When Satellites Mislead: Scientists Prescribe Caution
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Temperature-gleaning satellites are useful tools in the quest to diagnose global change, but only when their limitations are well understood. This is the message conveyed by scientists from the National Center for Atmospheric Research (NCAR) in Boulder, Colorado, in an article appearing in the journal Nature on March 13. NCAR is managed by the University Corporation for Atmospheric Research under sponsorship of the National Science Foundation.
The article provides new findings on an ongoing controversy involving the reliability of global temperature trends obtained via satellite.
NCAR's James Hurrell and Kevin Trenberth analyze a puzzling discrepancy between global temperature trends ascertained by surface instruments versus satellites in an article "Spurious trends in satellite microwave sounder units (MSU) temperatures from merging different satellite records."
The difference in trends has been a subject of spirited debate because of its implications for the projection and measurement of global warming. Since 1979, microwave sounder units (MSUs) have been deployed aboard polar-orbiting satellites. MSUs measure the brightness of oxygen in the earth's atmosphere and thus infer the temperature across the globe at various heights.
In their Nature article, Hurrell and Trenberth argue that the MSU data, while useful for many purposes, are poorly suited for gauging long-term surface temperature trends. MSUs monitor the globe more thoroughly than surface reports, which are concentrated over land and approximated over oceans. However, each MSU lasts only a few years, to be replaced by another deployment on a different satellite. According to the NCAR scientists, the transitions between satellites may be producing spurious temperature drops that mask an actual rise in global readings. "The surface and MSU records measure different physical quantities," write Hurrell and Trenberth, "so that decadal trends should not be expected to be the same." However, they add, "unreconciled discrepancies among the different records remain."
To study the matter further, the scientists focused on the tropics between 20 degrees N and S, where "noise" from short-term weather variations is lower than it is in temperate and polar zones. Hurrell and Trenberth compared simultaneous MSU records to each other, to sea-surface temperatures (SSTs), and to air temperatures simulated by an NCAR climate model using SSTs. They found that most of the difference between MSU and surface trends could be explained by two significant drops in MSU data for 1981 and 1991, years when satellite transitions took place.
Some analyses now cite only the MSU or only the surface data in reporting on global temperature trends, without noting the counterpart to each. Hurrell and Trenberth stress that both data sets are needed to unravel the mysteries of global climate. "The MSU data are excellent for analyzing year-to-year changes, but not necessarily for longer-term trends," says Hurrell.
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