An op-ed article in The Wall Street Journal a month ago claimed that a published study affirming 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 can tell you 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.

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.

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 Association 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 adviser, 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 Massachusetts Institute of Technology professor Richard Lindzen, the author of the Wall Street Journal article, who disagree with the rest of the scientific community.

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.

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 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. 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.

Climate-change deniers can imagine all the hypotheses they like; it will not change the facts.

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 debate in the scientific community over the likely rate of future change: not "whether" but "how much" and "how soon." And this is 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 the University of California, San Diego. She wrote this commentary for the Los Angeles Times.