Contrary Thermometers

Scientists are working to understand why the lower atmosphere isn't heating up as fast as some global warming models predict.

How could the globe be warming and not warming at the same time?

That's the riddle posed to climatologists by satellite and radiosonde (balloon-borne instrument for making atmospheric measurements) data which show that while the Earth's surface has been warming over the past decades, the lowest layer of the atmosphere shows a weaker warming trend.

The measurements are surprising, because computer simulations of the world's climate predict that the two lowest layers of the atmosphere -- which together form the "troposphere" -- should be warming faster than the Earth's surface.

"I think it points out that the atmosphere is more complex than the computer models currently simulate," says Dr. Roy Spencer, senior scientist for climate studies at the Global Hydrology and Climate Centre (GHCC) at NASA's Marshall Space Flight Centre. "However, it does not by itself substantially alter the expectation that some amount of global warming will occur in the future."

Spencer and Dr. John Christy, a professor of atmospheric science at the University of Alabama in Huntsville, are trying to account for the unexpected temperature patterns. By explaining the contrary behaviour of atmospheric and surface-level temperature trends, they hope to improve computer models used to simulate the world's climate. This would provide a better picture of how severe or mild global warming will be over the next century.

"I believe the data bolster the traditional scientific scepticism one must have when discussing predictions of the future," Christy said.

A recent National Research Council report states: "For the time period from 1979-1998, it is estimated that on average, over the globe, surface temperature has increased by 0.25 to 0.4 degrees C and lower to mid-tropospheric temperature has increased by 0.0 to 0.2 degrees C." These are stated as ranges because of measurement uncertainties in each. Current climate models predict that the layer of the atmosphere called the "lower troposphere" -- which extends from the surface to an altitude of about 5 miles -- would be warming at a slightly faster rate than the surface.

Global Warming Ranges

But satellite measurements of temperatures in the lower troposphere over the last 21 years don't agree with that prediction. Collected by the National Oceanic and Atmospheric Administration's TIROS-N series of weather satellites, the data show only a slight net warming of 0.05 degrees Celsius per decade.