Among its arguments, the EPA will probably say that the science on global warming is too uncertain to be the basis of expensive regulations. This threadbare argument has elicited two 'amicus curiae' briefs — documents filed with the court by someone not party to the case. One was from a group of scientists and one from four former EPA administrators. Both say there is a strong scientific consensus, and point out that nothing in science is ever completely certain.

The scientists' brief was filed by 18 researchers, including James Hansen, head of the NASA Goddard Institute for Space Studies in New York, and Nobel prizewinner Sherwood Rowland of the University of California, Irvine. They accuse the EPA of misrepresenting their findings, adding: "The evidence of these changes...has crystallized a remarkable consensus within the scientific community: climate warming is happening, and human activities are very likely a significant causal factor."

Even with a certain level of uncertainty, the precautionary principle should come out on top when the potential damages caused by inaction are so huge.

Another EPA argument, that greenhouse gases are not pollutants, has met with derision from environmentalists. "Carbon dioxide is clearly a pollutant — it is the dose that makes the poison," says Andrew Aulisi of the World Resources Institute, an environmental think-tank in Washington DC. "These are political appointees at the top of the agency taking their cues from the White House."

But lets not get too carried away; even if Massachusetts wins, the EPA will not start to regulate CO2 emissions from cars immediately: "Agency scientists would merely have to determine formally whether greenhouse gases are likely to endanger public health. If they decide this is the case, the agency may begin hammering out regulations. But all this would take years to unfold." ::Climate in court (paid registration required), ::U.S. Supreme Court to Weigh in on Global Warming