Zeroing In on Climate Change.(Column)

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

Just last October, when the U.S. Senate debated a bill that would have curbed emissions of carbon dioxide, the most critical greenhouse gas, Republican Sen. James Inhofe of Oklahoma helped defeat the measure by declaring that "the science underlying this bill has been repudiated." A few weeks later, one of Inhofe's favorite pieces of evidence looks like it has been torpedoed.

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 U.S. 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 ...