Weather Wise

WARMER or cooler? It depends on your viewpoint. From the ground, measurements indicate that the Earth has been getting steadily warmer - by between 0.3C and 0.6C over the past 100 years. This warming has been largely attributed to the effects of man-made carbon dioxide and other chemicals.

Controversially though, from space, things look a little different. Satellite measurements of temperatures in the lower atmosphere show no warming trend, and reveal a slight global cooling in the past three decades. Analysis of the satellite data reveals that the greatest impact on global temperatures seen recently has been the works of wholly natural events such as the extreme El Nino episodes of 1983 and 1997, and the eruptions of volcanoes like El Chichon and Mt Pinatubo.

Greenhouse advocates may doubt the satellite data, insisting the computer models which predict a steep temperature rise resulting from the near doubling of CO2 seen this century must be correct. In other words, the satellites must be wrong. Trouble is, the data have been checked and rechecked, and no errors have been found.

One answer may lie in the behaviour of the lower atmosphere in the tropics. It was assumed that radiation from the surface is the main process by which heat may be lost from the Earth. But there has been a growing realisation that evaporation and convection processes are more important as ways of transferring heat into space. The effects of water vapour - the most important greenhouse gas - are poorly understood. Areas of the atmosphere with extremely low humidities can act as holes in the greenhouse, allowing energy to freely radiate into space. We need to take into account such effects.