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One more argument against climate change collapsesOver the past few decades, the Earth's surface has been warming rapidly. But skeptics questioned whether the lower atmosphere was warming as well. Now researchers are convinced it isby HENRY HENGEVELDFor almost two decades, John Christy, a researcher at the University of Alabama, and Roy Spencer, a NASA scientist, have claimed that, since 1979, the lower five or so kilometres of the atmosphere have warmed very little. They have argued, in fact, that over many parts of the world they have actually been cooling.Their conclusions about lower atmospheric temperatures were based on their analysis of measurements made by sensors on board a sequence of nine orbiting weather satellites. Since 1979, these sensors have continuously recorded microwave radiation released by oxygen molecules within the atmosphere. The amount of radiation released by these molecules is related to the temperature of the air surrounding them. By measuring this radiation, recorded at different angles through the atmosphere as the satellites circle the Earth, the researchers can reconstruct the global average temperature throughout the lower atmosphere -- a long-term record that Christy claims is far more representative of global climate than that obtained from the large number of scattered surface climate stations used to reconstruct surface trends. Furthermore, he argues, these satellite data do not need to be corrected for human influences, like urban heat island effects or observing methods that change over time. Researchers also have another way of measuring atmospheric temperatures. As part of a global weather observing system, twice each day weather technicians launch balloon-borne instruments (called radiosondes) from different locations around the world to measure how overhead atmospheric conditions are changing from one day to the next. The data are collected to help weather forecasters predict weather. However, when corrected for changes in observing practices and instrument design, these records can also be assimilated to provide an independent assessment of how the atmospheric temperatures have changed over months and years. Like the satellite data, experts studying the trends in the radiosonde data also reported very modest warming in recent decades, much less than that at the surface. For years, climate change sceptics have used these satellite and radiosonde results to argue that the surface data must be wrong. After all, the atmospheric data was largely free of direct human heat contamination, and two independent approaches to measuring the trends agreed remarkably well. Both appeared to argue against any significant long-term change. Furthermore, they noted, all climate models predicted that, as surface temperatures warm to rising greenhouse gas concentrations, the lower atmosphere above it should warm even more. Hence, if the surface data was accurate, then the models must be wrong. By inference, so must their predictions for future climate! These apparent contradictions in data trends sufficiently worried American scientists that, some five years ago, the U.S. National Academy of Sciences convened a special panel of experts to assess what may be causing the difference. The panel concluded that errors in measurement and analysis might explain some, but not all, of the differences. Rather, they suggested, short-term climate events such as El Ninos and volcanic eruptions, were affecting surface temperatures differently than those for air parcels higher up in the atmosphere, causing short-term differences in trends. These differences should average out over longer time scales of three decades or more. Within the past two years, however, a very different story has begun to emerge. Perplexed by these ongoing contradictions between data sets and model projections, several international research groups began simultaneously to re-examine how Christy and Spencer had calculated their temperature trends from the satellite microwave signals -- and they found some serious problems. First, the original analysis had not adequately considered how radiation entering the troposphere from the stratosphere above -- which was cooling because of ozone depletion -- might have contaminated the signal. Second, errors caused by the gradual decay of the orbits of each satellite used had not been subtracted. More importantly, the original researchers had not properly removed errors caused by changes in orbit characteristics as one satellite was retired and another took over. As successive groups began to slowly address these errors, the difference between corrected satellite temperature trends for the lower atmosphere and that for the surface began to rapidly decrease. The most recent study, published by Mears and Wentz in the journal Science this past August, showed that the fully-corrected trend now indicates a warming trend of almost two-tenths of a degree Celsius per decade, remarkably similar to that of surface temperature trends. Until this summer, none of these studies had as yet addressed the other discrepancy -- that between surface data and the radiosonde records. However, in the same issue of Science in which the paper by Mears and Wentz appeared, researchers from Yale and Princeton Universities noted that radiosonde sensors used in the balloon flights were sensitive to exposure to the sun's heat during the ascent of the balloons, a factor that changed over time. While they also acknowledged that there were other problems with the quality of the radiosonde data, corrections for this one factor alone were enough to bring the radiosonde data in line with that for the surface. Although Christy has argued back that he thinks that some of the corrections for the satellite records have been exaggerated, the balance of evidence suggests otherwise.
One more little puzzle solved -- and one more plank that collapses in the shaky platform of climate change sceptics! BF Henry Hengeveld is Emeritus Associate, Science Assessment and Integration Branch/ACSD/MSC, Environment Canada
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