Global Climate Monitoring: The Accuracy of Satellite Data

March 12, 1997

Recently, much scientific debate has focused on the global temperature of the Earth's lower atmosphere as measured by orbiting satellites. And while these data are exceedingly precise, verified by multiple satellite observations, and balloon measurements taken in-situ, they reveal no discernable warming trend in the Earth's lower atmosphere over the last 18+ years.

Dr. Roy W. Spencer (NASA Marshall Space Flight Center) and Dr. John Christy (The University of Alabama in Huntsville) have used the Microwave Sounding Units (MSUs) flying aboard NOAA's TIROS-N weather satellites to construct a continuous record of lower tropospheric (from the surface to about 4 miles) temperatures since the first MSU was launched in late 1978. The lower tropospheric temperature trend has been calculated to be -0.04 degrees C/decade.

In the latest (March 13, 1997) edition of Nature, two scientists, James Hurrell and Kevin Trenberth, report that sea-surface temperatures monitored by buoys and ships at various locations in the tropics show, for the same period as the satellite record, a warming trend of +0.12 deg. C/decade, in apparent disagreement with the satellites. This so-called "disagreement" between satellite and surface temperature measurements> is not new.

Despite the fact that Hurrell and Trenberth estimate of the temperature of the atmosphere through a simple linear regression model based only on the sea surface temperatures, and a global climate model simulation with the same sea surface temperatures but no stratospheric volcanic aerosols, while the MSU data actually measure the temperature of the free atmosphere, Hurrell and Trenberth conclude that the satellite data must be wrong.

The recent paper's conclusion is based on two apparent "breaks" in the satellite versus sea-water temperature record, one in late 1981 and the other in late 1991. "During the first period, we had two separate satellites, operating simultaneously, and agreeing with each other to about 0.02 degrees C. So their estimate of the late 1981 break is inconsistent with these observations," observed Dr. Spencer, an atmospheric scientist at NASA.

"There isn't a problem with the measurements that we can find," Spencer explained. "In fact, balloon measurements of the temperature in the same regions of the atmosphere we measure from space are in excellent agreement with the satellite results." Dr. Christy explained further, "In particular, we've examined these two `breaks' claimed by Hurrell and Trenberth. Even in these disputed intervals, we find excellent agreement between the two independent, direct atmospheric temperature measurements from balloons and satellites."

The disagreement between satellites and surface-based thermometers, furthermore, is not geographically uniform. "Over Northern Hemisphere land areas, where the best surface thermometer data exist, the satellites and thermometers agree almost perfectly", said Dr. Christy of UAH. "It is primarily over the oceans where they disagree by a couple of tenths of a degree C. This is most likely a well-known phenomenon in which the temperature in the deep atmosphere is not as strongly linked to the surface temperature as it is over land."

While Hurrell and Trenberth attempt to account for possible differences between the surface and deep-layer measurements by forcing a computerized atmospheric general circulation model (GCM) with the observed sea-water temperatures, this methodology is likely flawed. "It is well known that GCMs will produce atmospheric temperatures that vary in lock-step with the surface temperatures. In addition, the GCM did not include the direct forcing on the atmosphere from this century's two largest volcanoes," noted Spencer. "The physics in these models is not refined enough to do anything else. The satellite measurements provide the first observational evidence that the surface and deep layer temperatures can vary slightly differently (a couple of tenths of a degree) over a decade or so."Spencer and Christy point out that the surface versus satellite temperature controversy will likely not die away soon. Through NASA's Earth Observing System, researchers will continue to improve our ability to monitor the Earth system so that we may understand the subtleties of variations in the global atmosphere as noted in the current discussion. It is only with direct observations of the earth that we will be able to sort out the issues of climate variability and change that affect the planet.

NASA/Marshall Space Flight Center News Center

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