But a team led by a University of Washington atmospheric scientist has used satellite data in a new and more accurate way to show that, for more than two decades, the troposphere has actually been warming faster than the surface. The new approach relies on information that better separates readings of the troposphere from those of another atmospheric layer above, which have disguised the true troposphere temperature trend.
"This tells us very clearly what the lower atmosphere temperature trend is, and the trend is very similar to what is happening at the surface," said Qiang Fu, a UW associate professor of atmospheric sciences.
He is lead author of a paper documenting the work published in the May 6 edition of the journal Nature. Co-authors are Celeste Johanson, a UW research assistant and graduate student in atmospheric sciences; Stephen Warren, a UW professor of atmospheric sciences and Earth and space sciences; and Dian Seidel, a research meteorologist with the National Oceanic and Atmospheric Administration's Air Resources Laboratory in Silver Spring, Md.
The team examined measurements from devices called microwave-sounding units on NOAA satellites from January 1979 through December 2001. The satellites all used similar equipment and techniques to measure microwave radiation emitted by oxygen in the atmosphere and determine its temperature.
Different channels of the microwave-sounding units measured radiation emitted at different frequencies, thus providing data for different layers of the atmosphere. In the case of the troposphere - which extends from the surface to an altitude of about 7.5 miles - it was believed there was less warming than what had been recorded at the surface.
The troposphere temperature was measured by channel 2 on the microwave sounding units, but those readings were imprecise because about one-fifth of the signal actually came from a higher atmospheric layer called the stratosphere.
"Because of ozone depletion and the increase of greenhouse gases, the stratosphere is cooling about five times faster than the troposphere is warming, so the channel 2 measurement by itself provided us with little information on the temperature trend in the lower atmosphere," Fu said.
Stratosphere temperatures are measured by channel 4 on the microwave units. Fu's team used data from weather balloons at various altitudes to develop a method in which the two satellite channels could be employed to deduce the average temperature in the troposphere. The scientists correlated the troposphere temperature data from balloons with the simulated radiation in the two satellite channels to determine which part of the channel 2 measurement had come from the cooling stratosphere and should be removed.
What remained indicated that the troposphere has been warming at about two-tenths of a degree Celsius per decade, or nearly one-third of a degree Fahrenheit per decade. That closely resembles measurements of warming at the surface, something climate models have suggested would result if the warmer surface temperatures are the result of greenhouse gases. The previous lack of demonstrable warming in the troposphere has prompted some to argue that climate models are missing unrecognized but important physical processes, or even that human-caused climate change is not happening.
One reason previous data have not shown enough warming in the troposphere, Fu said, is because the stratosphere influence on the channel 2 temperature trend has never been properly quantified, even though there have been attempts to account for its influence. Those attempts had large uncertainties, so many researchers had simply used the unadjusted channel 2 temperature trends to represent the temperature trends in the middle of the troposphere.
Fu's work is supported by the U.S. Department of Energy, the National Science Foundation and the National Aeronautics and Space Administration.
The findings, he said, could offer a new context for climate change discussion.
"I think everyone can understand our approach," he said. "I think this could convince not just scientists but the public as well."
For more information, contact Fu at 206-685-2070 or firstname.lastname@example.org