US scientists based at the Amundsen-Scott South Pole Station say they have measured the temperature of the atmosphere 30 to 110 kilometres (18 to 68 miles) over the pole and found it is 20 to 30 degrees Centigrade (36 to 54 degrees Fahrenheit relative) colder than computer models showed.

Various models are used to predict global climate and some assumptions have had to be made, including air temperatures over Antarctica.

Chester Gardner, a professor of electrical and computer engineering at the University of Illinois, Weilin Pan, a doctoral student at Illinois and Ray Roble of the US National Center for Atmospheric Research writing in the American Geophysical Union Letters say the models are wrong.

"Because of the obvious challenges, until now, the only temperature data we have had from either the North or South Poles has been from surface measurements and weather balloons that don't go any higher than about 20-30 kilometres (12-18 miles)," Gardner told AFP.

The researchers used a laser radar system from the South Pole to make the first measurements of the temperature higher up and found it was much colder than assumed.

Global warming could be caused by greater concentrations of carbon dioxide (CO2) which is a strong absorber of infrared radiation.

In the lower atmosphere CO2 absorbs the heat. While CO2 also emits heat other CO2 absorbs it. In the thinner stratosphere and mesosphere, much of the heat emitted by CO2 is radiated into space and so in the upper atmosphere the primary affect of CO2 is cooling.

"Thus as CO2 levels continue to rise in the atmosphere, we expect the lower atmosphere to continue warming while the upper atmosphere ... will cool."

During winter Antarctica receives little sunlight and its atmosphere is sealed off by a vortex of winds preventing warmer air from lower latitudes travelling to the pole.

"As a consequence the region cools to very low temperatures in winter, primarily by radiation of heat into space."

In May, June and July the stratopause was considerably colder than model predictions. The greatest difference occurred in July, when the measured stratopause temperature was about minus 17 degrees C (0 degrees F) to about 4 degrees C (40 degrees F) predicted by the models.

"Current global circulation models apparently over-predict the amount of down-welling, because they show warmer temperatures than we observed," Gardner said.

Their measurements will be a baseline for future temperature studies.

"We believe a major flaw in current models is the way they account for compressional heating associated with down welling over the polar cap in winter," Gardner said.

"Of course you and I are really not interested in what happens above the South Pole. We do care about what happens where we live. Models can help predict those changes due to rising CO2 levels but only if we believe they give accurate results.

"Our South Pole measurements will help the modellers and theoreticians better understand the atmosphere and incorporate that understanding in their models, making their future predictions more accurate."

SPACE.WIRE