Is there a scientific consensus on global warming? Naomi Oreskes from the Department of History and Science Studies Program, University of California at San Diego, thinks so. She published a study in the journal Science (December 3, 2004, volume 306, p. 1686) which argued that the scientific community had arrived at a consensus position on “anthropogenic climate change.” i.e. that global warming was occurring, and that “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”.

Her study looked at the scientific databases of the Institute for Scholarly Information (ISI) and searched on the keywords “climate change.” She then examined the abstracts of the 928 papers that were returned and classified them under six categories: explicit endorsement of the consensus position, evaluation of impacts, mitigation proposals, methods, paleoclimate analysis, and rejection of the consensus position.

Her results were that “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.” (my italics)

She is careful to point out that some of the authors of the minority 25% may not have agreed with the consensus view but none of those papers explicitly took such a stand. She also pointed out that scientific bodies such as the Intergovernmental Panel on Climate Change (IPCC, created in 1988 by the World Meteorological Organization and the United Nations Environmental Programme), the National Academy of Sciences (2001), The American Meteorological Society (2003) , the American Geophysical Union (2003), and the American Association for the Advancement of Science (AAAS) had all issued statements endorsing the consensus viewpoint.

This does not necessarily mean that there is complete unanimity among scientists about all aspects of this issue. Richard Lindzen, who is an MIT professor of meteorology and a member of the NAS panel on climate change that issued the report cited by Oreskes, argued in a Wall Street Journal op-ed on June 11, 2001, that as far as he was concerned, all he was agreeing with was that “(1) that global mean temperature is about 0.5 degrees Celsius higher than it was a century ago; (2) that atmospheric levels of carbon dioxide have risen over the past two centuries; and (3) that carbon dioxide is a greenhouse gas whose increase is likely to warm the earth (one of many, the most important being water vapor and clouds).” But he went on “we are not in a position to confidently attribute past climate change to carbon dioxide or to forecast what the climate will be in the future” and he argued that the case for reducing the level of carbon dioxide emissions, as called for by the Kyoto treaty in 1997, was not compelling. He argues that the process of warming we currently observe may be part of the normal cyclical variations of the Earth, and that other greenhouse gases (such as water vapor and methane) may be more important players in producing warming than carbon dioxide.

Lindzen repeated much of the same arguments in a critical review of the documentary An Inconvenient Truth, that appeared in the op-ed pages of the Wall Street Journal on June 27, 2006, where he also explicitly challenged Oreskes’ 2004 study.

In a previous post on belief preservation I wrote about the fact that there are many strategies that can be adopted to preserve one’s existing beliefs. The latest issue of Physics and Society, published by the American Physical Society (vol. 35, no.3, July 2006) illustrates this. It has a letter (p. 25) by a global warming skeptic who also argues for the “natural cycles” theory and also adds that the Earth is so big that human activity is unlikely to have an impact on it. Looking on the bright side, the author argues that some parts of the Earth are too cold now anyway, and that even if global warming should occur, we might be better off figuring out better crops that can be grown in warmer conditions, and taking steps to protect ourselves from the flooding that would ensue from the rise of ocean levels.

This raising of alternative speculative ideas against a scientific consensus is not uncommon and can confuse non-scientists into asking “Well, is there a scientific consensus or not?” This sense of confusion is encouraged by those industries (such as automobile and energy) that are the chief producers of carbon dioxide, and who oppose actions that would require them to reduce emissions. Such people know that if there is a sense of controversy over an issue, and especially if that issue has economic costs associated with it, the natural impulse of the general public is to wait until the dust settles and a clear policy emerges. So kicking up dust is a good strategy if you want nothing to be done. This is not unlike what was done by the tobacco industry concerning the adverse health effects of smoking (an effort which ultimately failed) and by intelligent design creationists concerning evolution (which is ongoing). These people take advantage of the media’s propensity to do “one the one hand, on the other hand” type stories, balancing the quotes of scientists warning of the dangers of warming with those of skeptics. This results in there being a much wider divergence in media coverage of the global warming issue than there is in the scientific community.

All these interests have used such strategies to dispute the conclusion that there is a scientific consensus that anthropogenic global warming is occurring. Oreskes addresses these arguments head-on in a recent Los Angeles Times op-ed on July 24, 2006:

[S]ome climate-change deniers insist that the observed changes might be natural, perhaps caused by variations in solar irradiance or other forces we don’t yet understand. Perhaps there are other explanations for the receding glaciers. But “perhaps” is not evidence.

The greatest scientist of all time, Isaac Newton, warned against this tendency more than three centuries ago. Writing in “Principia Mathematica” in 1687, he noted that once scientists had successfully drawn conclusions by “general induction from phenomena,” then those conclusions had to be held as “accurately or very nearly true notwithstanding any contrary hypothesis that may be imagined. . . “

Climate-change deniers can imagine all the hypotheses they like, but it will not change the facts nor “the general induction from the phenomena.”

None of this is to say that there are no uncertainties left – there are always uncertainties in any live science. Agreeing about the reality and causes of current global warming is not the same as agreeing about what will happen in the future. There is continuing debate in the scientific community over the likely rate of future change: not “whether” but “how much” and “how soon.” And this is precisely why we need to act today: because the longer we wait, the worse the problem will become, and the harder it will be to solve.

The fact that you never run out of alternative hypotheses and explanations for anything is an important point to realize. Philosopher of science Pierre Duhem addressed this way back in 1906 in his book The Aim and Structure of Physical Theory when he pointed out that you can never arrive at a correct theory by a process of eliminating all the possible alternatives because “the physicist is never sure that he has exhausted all the imaginable assumptions.”

It is easy to come up with alternative explanations for any phenomenon. That is why evidence plays such an important role in evaluating theories and scientists use published research in peer-reviewed journals as indicators of whether an idea has any merit or not. And Oreskes’ 2004 (peer reviewed) study in Science, showing that in the technical (peer-reviewed) journals a scientific consensus exists on anthropogenic climate change, has to be taken seriously. As she says in that paper:

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.

Sensible words. But if you prefer, you can always listen to George Bush’s ideas about global warming, courtesy of Will Ferrell.