6 May 2004Mark Henderson, Science Correspondent
 | POWERFUL evidence for global warming has been discovered by scientists funded by the US Government, demolishing the chief argument of sceptics who deny that the phenomenon is real. A new analysis of satellite data has revealed that temperatures in a critical part of the atmosphere are rising much faster than previously thought, strengthening the scientific consensus that the world is warming at an unnatural rate. The discovery resolves one of the most contentious anomalies in climate science, which has often been invoked by the Bush Administration to question whether man-made global warming is happening. |
While it is generally accepted that surface temperatures are increasing by an average of 0.17C (0.31F) per decade, satellites have been unable to detect a parallel trend in the troposphere — the lowest level of the atmosphere, extending 7.5 miles above the ground, in which most weather occurs.
This lack of tropospheric warming has long puzzled scientists, as it is predicted by all the major models of climate change. It has also been seized on by a small but vocal minority of scientists, who have used it to raise doubts about whether global temperatures are rising at all. The enigma, however, has been explained by a team led by Qiang Fu, of the University of Washington in Seattle.
His research reveals that the troposphere is warming almost precisely as the models predict it should: by about 0.2C (0.4F) per decade. Satellites have not previously detected the trend as they have been confused by colder temperatures in the atmospheric layer above.
The findings, details of which are published today in the journal Nature, provide one of the final pieces of proof that global warming is taking place, and that it is a human-induced phenomenon.
Sceptics have often argued that if temperatures are rising at all, this is down to natural variation in the climate as the world emerges from a “little Ice Age”. The tropospheric trend, however, is precisely what scientists would expect to see if man-made emissions of greenhouse gases were causing it to heat up.
“I think this could convince not just scientists but the public as well,” Dr Fu said.
Mike Hulme, director of the Tyndall Centre for Climate Change Research in Norwich, said: “It will become that much harder for people to claim that the world isn’t warming and that the warming isn’t caused by greenhouse-gas emissions.”
In their study, the Washington team examined atmospheric temperature data collected between January 1979 and December 2001 from satellites operated by the US National Atmospheric and Oceanic Administration.
These satellites used instruments known as microwave-sounding units to measure microwave radiation emitted by oxygen molecules, and thus to calculate the temperature.
The raw data for the troposphere, as measured by the instruments’ channel 2 setting, showed no pronounced warming trend.
Dr Fu realised, however, that about a fifth of the signal picked up on channel 2 in fact originated in the stratosphere — the higher level of the atmosphere between 10km and 50km above the Earth’s surface. This had skewed the data, as the stratosphere is known to be cooling rapidly.
“Because of ozone depletion and the increase of greenhouse gases, the stratosphere is cooling about five times faster than the troposphere is warming, so the channel 2 measurement by itself provided us with little information on the temperature trend in the lower atmosphere,” Dr Fu said.
His team then used measurements from weather balloons and from another channel on the microwave units to determine precisely how much of the channel 2 signal was coming from the stratosphere.
Once this stratospheric error was eliminated, the remaining data showed that the troposphere had indeed been warming, by about 0.2C (0.4F) a decade.
“This tells us very clearly what the lower atmosphere temperature trend is, and the trend is very similar to what is happening at the surface,” Dr Fu said.
The new tropospheric data does not suggest that the pace of global warming is increasing or decreasing. The research was funded by the US Government, through the Department of Energy, the National Science Foundation and Nasa.
The Intergovernmental Panel on Climate Change estimates that global temperatures will rise by an average of between 1.4C and 5.8C by the end of the century.
Dr Hulme said that while the results further confirm the overwhelming scientific consensus that man-made global warming is a proven phenomenon, he would be surprised if it were accepted by critics.
“I’m under no illusions that it will knock down the critics altogether,” he said. “In some quarters, people hold almost fundamentalist beliefs that are immune to carefully reasoned argument. A new paper that seems to take the legs away from one of their critiques may unfortunately not make much difference to their arguments.
“It is the totality of the evidence that has convinced the vast majority of experts that the planet is warming: surface temperature recordings, rises in sea level, retreating glaciers, shifting species domains.
“The compendium of evidence from all these different sources means the overwhelming majority of scientists feel justified in warning society about this.”
