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Fall 1998


This new feature reports developments in stories already covered in the UCAR Quarterly--by reporting experiment results, for example. This time, we follow up on "Global temperatures from space and the surface," in the Spring 1997 issue. The article is available on the Web.

Although globally averaged temperatures on the earth's surface have been steadily climbing (see p. 12), the lower-tropospheric temperature records from the satellite-based Microwave Sounding Units (MSUs) have shown a puzzling contradiction to the warming trend. The MSUs have recorded global cooling from 1979 to 1996 of 0.05 degrees C (0.09 degrees F) per decade in the lower troposphere. The reasons for this cooling have been a matter of debate among scientists; some have suggested problems with the instruments themselves, while others have believed that the satellite records offer evidence against global warming.

Now, Frank Wentz and Matthias Schabel of Remote Sensing Systems (Santa Rosa, California) have found that including the satellites' orbital decay in temperature calculations appears to explain the anomalous cooling. Their results were reported in the 13 August issue of Nature; a commentary on the subject by James Hansen and four coauthors from NASA Goddard Institute for Space Studies appeared in the 14 August Science.

During solar maximum, increases in ultraviolet radiation in the atmosphere increase the drag on satellites, pulling them down to a lower altitude. This changes the viewing angle as the MSU scans off to the sides of the nadir (straight down). The instrument makes its temperature measurement by subtracting the weighted side-scan reading from the weighted nadir temperature. Thus if it is looking into a different area with a warmer temperature, the difference between the side and nadir values decreases, creating a cooling that is the result only of the change in the satellite's position, not of any actual change in atmospheric temperature.

The MSU lower-troposphere record shows two intervals, around 1981 and 1991, with much greater cooling than the rest of the record. These periods correspond with the two most recent solar maximums, when drag on the satellites would be greatest--causing the satellite to drop about 2 km (1.2 mi) per year, compared with only 0.3 km (0.2 mi) per year during solar minimum. When Wentz and Schabel added the effect of this orbital decay to the temperature calculations, they got a correction of +0.12 degrees C (0.22 degrees F) per decade. Adding this correction to the record produced a global warming trend of 0.07 degrees C (0.13 degrees F) per decade--a number that is close to the measured surface trend.

In Nature, Wentz and Schabel note that their recalculation does not answer all questions on the subject and that more analyses should be done. They say, "We advise caution with respect to the significance placed on the absolute trend of +0.07 K per decade."

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Edited by Carol Rasmussen, carolr@ucar.edu
Prepared for the Web by Jacque Marshall
Last revised: Tue Apr 4 15:03:21 MDT 2000