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Revisiting the satellite record

Global warming skeptics have long pointed to satellite data showing cooling in the tropical atmosphere as evidence that either climate models or measurements of surface warming, or both, are unreliable. New research suggests, however, that it is the analysis of the satellite data that was wrong.

New research is revising the temperature record of the tropical troposphere, the bottom layer of the atmosphere visible in this sunset over Hawaii, and is showing that it is consistent with the warming trend measured on Earth’s surface. Image by Commander John Bortniak, NOAA Corps (ret.).

Basic atmospheric physics maintains that the lower troposphere, the bottom layer of the atmosphere, would warm in pace with a warming at Earth’s surface. In the tropics, where hot, moist air rises and releases heat as it condenses into clouds, the warming would be amplified.

But since 1992, a team led by John Christy of the University of Alabama in Huntsville has been reporting that while surface thermometers show a warming trend, temperatures recorded by weather satellites indicate the troposphere is either not warming as fast as predicted globally or is actually cooling in the tropics. These data were supported by weather balloon, or radiosonde, data that also appeared to show a cooling trend.

Now, three independent research teams have reported errors in weather balloon data and in the way the satellite data were corrected, and they say in the Aug. 11 Science Express that the cooling trend was actually an artifact of those errors. The new studies add to the growing body of evidence indicating that the troposphere is, as predicted, warming in pace with the surface (see Geotimes, July 2004).

The three studies “bolster the case that the models are doing something right and that there are problems with both the earlier version of the satellite record and the radiosonde record,” says Kevin Trenberth, an atmospheric scientist and head of the Climate Analysis Section at the National Center for Atmospheric Research in Boulder, Colo.

The first study, by Carl Mears and Frank Wentz of Remote Sensing Systems in Santa Rosa, Calif., a company that analyzes satellite data for NASA, found a problem with the way the Alabama group corrected for satellite drift in data collected from 1979 to 2003. Over time, polar orbiting weather satellites — which scan from left to right across a swath of ground — “drift” in their orbits and begin crossing the equator at a later time each day.

“NOAA 11 started off making observations around 1:30 in the afternoon. About six years later it’s making observations around 6:00 at night,” Trenberth says. “It experiences a cooling because it’s looking at a later time of day.”

The Alabama team used temperatures taken on either side of the swath to calculate that day’s mean warming or cooling rate, which was then used to correct for the drift in observation time. However, such measurements are extremely sensitive and depend on whether the satellite was flying straight and level or was tilted, Mears says. Even a hundredth of a degree tilt can cause a significant error in the calculation, he says, and over time, can produce a spurious cooling trend.

The Alabama researchers responded to the new work in an Aug. 11 press release and agreed, for the first time, that the tropical troposphere is actually warming rather than cooling, but not as much as Mears and Wentz found: “We didn’t take into account the fact that there is a diurnal change between the readings just on each side of the swath, especially in the tropics,” they wrote.

Mears and Wentz used numerical models of the vertical structure of the atmosphere to account for the orbital drift in the same raw satellite data that Christy’s team had used. They found a warming trend of 0.193 degrees Celsius per decade globally and 0.189 degrees per decade in the tropics — 0.20 degrees higher than the Alabama team’s temperatures. “Our new satellite data agree much better with the climate models than the previous satellite data,” Mears says.

The second study, by Steven Sherwood of Yale University and colleagues, found that since the 1970s, improved balloon instruments have reduced the amount of error associated with taking temperatures in direct sunlight. The cooling trend arose, Sherwood’s team found, because larger errors in earlier years’ data made more recent temperatures seem comparatively cooler. “The paper by Sherwood et al. shows that there are severe flaws in the radiosonde record, which we have long suspected,” Trenberth says, “and they were able to find the smoking gun.”

The third study, by Benjamin Santer of Lawrence Livermore National Laboratory and colleagues, ran 50 simulations of 20th century warming using 19 different climate models, and each found an amplified warming in the tropical troposphere similar to that found by Mears and Wentz, thus supporting the new dataset’s accuracy.

The discovery of the errors “should put the [global warming] debate largely to rest, but it likely won’t,” Trenberth says. Mears agrees: “The true skeptics probably won’t be convinced, but maybe a few more policy-makers will be.”

Sara Pratt
Geotimes contributing writer

Link:
"Recalculating the warming trend," Geotimes, July 2004

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