Scientists have also refined their understanding of other factors that could accelerate or temper climate change. At one point, researchers thought warming would cause more water to evaporate and form clouds, which cool the atmosphere. They recently discovered this was not the case. They also have begun to grasp the complex role that aerosols -- the fine particles emitted by cars, power plants and other sources -- play. Lighter-colored aerosols, such as car exhaust and power plant pollution, reflect sunlight and have a cooling effect, while darker ones, such as soot, absorb it. Both types of emissions will affect warming in the future, though scientists are still gauging their influence.
Other researchers have documented concrete indications of global warming's effects, such as these: Plants worldwide are blooming an average 5.2 days earlier per decade, according to Stanford University senior fellow Terry L. Root; and the opossum, an animal that confined its range to the South as recently as the Civil War, can now be found as far north as Ontario.
When all these indicators "line up in the same direction, what's the possibility that's all an accident?" asked Stephen H. Schneider, who co-directs Stanford University's Center for Environmental Science and Policy and advocates stricter carbon limits.
But some scientists do question the evidence. John R. Christy, an atmospheric science professor at the University of Alabama in Huntsville, said that despite a recent study suggesting the Arctic is warming much faster than the rest of the globe, the hottest years for Arctic temperatures in recorded history are 1937 and 1938, and current Greenland temperatures are not higher than they were 75 years ago. And Myron Ebell, who directs global warming and international environmental policy for the free-market Competitive Enterprise Institute, said some studies cast doubt on a U.N. pronouncement in 2001 that the 20th century was likely the warmest in a millennium.
Christy said, given the economic costs of imposing tighter controls on energy production, "The Bush administration is doing a more reasonable approach, considering that mandating carbon restrictions will have no measurable effect on what the climate will do."
Several senior administration officials said that while they agree that greenhouse gases such as carbon dioxide contribute to climate change, restricting these emissions right now would cost jobs. Instead, they said, the government should continue to focus its efforts on promoting technologies that will curb pollution. One example is FutureGen, a $1 billion, decade-long power plant project to convert coal into gas and store carbon emissions underground. Bush has also sponsored a $1.7 billion, five-year hydrogen car project aimed at eliminating carbon dioxide emissions from cars.
"The U.S. position is maybe the only rational position, to identify and promulgate application of new technologies," said White House science adviser John H. Marburger III. "To do anything meaningful [on limiting greenhouse gas emissions] requires a dramatic cessation or reduction of economic activity. It's simply not practical at the present time."
Advocates of limiting greenhouse gases, however, remain optimistic they will eventually prevail. Christine Todd Whitman, Environmental Protection Agency administrator in Bush's first term, said mandatory carbon dioxide reductions are "going to happen at some point," in part because multinational corporations will demand that U.S. policy mirror European standards.
Larry J. Schweiger, president of the National Wildlife Federation, said Bush has an opportunity to outline a new climate policy in his second term.
"If President Bush personally sits down with the scientists and hears what has happened since he first came to office, we can work together to make progress on global warming," he said. "The president has an opportunity to leave behind a strong legacy of addressing one of the biggest challenges the world has ever faced. He shouldn't squander that opportunity."