scientifically", this would probably be one very good way to do it.

I understand the IPCC's reticence to get into the business of assigning probabilities and costs to possible future scenarios and susing this to calculate expected cost of future warming, but I believe it would help greatly in the "political debate" about AGW. It would put the focus on cost -- something most politicians can understand -- rather than on sea level rise, which most politicians have no clue about.

Perhaps (hopefully) the IPCC have changed their approach. We shall see when their new report comes out. said...

Dr. Dressler, just found your new blog via SciGuy Eric Berger at the Chron. Glad to see this blog!

The issue of "settled science" has come up a number of times at Berger's blog re global warming, and while I am skeptical of the apocalyptic predictions, I am also constantly reminding the other commenters at SciGuy that we don't need to envision an impending global disaster to justify local, national and international efforts in the public and private sectors to reduce pollution.

The health risks and associated costs to the individual person have already reached the catastrophic level, and in fact did so for many millions of people long before we realized just how dangerous pollution could be.

As a mental excersize, just add up the US Medicare bill since the creation of the program, discount it by about 20% and you will be looking at "just one" --- and not by any means the largest -- of the cost factors involved in pollution as a public health problem.

Do I care that some retiree 80 years from now might lose his waterfront as a result of rising sea levels?


On the other hand, as a taxpayer do I care that billions will be spent on treating illnesses related to pollution?

As an insured person do I care that my rates are driven to exhorbitant levels to pay for treating illnesses caused by exposure to pollution?

And as a man living in a highly-industrialized area who would like to stay healthy and not die in agony while attached to an oxygen tank, do I care what is in the air I breathe?

Well, hell yes! :^D

Dr. J said...

tyler5, I can agree with you about true air pollution, such as NOx, SOx, Hg, O3, etc., that have solid scientific cases for cause-effect on human health and thus solid regulatory policy actions (even though not enforced nearly as strictly or consistently as they should be) underway targeted to improved public health. However, CO2 is not even in the same ballpark as real pollution and can't be lump into real, dangerous health risks by any stretch of the imagination.

Daniel Collins said...

“Is the science settled?” is a very ambiguous question. The science of climate change has so many constituents that it can really only be settled when doubt in each of these constituents has fallen by the wayside. But this won’t happen all at once. Some things are bound to become settled sooner than others (by a process known as flocculation). And like the science, some people will remain in suspense until the very end, with others long before understanding the gravity of the situation - unsettling though this may be.

As with hurricanes, we should act on those aspects that are settled, and continue to sort out those that are not. In so doing, we can come to grasp with the two hands that Roger raised above. said...

Dr. J, thanks for your insight and I understand completely what you are pointing out, however, much of the co2 is a byproduct of processes which create a variety of emission problems and contribute overwhelmingly to our national health problems.

For example, driving a vehicle with a gasoline or deisel engine is, I would guess, the most serious personal contribution to air pollution most Americans will make and is a staggering source of dangerous air pollutants as well as co2 emissions. said...

PS, joe-6-pack, I am very interested in your viewpoint and hope you will expand on your viewpoint.

Personally, I don't trust the "records" of the past against which we are comparing the results of the last thirty years of hi-tech data gathering and I strongly suspect the supposed temp "spike" is an artifact of this comparison and not a real phenomenon.

Andrew Dessler said...

Everyone, thanks for your comments.

Mark H.-

I think a problem with the debate is that everyone has a different vision of what "action" means. When I say we need to take action, whatever policy we adopt needs to continue to allow economic growth for all countries. Otherwise, no one will adopt the plan. The challenge is how to structure a plan that allows economic growth while also cutting GHG emissions.

Also, I agree with your example of Saddam and nuclear weapons. The problem with that type of analysis is that we have no idea what the odds are that Saddam would have used the weapon. Thus, one is in a situation similar to climate change, where the probabilities are not well known.

To an anonymous commenter (8:49 AM):
The decision to look at things from a cost-benefit perspective is itself a moral choice. Some evangelical Christians, for example, take their motivation from the Bible, that we should be good stewards of the Earth. They would not care at all whether the costs outweigh the benefits. Again, how one chooses to evaluate policy options is a choice that we must make as a society.


By talking about a "retiree losing his waterfront property," I think you're trivializing the possible impacts of climate change. At the high end, they are substantially more severe than that. Would be more interested in heading off climate change if you thought your descendants might starve? I think it's important that people recognize the possible worst-case scenarios when they consider our responses.


Anonymous said...

Andrew posted "The decision to look at things from a cost-benefit perspective is itself a moral choice."

It may have a moral component (as every policy decsion does) but. let's face it, much of the driving force in our society is economics and it might facilitate decision-making if policy makers knew what the "expected cost" of doing nothing to mitigate globalwarming is vs the expected cost if something is done and the cost associated with the mitigation.

It seems to me that one of the reasons that the politicians in this country can not agree on what (if anything) to do is that they have no concrete cost numbers to work with.

It is all well and good to argue that "We have amoral obligation to do something", but that it very nebulous and does little to nail down exactly what 9if anything) should be done.

Andrew Dessler said...

Anon. (9:42 AM):

There are LOTS of estimates of the costs of GHG reductions and the costs of climate change impacts. Just look at chap. 4 of my book or at the IPCC's Third Assessment Report, working groups II and III.

The problem is that they are highly uncertain --- e.g., the marginal cost of the climate impacts of emitting one ton of carbon to the atmosphere ranges from $10 to $100.

So we're back to the same issue. Do we wait for better estimates of costs? Or do we take action now, despite the uncertainty?

That's a value choice we have to make as a society.


Dr. J said...

It may be true that some decision is coming from our "policy" makers (i.e. political hacks in DC) sometime soon about what actions to take on the hypothesis of AGW. However, to date even the most benign and modest steps have been firmly defeated in the Senate and House. As a skeptic, that's fine with me, but I know others are becoming anxious about doing something, anything (Arnold and Tony's agreement comes to mind). I think anything close to Kyoto is politically impossible for decades, and many pro-AGW forces claim Kyoto is nothing and we need a 10 or 20X Kyoto. My vote is to do nothing other than continue to clean up real pollution aggressively and invest in alternate energy sources as they become economically viable, and if that leads to CO2 reductions, fine, but the object should be a cleaner environment and healthier air, along with more sustainable energy sources with less geopolitical volatility. By targeting real pollution and real energy issues, we can get away from the political quagmire and scientific stalemate and mess that AGW has brought us. said...

Dr. Dessler,

Thanks for your reply!

You ask:

Yes, I certainly would, and I would act on it! :^D

My comments were observations about the current practical political side of the problem. I hope I can contribute something to the discussion of that side of it here.

In the above comments, I was speaking in the voice of the proverbial "man in the street" whose knowledge of climate science is restricted to what he has gleaned from press reports, National Geographic science shows and History channel specials. The general public has long been aware that pollution (for tens of millions of us, based on daily experience with it) is a serious threat and it was not much of a jump at all for "the man in the street" to understand that pollution can have global effects on the climate.

The state of the public mind is, at this time, that the great majority of the public, based on their general knowledge of pollution and global warming issues as well as upon their daily experiences with pollution, are ready for action on the **low end** of the models, but remain skeptical of the catastrophic predictions.

To put it another way, in the public mind the question of the low-end models is "settled." :^D

The cautious skepticism among the general public about the high-end predictions has a lot to do with the fact that the findings of your science have for quite some time been "mediated by the media" as well as by the political activists, professional bureaucrats and politicians, and the public doesn't trust these people as far as they can pick them up and throw them! :^D

(On the other hand, the public's trust for scientists is very high and they will listen to you when you speak directly to them.)

What I'm getting at is this, on the political side it's possible to reach a provisional concensus on action, at this point in the public discussion, at the low end of the models. And this recommends a provisional political strategy which recognizes the current state of the public debate and moves things forward for now as far as possible under the current circumstances.

Anonymous said...

Andrew said: "he problem is that they [costs] are highly uncertain.

That was actually my point of using the example of the insurance industry's estimates of "expected costs".

One does not have to have exact figures for either cost or probability of occurrence of various scenarios to estimate "expected costs".

The insurance industry certainly does not wait around for precise numbers for the probability of future hurricanes of a given magnitude (and of the associated cost) before setting their rates.

The IPCC may have made numerous estimates of costs, but they have not (that i am aware of) done the sort of "expectation" analysis that the insurace industry bases its rates on.

In order to do that, IPCC would actually have to assign probabilites to the various scenarios. Though that may be a very uncertain undertaking, that does not mean the result would have no value to policymakers.

Even bracketing the "expected cost" within some range would give people a basic idea of whether certain policy options were likely to be worthwhile from a cost-benefit standpoint.