Global Warming Real, Panel Says
Despite major conflicts in temperature records, a National Research Council panel concluded Wednesday that the warming of the Earth’s surface is “undoubtedly real,” and that surface temperatures in the last two decades have risen at a rate substantially greater than the average for the last 100 years.
The 11 climate experts on the panel spent nine months investigating the troubling differences in global warming as measured at Earth’s surface and in the upper atmosphere. The discrepancies had caused some scientists to question whether a long-term warming trend was just an artifact of poor record-keeping and faulty satellite measurements.
Because global warming is a long-term process that can be masked by year-to-year changes in climate, warming trends are most clearly revealed by surface temperature measurements, which have been recorded daily at hundreds of locations for more than a century.
These data indicate that the Earth is, in fact, warming, the panel said. By the best ground measurements, Earth’s surface temperature has risen about 0.7 to 1.4 degrees Fahrenheit in the last century. But data collected by satellites and balloon-borne instruments since 1979 indicate little if any warming of the atmospheric layer extending up to about 5 miles from the Earth’s surface.
Most climate models of global warming--caused by the buildup of greenhouse gases like carbon dioxide--generally predict that temperatures should increase in the upper air as well as at the surface. Critics of the global warming theory often have cited the satellite and balloon data as evidence that the threat of climate change has not been proven.
However, after examining all available ground and atmospheric temperature records, the panel concluded that the difference in surface and atmospheric temperatures may be real, but it does not undercut the case for global warming.
“In the opinion of the panel, the warming trend . . . during the past 20 years is undoubtedly real and is substantially greater than the average rate of warming during the 20th century,” the group concluded. “The disparity between surface and upper air trends in no way invalidates the conclusion that the surface temperature has been rising.”
Panel Chairman John M. Wallace, director of the University of Washington’s environment program, emphasized that the group was not asked to address the cause of the rising temperatures or whether human influences, such as the burning of fossil fuels or greater urbanization, might be involved.
“It is a reaffirmation of the fact that the surface temperature is rising and has risen substantially in the past 20 years. We are not saying that rise is due to greenhouse gases nor are we saying it is going to continue,” Wallace said.
In addition, the panel warned that temperature records for any single 20-year period may not be representative of a long-term climate trend.
“Twenty years is a short time,” Wallace said. “This particular 20 years had two big volcanic eruptions, which affected the atmosphere, and the two biggest El Nino events on record. That may make the trends different from what we might otherwise see.”
A combination of human activities and natural causes has contributed to rising surface temperatures, while other human and natural forces actually may have cooled the upper atmosphere. Natural events such as the eruption of Mt. Pinatubo in 1991 tended to decrease atmospheric temperature for several years. And burning coal and oil for energy produces tiny aerosol particles in the atmosphere that can have a cooling effect.
Upper-air temperatures also can be reduced by depletion of ozone in the stratosphere caused by chlorofluorocarbons and other chemicals being emitted into the atmosphere.
When these variables are accounted for in atmospheric models, satellite and balloon data more closely align with surface-temperature observations, Wallace said.
Some environmentalists said the report effectively countered those who have argued that there is no global warming trend.
“It totally deflates the argument of the so-called skeptics [who] 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, president and 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 was part of a natural trend.
