From space, climate change is obvious. For more than 20 years satellite images have shown springtime greenery bursting forth earlier and earlier in that season. Thanks to global warming, the growing season is lengthening in many parts of the world. And though this may boost crop yields in some areas, it will have a host of other, less benign impacts, such as transforming the eastern Amazon from rain forest to savanna, according to the second report from the Intergovernmental Panel on Climate Change (IPCC).

Permafrost is melting, imperiling roads from Alaska to Patagonia. Mountain snowpacks thaw earlier in the year, imperiling water supplies in the southwestern U.S., China and India, among other places. And rising temperatures are causing plant and animal species to migrate in search of cooler climes, whether they live in the ocean or on land; there are now new species of plants in formerly forbidding Antarctica. "For the first time, the anthropogenic warming that Working Group I has identified has been linked to [its] impacts," says Cynthia Rosenzweig, a coordinating lead author on the Working Group II report and head of the climate impact group at the NASA Goddard Institute for Space Studies at Columbia University. "It is likely that anthropogenic warming has had a discernible influence on many physical and biological systems."

In fact, the IPCC analysis of peer-reviewed science published since the last report in 2001 shows that nearly 90 percent of such studies confirm what would be expected to change as a result of warming temperatures. From increased incidence of fires and pest outbreaks in forests worldwide to a decrease in the extent of the polar ice sheets, observations back the climate change hypothesis. "What we've seen in the past 50 to 75 years is a compelling story that not only is the climate changing, but it is changing because of human activity," says Bill Easterling, professor of geography and Earth systems science at Pennsylvania State University and another coordinating lead author. "In agriculture, we're seeing changes in the dates that farmers plant certain crops to take advantage of earlier spring warmth."

Agriculture may be the man-made system most vulnerable to climate change impacts. Increased levels of the greenhouse gas carbon dioxide (CO₂) will help fertilize fields and warmer weather will boost wheat, corn and rice yields in North America while causing them to decline precipitously in tropical regions, according to the report. Other effects are anticipated as well: increased flooding of coastal regions, a rise in malnutrition and disease, and exacerbated water shortages in already dry regions, among others. "The glaciers that [people in Pakistan] depend upon are predicted to disappear in 35 years or so. This is not that far off," says Kathleen Miller, a lead author for the report and scientist with the Institute for the Study of Society and Environment at the National Center for Atmospheric Research in Boulder, Colo. "We've got some real humanitarian crises looming on the horizon."

The true extent of the impacts are only beginning to be understood, or even researched. The majority of studies on which this report relies were conducted in North America, Europe or Australia, and there is a paucity of data from the developing world where impacts are likely to be most severe. An additional one degree Celsius of warming—all but certain due to greenhouse gases already emitted—would make water scarce for an additional 1.2 billion people in Asia, according to fellow report lead author Gary Yohe, an economist at Wesleyan University in Middletown, Conn. And, it is also clear that rising CO₂ levels contribute to increased acidity in the world's oceans, although those effects have not yet been documented, according to the report.

The authors of the report recommend adopting a "mixed portfolio" of adaptation and mitigation efforts to combat the problem. "Certainly adaptation has to be an immediate priority. The reason for that is that it is already way too late to take the climate change in the pipeline out of the pipeline," says report coordinating lead author Stephen Schneider, a climatologist at Stanford University. "There are a range of things that we can do that make sense with or without climate change. They don't climate-proof you, they reduce your susceptibility."

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