SOME YEARS back, MIT professor William Thilly spoke in Woburn about a test he hoped to develop that would show definitively whether a person’s DNA had been altered by exposure to industrial pollutants. When he was asked about the possibility of introducing such test results in court (eight families had just come out on the short end of an out-of-court settlement following the trial that was later made famous in A Civil Action), Thilly replied that he would hope to stay off the witness stand. "For every PhD," he explained, "there is an equal and opposite anti-PhD."

The audience laughed, but his point — that a scientific expert can be found who’ll testify to just about anything — was a valid one. And it is directly relevant to the science of global warming.

The theory behind global warming is fairly well known. According to a recent report by the National Academy of Sciences, we have been pumping ever-increasing amounts of greenhouse gases (principally carbon dioxide) into the atmosphere since the beginning of the Industrial Revolution. These gases, which trap heat in the atmosphere rather than letting it escape into space, caused a one-degree temperature rise during the 20th century, which manifested itself through such evidence as retreating glaciers, thinning Arctic ice, rising sea levels, lengthening growing seasons in some areas, and the earlier arrival of migratory birds. What’s more, the worst is yet to come: over the next century, according to the report, computer models show that the average global surface temperature will rise by 2.5 to 10.4 degrees.

For Bush, the report was particularly painful: he had ordered it rather than accept an international report prepared by the Intergovernmental Panel on Climate Change (IPCC). Instead of contradicting the IPCC report and giving Bush a way out, the National Academy essentially ratified it, putting the president on the spot.

But even though many scientists say that consensus on global warming is strong and getting stronger, critics remain. Some of those critics say the National Academy findings themselves support their doubts, claiming that a government-drafted press release used by most journalists papers over the uncertainties contained in the report. And it is a fact that one of the 11 "top climate scientists" who contributed to the report is MIT professor Richard Lindzen, who has outspokenly criticized the view that human-caused global warming is a serious problem.

Global-warming critics cite a varied range of arguments. One well-known naysayer, Sallie Baliunas, of the Harvard-Smithsonian Center for Astrophysics, attributes rising temperatures to changes in the magnetic output of the sun. She and others note that there have been times in the past when temperatures were much warmer than they are today — such as the period between the years 800 and 1200, when Greenland, or parts of it anyway, were green. The inadequacies of computer modeling are discussed in loving detail. Some skeptics contend that temperatures worldwide actually went down between 1945 and ’70, when CO2 emissions were roaring along. There’s also an argument that because temperature measurements taken by satellites show no increase in warming, those taken at the surface should be discounted, since they can be affected by such localized factors as urbanization.

Each one of these arguments brings a counterargument, which in turn brings a counter-counterargument. The point is that there are two sides to the debate. And though the consensus opinion clearly lies with those scientists who believe global warming is real, substantially human-made, and dangerous as all hell, the opposition includes some well-credentialed people.

"The firm belief I have is that the science is a long way from settled. It is speculative on both sides of the issue," says Peter Leavitt, a certified consulting meteorologist based in Newton and the former CEO of Weather Services Corporation. "The whole global-warming issue is being managed by both its proponents and opponents the way a political campaign is being managed." The "obvious logical argument," Leavitt adds, is to continue studying the issue, and to hold off on drastic action until more is known.

AS LEAVITT suggests, the debate over global warming looks, at times, like a political campaign — and an unusually down-and-dirty one at that. Take the case of S. Fred Singer and Ross Gelbspan. Singer is the president of the Science and Environmental Policy Project (SEPP) and a leading global-warming skeptic. Gelbspan is the author of The Heat Is On: The High Stakes Battle over Earth’s Threatened Climate (Addison-Wesley, 1997).

Gelbspan’s book is not just about global warming, but also about what he calls the "disinformation" campaign conducted by the global-warming skeptics, who, he charges, receive much of their funding from industry groups. "The science has become so robust that these guys are laughingstocks in the scientific community, but they really do have a strong foothold in the White House," says Gelbspan, who’s based in Brookline. "Singer," he adds, "is the most reckless of these skeptics." Gelbspan cites ExxonMobil’s contributions to SEPP, which are openly listed on the oil company’s Web site. Gelbspan notes that Singer, in a letter to the Washington Post earlier this year, denied having received any oil-industry money in nearly two decades.