Sorry , I got my hands on a text copy...
Science 3 December 2004:
Vol. 306. no. 5702, p. 1686
DOI: 10.1126/science.1103618
Essays on Science and Society
Also see the archival list of the Essays on Science and Society.
BEYOND THE IVORY TOWER:
The Scientific Consensus on Climate Change
Naomi Oreskes*
Policy-makers and the media, particularly in the United States, frequently assert that climate science is highly uncertain. Some have used this as an argument against adopting strong measures to reduce greenhouse gas emissions. For example, while discussing a major U.S. Environmental Protection Agency report on the risks of climate change, then-EPA administrator Christine Whitman argued, "As [the report] went through review, there was less consensus on the science and conclusions on climate change" (1). Some corporations whose revenues might be adversely affected by controls on carbon dioxide emissions have also alleged major uncertainties in the science (2). Such statements suggest that there might be substantive disagreement in the scientific community about the reality of anthropogenic climate change. This is not the case.
The scientific consensus is clearly expressed in the reports of the Intergovernmental Panel on Climate Change (IPCC). Created in 1988 by the World Meteorological Organization and the United Nations Environmental Programme, IPCC's purpose is to evaluate the state of climate science as a basis for informed policy action, primarily on the basis of peer-reviewed and published scientific literature (3). In its most recent assessment, IPCC states unequivocally that the consensus of scientific opinion is that Earth's climate is being affected by human activities: "Human activities ... are modifying the concentration of atmospheric constituents ... that absorb or scatter radiant energy. ... [M]ost of the observed warming over the last 50 years is likely to have been due to the increase in greenhouse gas concentrations" [p. 21 in (4)].
IPCC is not alone in its conclusions. In recent years, all major scientific bodies in the United States whose members' expertise bears directly on the matter have issued similar statements. For example, the National Academy of Sciences report, Climate Change Science: An Analysis of Some Key Questions, begins: "Greenhouse gases are accumulating in Earth's atmosphere as a result of human activities, causing surface air temperatures and subsurface ocean temperatures to rise" [p. 1 in (5)]. The report explicitly asks whether the IPCC assessment is a fair summary of professional scientific thinking, and answers yes: "The IPCC's conclusion that most of the observed warming of the last 50 years is likely to have been due to the increase in greenhouse gas concentrations accurately reflects the current thinking of the scientific community on this issue" [p. 3 in (5)].
Others agree. The American Meteorological Society (6), the American Geophysical Union (7), and the American Association for the Advancement of Science (AAAS) all have issued statements in recent years concluding that the evidence for human modification of climate is compelling (8).
The drafting of such reports and statements involves many opportunities for comment, criticism, and revision, and it is not likely that they would diverge greatly from the opinions of the societies' members. Nevertheless, they might downplay legitimate dissenting opinions. That hypothesis was tested by analyzing 928 abstracts, published in refereed scientific journals between 1993 and 2003, and listed in the ISI database with the keywords "climate change" (9).
The 928 papers were divided into six categories: explicit endorsement of the consensus position, evaluation of impacts, mitigation proposals, methods, paleoclimate analysis, and rejection of the consensus position. Of all the papers, 75% fell into the first three categories, either explicitly or implicitly accepting the consensus view; 25% dealt with methods or paleoclimate, taking no position on current anthropogenic climate change. Remarkably, none of the papers disagreed with the consensus position.
Admittedly, authors evaluating impacts, developing methods, or studying paleoclimatic change might believe that current climate change is natural. However, none of these papers argued that point.
This analysis shows that scientists publishing in the peer-reviewed literature agree with IPCC, the National Academy of Sciences, and the public statements of their professional societies. Politicians, economists, journalists, and others may have the impression of confusion, disagreement, or discord among climate scientists, but that impression is incorrect.
The scientific consensus might, of course, be wrong. If the history of science teaches anything, it is humility, and no one can be faulted for failing to act on what is not known. But our grandchildren will surely blame us if they find that we understood the reality of anthropogenic climate change and failed to do anything about it.
Many details about climate interactions are not well understood, and there are ample grounds for continued research to provide a better basis for understanding climate dynamics. The question of what to do about climate change is also still open. But there is a scientific consensus on the reality of anthropogenic climate change. Climate scientists have repeatedly tried to make this clear. It is time for the rest of us to listen.
References and Notes
1. A. C. Revkin, K. Q. Seelye, New York Times, 19 June 2003, A1.
2. S. van den Hove, M. Le Menestrel, H.-C. de Bettignies, Climate Policy 2 (1), 3 (2003).
3. See www.ipcc.ch/about/about.htm.
4. J. J. McCarthy et al., Eds., Climate Change 2001: Impacts, Adaptation, and Vulnerability (Cambridge Univ. Press, Cambridge, 2001).
5. National Academy of Sciences Committee on the Science of Climate Change, Climate Change Science: An Analysis of Some Key Questions (National Academy Press, Washington, DC, 2001).
6. American Meteorological Society, Bull. Am. Meteorol. Soc. 84, 508 (2003).
7. American Geophysical Union, Eos 84 (51), 574 (2003).
8. See www.ourplanet.com/aaas/pages/atmos02.html.
9. The first year for which the database consistently published abstracts was 1993. Some abstracts were deleted from our analysis because, although the authors had put "climate change" in their key words, the paper was not about climate change.
10. This essay is excerpted from the 2004 George Sarton Memorial Lecture, "Consensus in science: How do we know we're not wrong," presented at the AAAS meeting on 13 February 2004. I am grateful to AAAS and the History of Science Society for their support of this lectureship; to my research assistants S. Luis and G. Law; and to D. C. Agnew, K. Belitz, J. R. Fleming, M. T. Greene, H. Leifert, and R. C. J. Somerville for helpful discussions.
Science 13 May 2005:
Vol. 308. no. 5724, pp. 952 - 954
DOI: 10.1126/science.308.5724.952
Letters
Consensus About Climate Change?
In her essay "The scientific consensus on climate change" (3 Dec. 2004, p. 1686), N. Oreskes asserts that the consensus reflected in the Intergovernmental Panel on Climate Change (IPCC) appears to reflect, well, a consensus. Although Oreskes found unanimity in the 928 articles with key words "global climate change," we should not be surprised if a broader review were to find conclusions at odds with the IPCC consensus, as "consensus" does not mean uniformity of perspective. In the discussion motivated by Oreskes' Essay, I have seen one claim made that there are more than 11,000 articles on "climate change" in the ISI database and suggestions that about 10% somehow contradict the IPCC consensus position.
But so what? If that number is 1% or 40%, it does not make any difference whatsoever from the standpoint of policy action. Of course, one has to be careful, because people tend to read into the phrase "policy action" a particular course of action that they themselves advocate. But in the IPCC, one can find statements to use in arguing for or against support of the Kyoto Protocol. The same is true for any other specific course of policy action on climate change. The IPCC maintains that its assessments do not advocate any single course of action.
So in addition to arguing about the science of climate change as a proxy for political debate on climate policy, we now can add arguments about the notion of consensus itself. These proxy debates are both a distraction from progress on climate change and a reflection of the tendency of all involved to politicize climate science. The actions that we take on climate change should be robust to (i) the diversity of scientific perspectives, and thus also to (ii) the diversity of perspectives of the nature of the consensus. A consensus is a measure of a central tendency and, as such, it necessarily has a distribution of perspectives around that central measure (1). On climate change, almost all of this distribution is well within the bounds of legitimate scientific debate and reflected within the full text of the IPCC reports. Our policies should not be optimized to reflect a single measure of the central tendency or, worse yet, caricatures of that measure, but instead they should be robust enough to accommodate the distribution of perspectives around that central measure, thus providing a buffer against the possibility that we might learn more in the future (2).
Roger A. Pielke Jr.
Center for Science and Technology Policy Research
University of Colorado
UCB 488
Boulder, CO 80309-0488, USA
References
1. D. Bray, H. von Storch, Bull. Am. Meteorol. Soc. 80, 439 (1999).
2. R. Lempert, M. Schlesinger, Clim. Change 45, 387 (2000).
Response
Pielke suggests that I claimed that thereare no papers in the climate literature that disagree with the consensus. Not so. I simply presented the research result that a sample based on the keywords "global climate change" did not reveal any, suggesting that the existing scientific dissent has been greatly exaggerated and confirming that the statements and reports of leading scientific organizations--including the U.S. National Academy of Sciences--accurately reflect the evidence presented in the scientific literature.
Pielke is quite right that understanding the results of scientific research does not implicate us in any particular course of action, and the purpose of my Essay was not to advocate either for or against the Kyoto accords or any other particular policy response. A full debate on the moral, social, political, ethical, and economic ramifications of possible responses to climate change--as well as the ramifications of inaction--would be a very good thing. But such a debate is impeded by climate-change deniers. In this respect, I am in complete agreement with Pielke's conclusion, which was precisely the point of my Essay: Proxy debates about scientific uncertainty are a distraction from the real issue, which is how best to respond to the range of likely outcomes of global warming and how to maximize our ability to learn about the world we live in so as to be able to respond efficaciously. Denying science advances neither of those goals.
Naomi Oreskes
Department of History and Science Studies Program
University of California at San Diego
La Jolla, CA 92093, USA
The author is in the Department of History and Science Studies Program, University of California at San Diego, La Jolla, CA 92093, USA. E-mail:
[email protected]