Paper - Science, Technology, and Public Policy Program, Belfer Center

Climate-Change Skeptics Revisited

| August 5, 2008


STPP Director John P. Holdren's August 4, 2008, op-ed, "Convincing Climate Change Skeptics", which appeared in both the Boston Globe and International Herald Tribune, has generated much criticism. Professor Holdren has written this essay in response.

I did not expect that my op-ed in Monday’s Boston Globe, to which the editors gave the title “Convincing the Climate -Change Skeptics”, would actually convince many skeptics. It was aimed more at reinforcing the resolve of the majority in the public and the policymaking community who, betting on the scientific consensus, are ready to move forward with a serious approach to dealing with the problem but are being slowed down by the illfounded skepticism of a minority. That is why my own title for the piece was “Climate-Change Skeptics Are Dangerously Wrong”.

I am being castigated by many respondents for resorting to reference to authority rather then providing substantive responses to the specific arguments of climate-change deniers. I suggest that this criticism is in part based on a misunderstanding of what is possible within the length constraint of an op-ed piece. The “top ten” arguments employed by the relatively few deniers with credentials in any aspect of climate-change science (which arguments include “the sun is doing it”, “Earth’s climate was changing before there were people here”, “climate is changing on Mars but there are no SUVs there”, “the Earth hasn’t been warming since 1998”, “thermometer records showing heating are contaminated by the urban-heat-island effect”, “satellite measurements show cooling rather than warming”) have all been shown in the serious scientific literature to be wrong or irrelevant, but explaining their defects requires at least a paragraph or two for each one.

This cannot be done in the 700 words of an op-ed piece. But there are plenty of other forums where it can be…and has been. Persuasive refutations are readily available not only at a high scientific level in (among others) the reports of the Intergovernmental Panel on Climate Change (, the UN Scientific Expert Group on Climate Change and Sustainable Development (, the US National Academy of Sciences (, the US National Center for Atmospheric Research (, and the UK Meteorological Office ( — as well as on a myriad of websites run by serious climatologists (e.g.,,, — but also in a form boiled down for the intelligent layperson by organizations skilled in scientific communication, such as the BBC (, the New Scientist magazine (, and the promising new Climate Central organization ( featuring The Weather Channel’s climatologist, Heidi Cullen.1
Any skeptic who actually wants to know what’s wrong with the standard deniers’ arguments can easily find out.

I provided all the above-mentioned references and more in a longer essay on climate-change skepticism that I wrote in June in response to requests for an explanation of the apparent continuing influence of deniers in the U.S. policy process, and from which I abstracted the op-ed I submitted to The Globe. The references wouldn’t fit within the op-ed word limit without losing too much else that I thought needed to be said.

Even more regrettably, I agreed to a further shortening of what I submitted by the editors at The Globe. I regret agreeing to it because it’s clear (from the responses I’m receiving) that the resulting omission of a sentence about the value of skepticism in science left the impression that I am unaware of the positive role that healthy skepticism has played in the scientific enterprise over the centuries. The omitted sentence was in the middle of a passage that in the original read as follows (omission italicized):

All three factions are wrong, but the first is the worst. We should really call them“deniers” rather than “skeptics”, because they are giving the venerable tradition ofskepticism a bad name. Their arguments, such as they are, suffer from two huge deficiencies.

As my original reference to “the venerable tradition of skepticism” indicates, I am in fact well aware of its valuable and indeed fundamental role in the practice of science. Skeptical views, clearly stated and soundly based, tend to promote healthy re-examination of premises, additional ways to test hypotheses and theories, and refinement of explanations and arguments. And it does happen from time to time — although less often than most casual observers suppose — that views initially held only by skeptics end up overturning and replacing what had been the “mainstream” view.

Appreciation for this positive role of scientific skepticism, however, should not lead to uncritical embrace of the deplorable practices characterizing what much of has been masquerading as appropriate skepticism in the climate-science domain. These practices include refusal to acknowledge the existence of large bodies of relevant evidence (such as the proposition that there is no basis for implicating carbon dioxide in the global-average temperature increases observed over the past century); the relentless recycling of arguments in public forums that have long since been persuasively discredited in thescientific literature (such as the attribution of the observed global temperature trends to urban-heat-island effects or artifacts of statistical method); the pernicious suggestion that not knowing everything about a phenomenon (such as the role of cloudiness in a warming world) is the same as knowing nothing about it; and the attribution of the views of thousands of members of the mainstream climate-science community to “mass hysteria” or deliberate propagation of a “hoax”.

The purveying of propositions like these by a few scientists who do or should know better — and their parroting by amateur skeptics who lack the scientific background or the motivation to figure out what’s wrong with them—are what I was inveighing against in the op-ed and will continue to inveigh against. The activities of these folks, whether witting in the case of the scientists or unwitting in the case of their gullible adherents, have nothing to do with respectable scientific skepticism.

It also needs to be understood by publics and policymakers alike that, while it can never be guaranteed that a mainstream scientific position will not be overturned by new data or insight, the likelihood of this occurring gets smaller as the size and coherence of the body of data and analysis supporting the mainstream position get larger. The lines of evidence and analysis supporting the mainstream position on climate change are diverse and robust — embracing a huge body of direct measurements by a variety of methods in a wealth of locations on the Earth’s surface and from space, solid understanding of the basic physics governing how energy flow in the atmosphere interacts with greenhouse gases, insights derived from the reconstruction of causes and consequences of millions of years of natural climatic variations, and the results of computer models that are increasingly capable of reproducing the main features of Earth’s climate with and without humaninfluences.

The public and the policymakers who are supposed to act on the public’s behalf are constantly having to make choices in the absence of complete certainty about threats and outcomes. If they are smart, they make those choices on the basis of judgments about probability: Which position is more likely to be right? On climate change, the probability is high that the scientific mainstream is right about its main conclusions, even if all the details are not yet pinned down. Those main conclusions are that climate is changing in ways unusual against the backdrop of natural variability; that human activities are responsible for most of this unusual change; that significant harm to human well-being is already occurring as a result; and that far larger — perhaps catastrophic — damages will ensue if serious remedial action is not started soon.

The rationale for calling the attention of the public and policymakers — the audiences for an op-ed — to the number, diversity, and distinction of scientists and scientific organizations embracing these conclusions is to inform them of the extent to which this is the view of the most qualified people and groups that have studied the matter. Given the unavoidable fact that most people do not have the training (or the time) to reach an independent conclusion on a scientific matter of this kind, knowing where most of the people who do have the training and who have taken the time come down on the matter is the best guide available on where the public and its policymakers should place their bets.


1 Truth in advertising: I was one of five Coordinating Lead Authors of the UN Scientific Expert Group report (which had altogether 18 authors from 11 countries), and I am a member of the Board of Climate Central. I have not been involved in the work of the IPCC.

For more information on this publication: Please contact Science, Technology, and Public Policy
For Academic Citation: Holdren, John P.. “Climate-Change Skeptics Revisited.” Paper, Science, Technology, and Public Policy Program, Belfer Center, August 5, 2008.