Consensus reached on climate change causes.

While the politics of global warming inspire division and argument among nations, the world's scientists have reached a consensus on what causes climate change. Hundreds of top researchers from more than 80 countries agreed last week that carbon dioxide and other greenhouse gas pollutants represent the most important forces currently altering the climate, drowning out other effects that can cool the globe.

To produce the authoritative document, the Intergovernmental Panel on Climate Change (IPCC) enlisted more than 140 scientists to write the report and then sent the work to 230 reviewers. "We involved virtually all the scientists in the world who have done serious research in this area," British climate researcher John Houghton, who cochaired the science working group that produced the assessment, told SCIENCE NEWS.

The IPCC report notes that scientists still face many uncertainties regarding climate change. These outstanding questions, however, have not contradicted the basic conclusions that the group reached in its first major report in 1990. At the time, the IPCC stated with confidence that greenhouse gas emissions would warm the climate. It predicted that without any limits on these emissions, the climate would warm by roughly 1[degrees]C by 2025 and 3[degrees]C by the year 2100.

"It is interesting that in this very uncertain area, over a period of 5 years, the essential story remains the same. There's been no evidence that's come to light to destroy those basic findings," Houghton says.

Since the initial report, some researchers have explored whether changes in solar radiation or emissions of light-blocking sulfur pollution could offset warming caused by greenhouse gases.

The new study concludes that solar variations and sulfur pollution are too weak to counter the warming influence of carbon dioxide, methane, chlorofluorocarbons, nitrous oxide, and other heat-trapping gases. Although sulfur droplets and smoke from fires do reflect sunlight back toward space, such pollutants do not exert a uniform cooling effect because they have a spotty distribution around the globe.

The report also offered new information concerning efforts to curb emissions of greenhouse gases. In 1992, 155 nations signed a treaty in Rio de Janeiro pledging to stabilize atmospheric concentrations of greenhouse gases at an unspecified level. Toward that goal, developed nations agreed in a nonbinding way to scale back their emissions to 1990 amounts by the year 2000. The treaty does not say whether countries must cap their emissions after that time.

The new IPCC assessment concludes that the limits discussed in the Rio treaty will not stop the atmospheric accumulation of greenhouse gases. To stabilize concentrations at today's amounts or even twice those, nations will need to decrease their emissions to well below 1990 levels, Houghton says.

International negotiators are currently considering whether to set binding emissions limits and whether to require further cuts beyond the year 2000. Countries that signed the Rio treaty will take up those contentious issues next March in Berlin. But the deadline for draft treaties closes in a week. If that date passes without any submissions, no strict limits on gas emissions will emerge from Berlin.