An Op-Ed article in the Wall Street Journal a month ago claimed that a published study affirming the existence of a scientific consensus on the reality of global warming had been refuted. This charge was repeated again last week, in a hearing of the House Committee on Energy and Commerce.

I am the author of that study, which appeared two years ago in the journal Science, and I’m here to tell you that the consensus stands. The argument put forward in the Wall Street Journal was based on an Internet posting; it has not appeared in a peer-reviewed journal — the normal way to challenge an academic finding. (The Wall Street Journal didn’t even get my name right!)

My study demonstrated that there is no significant disagreement within the scientific community that the Earth is warming and that human activities are the principal cause.

Papers that continue to rehash arguments that have already been addressed and questions that have already been answered will, of course, be rejected by scientific journals, and this explains my findings. Not a single paper in a large sample of peer-reviewed scientific journals between 1993 and 2003 refuted the consensus position, summarized by the National Academy of Sciences, that “most of the observed warming of the last 50 years is likely to have been due to the increase in greenhouse gas concentrations.”

Since the 1950s, scientists have understood that greenhouse gases produced by burning fossil fuels could have serious effects on Earth’s climate. When the 1980s proved to be the hottest decade on record, and as predictions of climate models started to come true, scientists increasingly saw global warming as cause for concern.

In 1988, the World Meteorological Assn. and the United Nations Environment Program joined forces to create the Intergovernmental Panel on Climate Change to evaluate the state of climate science as a basis for informed policy action. The panel has issued three assessments (1990, 1995, 2001), representing the combined expertise of 2,000 scientists from more than 100 countries, and a fourth report is due out shortly. Its conclusions — global warming is occurring, humans have a major role in it — have been ratified by scientists around the world in published scientific papers, in statements issued by professional scientific societies, and in reports of the National Academy of Sciences, the British Royal Society and many other national and royal academies of science worldwide. Even the Bush administration accepts the fundamental findings. As President Bush’s science advisor, John Marburger III, said last year in a speech: “The climate is changing; the Earth is warming.”

To be sure, there are a handful of scientists, including MIT professor Richard Lindzen, the author of the Wall Street Journal editorial, who disagree with the rest of the scientific community. To a historian of science like me, this is not surprising. In any scientific community, there are always some individuals who simply refuse to accept new ideas and evidence. This is especially true when the new evidence strikes at their core beliefs and values.

Earth scientists long believed that humans were insignificant in comparison with the vastness of geological time and the power of geophysical forces. For this reason, many were reluctant to accept that humans had become a force of nature, and it took decades for the present understanding to be achieved. Those few who refuse to accept it are not ignorant, but they are stubborn. They are not unintelligent, but they are stuck on details that cloud the larger issue. Scientific communities include tortoises and hares, mavericks and mules.

A historical example will help to make the point. In the 1920s, the distinguished Cambridge geophysicist Harold Jeffreys rejected the idea of continental drift on the grounds of physical impossibility. In the 1950s, geologists and geophysicists began to accumulate overwhelming evidence of the reality of continental motion, even though the physics of it was poorly understood. By the late 1960s, the theory of plate tectonics was on the road to near-universal acceptance.

Yet Jeffreys, by then Sir Harold, stubbornly refused to accept the new evidence, repeating his old arguments about the impossibility of the thing. He was a great man, but he had become a scientific mule. For a while, journals continued to publish Jeffreys’ arguments, but after a while he had nothing new to say. He died denying plate tectonics. The scientific debate was over.

So it is with climate change today. As American geologist Harry Hess said in the 1960s about plate tectonics, one can quibble about the details, but the overall picture is clear.

Yet some 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.

Naomi Oreskes is a history of science professor at UC San Diego.

© 2006 Los Angeles Times