Balloon Data

Excerpts from The Times, 13 August 1998
Science editor Nigel Hawkes reports on conflicting evidence over global warming

American scientists claim to have resolved one of the biggest puzzles over global warming. They say that evidence from satellites suggesting that the atmosphere is cooling, rather than warming, is the result of an error. The satellite scientists acknowledge the error, but say that it makes no difference - because they have found another error that cancels it out. They claim that, once both corrections are incorporated into the data, the atmosphere really is cooling - and so the puzzle remains. US weather satellites have measured the temperature of the atmosphere since 1979 and, in contrast to observations at ground-level, have shown a small decline. This has cast doubt over whether global warming is actually happening, and has been used by critics of the Global Climate Treaty.

But according to the new analysis, reported in Nature, the satellite data is wrong because it fails to take account of the slow decay of the satellites' orbits, which brings them slightly closer to Earth every year. The decline is small - three quarters of a mile a year - but it makes a significant difference, report Frank Wentz and Matthias Schabel, of Remote Sensing Systems, a company based in Santa Rosa, California. When the satellite is looking straight down, the annual change in height makes virtually no difference to the accuracy of its thermometers. They measure the temperature of the atmosphere by detecting microwave radiation emitted by oxygen atoms - the hotter the atoms, the stronger the radiation.

But when the instruments are looking sideways, towards the edge of the Earth, the angle of view matters. Small changes in that angle, caused by the decay of the orbit, can have significant effects on the temperature recorded. When allowances are made for these effects, the two scientists find that, rather than showing a decline in temperature in the lower troposphere of 0.05C per decade, the satellites show an increase of 0.07C per decade. While this is only half the rate of increase observed at the surface, it removes an anomaly in the satellite measurements, which had shown warming at some levels of the troposphere and cooling at other levels, in conflict with climatologists' expectations.

John Christy, of the University of Huntsville in Alabama, one of the scientists responsible for measuring the satellite temperatures, concedes that the decay theory is right. But he says that, on re-examining the data, he has found two other sources of error that, by coincidence, cancel out the effects of orbital decay. These errors are caused by the slow movement of the orbit, which means that the satellites do not cross the Equator at the same time every day, and by slowly accumulating instrument errors. "When these changes are also included, the result is a continued fall in satellite-measured temperatures," Simon Brown, of the Hadley Centre for Climate Prediction and Research at the Meteorological Office, said. "So there is still a contradiction between ground-based and satellite-based temperatures."

The problem is made more acute by the fact that balloon-based measurements of the temperature of the atmosphere during the same period - 1979-95 - back the satellite data, and show a small cooling. Looked at over a longer period, from the 1950s, the balloon data showed an increase, Dr Brown said.