New find points to global warming

By Lidia Wasowicz UPI Science Writer Copyright 1998 United Press International August 12, 1998

A study indicates "kinks" in the orbit of a satellite, rather than Mother Nature, are responsible for a troubling record of cooling in the lowest layer of Earth's atmosphere that had scientists scratching their heads and greenhouse skeptics rubbing their hands. The previous apparently erroneous measurements were difficult to reconcile with the rising temperatures on the planet's surface. At the same time, they provided fodder for those casting suspicion on global warming. "The significance of the findings is that the corrected air temperature record now agrees with other data sets and, thereby, strengthens the case for global warming," lead study author Frank Wentz of Remote Sensing Systems in Santa Rosa, Calif., tells UPI. "A record of cooling in the lower troposphere seems to have more to do with kinks in the orbit of the satellite making the measurements than any real climatic effect," Wentz and his colleague Matthias Schabel write in the British journal Nature. The 17-year lower-tropospheric temperature record derived from the satellite Microwave Sounding Unit had shown a global cooling trend, from 1979 to 1995, of 0.05 kelvin (0.09 degrees Fahrenheit) per decade. In contrast, air temperatures measured at Earth's surface have risen by approximately 0.13 K per decade over the same period. Even though the data sets are not directly comparable, and the warming trend seen at the surface is expected to diminish with altitude, some have regarded the cooling trend in the lower troposphere as suspiciously excessive, the authors note. In the meantime, the discrepancy has sparked a heated debate in the climate community about possible instrumental problems -- and even the existence of global warming. Wentz and Schabel offer a solution by revealing a fatal flaw in the way data on atmospheric temperatures from polar-orbiting weather satellites have been analyzed: the failure to take into account how atmospheric drag causes the satellite to drop in altitude over time. Once the correction for this drop is included in the calculations, it turns out the resulting tropospheric temperature chart meshes quite nicely with ground-based data, weather balloon records, information from other satellites and climate models, which show warming at the planet's surface as well as through the lower troposphere, the lowest layer of the atmosphere. "Our corrections add 0.12 K (0.22F) a decade, so the new corrected trend is now a warming -- not a cooling -- of 0.07 K (0.13 F) per decade," Wentz tells UPI. "A change of 0.12 K a decade is very significant." Significant not just for scientists, but for the world at large. Last December, leaders from around the globe met in Kyoto, Japan, to discuss what measures should be taken in view of mounting evidence that a titanic shift in Earth's climate is under way. While most scientists agree, after years of debate, that humans and their addiction to fossil fuels are at least partly to blame, debate over what to do about it has been sprinkled with skepticism about the seriousness of the situation. "The Kyoto treaty has major political and policy implications," Wentz says. "The supposed tropospheric cooling derived from satellites -- before realizing the orbit decay correction was necessary -- was the strongest argument of the greenhouse skeptics. Now that we understand the orbit decay correction, there is an even stronger consensus that global warming is indeed a reality." "Our paper clearly shows this cooling is spurious, an artifact of orbit decay," Wentz says. "In this sense, the paper resolved a major question in the debate on global warming and reconciled the satellite temperature with other satellite data, in situ, and climate models." If the corrected profile is confirmed, scientists will likely rely even more on such satellite data for precise analyses of how such factors as volcanic eruptions and greenhouse gases force change in global climate, James Hansen of the National Aeronautics and Space Administration's Goddard Institute for Space Studies in New York writes in Thursday's issue of Science. "There were many climatologists who were puzzled and confused over the apparent disagreement of the satellite temperatures versus other climate data sets," Wentz says. "Our paper was very well received by the climate community, which had been looking for a reconciliation of this discrepancy." "It's important that this effect has been brought to light," Dian Gaffen of the National Oceanic and Atmospheric Administration's Air Resources Laboratory in Silver Spring, Md., told UPI in an interview. "This important revelation resolves the discrepancies for the time being -- but I'm not sure it's end of the matter," she said. "There'll be further adjustments to the data sets, and possibly some might question these conclusions." She cautioned the findings entail no more than two decades worth of data, since the weather satellites were deployed only in 1979. "This is only short-term data so it's important to keep it in perspective. We have other data sets going back much farther in time, and in figuring out how climate is behaving, the longer the time frame of the data you have, the more confident you can be in its accuracy."

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