A worldwide rise in temperatures at the Earth's surface is "undoubtedly real" and appears to have accelerated in recent decades, an independent scientific panel concluded in a major new report yesterday.
The panel estimated the increase in temperatures over the past century at between 0.7 and 1.4 degrees Fahrenheit--a 30 percent increase from earlier projections that reflects record-shattering high temperatures in the late 1990s.
Partly deflating a key argument used by skeptics of global warming, the report also dismissed as insignificant a glaring contradiction between two sets of measurements that have tracked temperature change over recent decades. Although land-based weather stations have shown a rise in temperatures, satellites that record temperatures in the upper atmosphere have shown little change in 20 years.
"The difference between the surface and upper-air trends in no way invalidates the conclusion that the Earth's temperature is rising," said John M. Wallace, a professor of atmospheric sciences at the University of Washington and chairman of the panel.
However, Wallace said, the disparity between the two sets of measurements points up weaknesses in scientists' ability to predict future warming. Most climate-forecasting models have predicted corresponding increases in temperature on land and in the air.
"There really is a difference between temperatures at the two levels that we don't fully understand," Wallace said in a telephone news conference on the eve of the report's official release today at the annual meeting of the American Meteorological Society in Long Beach, Calif.
The 11-member panel, which was organized by the National Academy of Sciences's National Research Council (NRC), included the two scientists responsible for tracking satellite temperature measurements. Roy Spencer, of NASA's Marshall Space Flight Center, and John R. Christy, of the University of Alabama in Huntsville, had been among a vocal minority of atmospheric scientists who argued that the Earth's warming--if it were occurring at all--was likely very mild. While concurring with the panel's conclusions, Christy said that predicting future climate trends remains fraught with peril.
"It is still not clear whether this is a representative sample that will tell us how the greenhouse effect will be played out in the future," he said.
The report did not attempt to explain the reasons for the warming. Many scientists believe the century's warming is at least partly the result of man-made pollution--higher levels of carbon dioxide and other "greenhouse" gases that trap the sun's heat like an insulating blanket. Further warming could disrupt agriculture and cause sea levels to rise, swamping coastal cities, scientists say.
The NRC panel's chief task was to attempt to reconcile the differences between the observed temperature changes on land and in the troposphere, the layer of the atmosphere that extends six to nine miles above Earth. For years, the lack of notable warming in the 20-year satellite record has provided potent ammunition to global-warming skeptics, who view the land-based temperature measurements as unreliable.
In recent months, small computing errors were detected in satellite readings that, when corrected, showed a slight warming trend in the upper atmosphere. Still, a "substantial disparity remains," the report said.
Although the difference cannot be fully explained, the NRC panel found a variety of factors that could have slowed the rate of warming high above Earth. Possible explanations include atmospheric cooling from volcanic eruptions in the 1990s and the depletion of Earth's ozone layer.
Also, the 20-year satellite record is too short to be reliable in analyzing long-term trends, the report said.
The NRC's study was immediately hailed by environmental groups that have called for the reduction of greenhouse gases to slow the warming trend.
"It totally deflates the argument of the so-called skeptics that had used the apparent difference between ground-based and satellite data to argue that we really didn't know whether the world is warming or not," said Michael Oppenheimer, an atmospheric scientist with the Environmental Defense Fund.
But Arthur Robinson, the president of and a professor of chemistry at the Oregon Institute of Science and Medicine, called the report a "political document" and evidence that the "National Academy Board has pretty much been taken over by enviros." He contended that any global warming is part of a natural trend.
"One must not lose track of the fact that the Earth's temperature has been warming now for 300 years, not just 50 years. And there was no coal or gas 300 years ago," he said.
Staff writer Rick Weiss contributed to this report.