The heat is on. A model that takes into account human activity best explains Earth's increased temperature during the last century and a half.

Humans Cause Earth's Fever

It's not the sun, not natural climate fluctuations, not some bug in a computer model. Today, experts stated more firmly than ever that human activity is heating up the planet. Eschewing its earlier vagueness, the U.N.-sponsored Intergovernmental Panel on Climate Change (IPCC) flat out declared that "most of the observed warming over the last 50 years is likely to have been due to the increase in greenhouse gas concentrations."

The report bases its conclusions on a series of studies published since the last IPCC report, in 1995, which were "pretty impressive," says atmospheric scientist and U.S. IPCC delegate Daniel Albritton of the National Oceanic and Atmospheric Administration in Boulder, Colorado. For instance, climatologists have extracted millennium-long temperature records from tree rings and other sources. With this long perspective, the northern hemisphere warming of the 20th century "is likely to have been the largest of any century during the past 1000 years," the report finds, and "is unlikely to be entirely natural in origin."

The panel's report was approved Sunday at the end of a meeting of 100 participating governments in Shanghai. But it's still vague about how bad things could get in the future. In the 1995 report, researchers estimated that warming might range from 1.0°C to a hefty 3.5°C during the next century. Now, the projections range from a modest 1.4°C to a staggering 5.8°C--enough to thoroughly dry out continental interiors. The reason for the hike is that IPCC members considered scenarios in which countries drastically cut emissions of the sulfurous pollution that leads to corrosive acid rain. These emissions also form a cooling haze over large parts of the world, however; without that protective umbrella, the greenhouse world scorches.

During the last climate talks in The Hague in November (Science, 1 December 2000, p. 1663), negotiations broke down after the U.S. refused to make drastic cuts in its carbon dioxide production. Although the Bush Administration hasn't spelled out its position on global warming, many scientists believe the new report may make a difference. "It's hard to see how the new Administration couldn't take it seriously," says atmospheric physicist Michael Oppenheimer of Environmental Defense in New York City.

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Intergovernmental Panel on Climate Change

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