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Until recently, the computer models weren't working very well. When the scientists tried to simulate what they believe has been happening over the past century or so, the results didn't mesh with reality; the models said the world should now be warmer than it actually is. The reason is that the computer models had been overlooking an important factor affecting global temperatures: aerosols, the tiny droplets of chemicals like sulfur dioxide that are produced along with CO2 when fossil fuels are burned in cars and power plants. Aerosols actually cool the planet by blocking sunlight and mask the effects of global warming. Says Tom Wigley, a climatologist at the National Center for Atmospheric Research and a member of the international panel: "We were looking for the needle in the wrong haystack."
Once the scientists factored in aerosols, their models began looking more like the real world. The improved performance of the simulations was demonstrated in 1991, when they successfully predicted temperature changes in the aftermath of the massive Mount Pinatubo eruption in the Philippines. A number of studies since have added to the scientists' confidence that they finally know what they're talking about--and can predict what may happen if greenhouse gases continue to be released into the atmosphere unchecked. Just last week, a report appeared in Nature that firmly ties an increase in the severity of U.S. rainstorms to global warming.
In general, the news is not good. Over the next century, says the IPCC report:
Sea levels could rise up to 3 ft., mostly because of melting glaciers and the expansion of water as it warms up. That could submerge vast areas of low-lying coastal land, including major river deltas, most of the beaches on the U.S. Atlantic Coast, parts of China and the island nations of the Maldives, the Seychelles and the Cook and Marshallislands. More than 100 million people would be displaced.
Winters could get warmer--which wouldn't bother most people--and warm-weather hot spells like the one that killed 500 in Chicago this past summer could become more frequent and more severe.
Rainfall could increase overall--but the increase wouldn't be uniform across the globe. Thus areas that are already prone to flooding might flood more often and more severely, and since water evaporates more easily in a warmer world, drought-prone regions and deserts could become even dryer. Hurricanes, which draw their energy from warm oceans, could become even stronger as those oceans heat up.
Temperature and rainfall patterns would shift in unpredictable ways. That might not pose a problem for agriculture, since farmers could change their crops and irrigate. Natural ecosystems that have to adapt on their own, however, could be devastated. Observes Oppenheimer dryly: "They cannot sprout legs and move to another climate." Perhaps a third of the world's forests, he says, might find themselves living in the wrong places.