Thermometers on the ground, measuring the near-surface air temperature, demonstrate a marked increase in globally-averaged temperature over the past two decades. Computer models of global warming predict that the temperature trend in the Earth's thick lower atmosphere, called the lower troposphere, should be experiencing an even more pronounced warming that increases smoothly with altitude. And yet, satellite observations of the temperature of the Earth's lower troposphere do not reveal any overall warming trend. Although interpreted by some as a controversy, research from NASA/Marshall Space Flight Center and the Global Hydrology and Climate Center now suggests that the temperature structure of the atmosphere is more complex than we (and our computer models) originally thought.
These results will be presented today (February 6) at the 77th meeting of the American Metorological Society in Long Beach, California in a special session dedicated to the scientific study of global warming.
Dr. Roy Spencer, a scientist at NASA/Marshall and principal author on the paper, has been monitoring the temperature of layers in the Earth's atmosphere from space. Along with Dr. John Christy of the University of Alabama in Huntsville, Spencer has produced a temperature record spanning 18 years. Acquired from Microwave Sounding Unit (MSU) instruments flying aboard the TIROS series of weather satellites. Their data show temperature variations in the lower troposphere, a region from the surface to about 5 miles into the atmosphere.
"The temperatures we measure from space are actually on a very slight downward trend since 1979 in the lower troposphere. We see major excursions due to volcanic eruptions like Pinatubo, and ocean current phenomena like El Nino, but overall the trend is about 0.05 degrees Celsius per decade cooling," Spencer remarked.
However, temperature measurements on land and ocean are up. "Thermometers taking the temperature at the surface show a warming trend of about +0.10 to +0.15 degrees Celsius per decade," Spencer continued. "Current computer models of global warming always predict that the temperature variations at the surface should increase smoothly with height as you go up through the lowest 8 miles of the atmosphere." This should make the temperature trend in the troposphere not only upward, but more pronounced than on the surface.
But the space-based measurements show a more complex vertical structure, with cooling in the lower portion of this deep layer and warming in the upper portion. Spencer and co-author Dr. William Braswell of Nichols Research Corporation have great confidence in the quality of their satellite data. "We've concluded there isn't a problem with the measurements," 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."
"Instead, we believe the problem resides in the computer models and in our past assumptions that the atmosphere is so well behaved. These models just don't handle processes like clouds, water vapor, and precipitation systems well enough to accurately predict how strong global warming will be, or how it will manifest itself at different heights in the atmosphere," remarked Spencer.
These poorly modeled processes are all related to convection. This is the continual overturning of the atmosphere that occurs as water, evaporated from the Earth's surface, carries excess heat energy into the upper atmosphere where it can be more efficiently radiated to outer space. This convective redistribution, the scientists theorize, may be part of what causes the interesting height-dependent structure in the temperature variations seen in the MSU data. Spencer says that the models also suffer from "numerical diffusion," wherein water vapor in the lower atmosphere is allowed to unrealistically diffuse into the upper atmosphere, where it acts as a greenhouse blanket. "All of these effects together make the computer-modeled atmosphere look much more vertically uniform than it probably is," Spencer concluded.