Zeroing in on climate change

Although temperature measurements taken on the ground have shown a clear warming trend at least back to the 1950s, important satellite data have until now shown no evidence of warming whatsoever. That's a discrepancy big enough to drive an SUV through. In last week's issue of the Journal of Climate, however, scientists have essentially resolved it. In a thorough analysis of the data going back 24 years, they've found that the atmosphere has in fact been warming about 0.1 degrees Celsius per decade. That data has become another piece of evidence confirming a rise in the temperature in the past few decades.

The study concentrated on a set of NASA satellites that beam back temperature measurements of Earth's troposphere -- the lower atmosphere up to about 15 kilometers. If there had been any climate change at all, it should have shown up in that part of the atmosphere near the ground, but studies in the 1990s failed to find any. There were plenty of reasons why: satellites can be finicky and prone to subtle errors. Scientists have to compensate for factors that can skew the data, like changes in altitude over time. A team of scientists from Remote Sensing Systems in California took a more thorough look at the data, fitting together data from two series of readings that overlapped in the 1980s. They also worked with US government scientists to compare their new analysis with computer programs that re-create the past century of climate change, absent the action of carbon dioxide emissions. By the end of the 20th century, they concluded, the patterns of temperatures had drifted significantly higher than natural variability -- a fingerprint, as it were, left by manmade global warming.

When it comes to predicting how much temperatures will rise this century, there's still plenty of uncertainty to go around. Back in 2001, the Intergovernmental Panel on Climate Change concluded that the temperatures might rise anywhere from 1.5 to 5.8 degrees Celsius -- a huge range. Recently scientists' predictions have begun to converge on a narrower range, and the forecasts have gotten more modest. James Hansen of the NASA Goddard Institute for Space Studies in New York has pointed out that in recent years the actual rise of greenhouse gases hasn't accelerated as fast as the IPCC predicted. Carbon-dioxide emissions increased 4.7 percent a year from 1945 to 1973, but since then, the average increase has been only 1.4 percent a year. The rate for methane, another powerful greenhouse gas produced in landfills and rice farming, is barely increasing at all. Hansen thinks that even if nothing is done, the planet would warm only 1.5 degrees by 2050.

Can we handle even this modest increase? Studies of ancient climates, as revealed in ice-core samples taken from glaciers, suggest that even a modest rise would make the global average temperature as high as it's been in the past 150,000 years. There's a risk that ice shelves will begin to melt, raising sea levels and flooding coastlines; earlier this month, Australian scientists reported that the sea ice around Antarctica has already shrunk by 20 percent since 1950.

There's always a chance that scientists have left something out that will confound their predictions. At a climate meeting in Berlin this spring, Nobelist Paul Crutzen and colleagues raised serious concerns that pollution is masking the full impact of greenhouse-gas emissions. Aerosols -- haze, smog and other fine particles -- can reflect sunlight back into space, creating a cooling shield over the atmosphere. Even poor countries are working hard to cut down on air pollution for the health of their citizens, even as carbon levels rise.

If they succeed in making the air cleaner, temperatures may soar -- perhaps by as much as seven to 10 degrees Celsius. This uncertainty makes it hard to prepare for the future. If you're trying, for instance, to protect a nation's supply of drinking water, what you want are probabilities -- to be able to say, for example, that there is only a 5 percent chance that the planet will heat more than four degrees. These sorts of percentages would require fine-tuning of complex computer climate models -- which in turn requires far more processing power than any research center currently has. An English team of meteorologists has set up a program to find that power among the thousands of idle PCs on the Internet. Since September, they've enlisted more than 18,000 computers to help crunch climate data (www.climatep-rediction.net). If global warming is going to affect us all, we might as well pitch in to figure out what the future holds.

Carl Zimmer is author of the book "Soul Made Flesh," which is due out in January.

(c) 2003, Newsweek Inc. All rights reserved. Reprinted by permission.