Wednesday, 9 December 2009

Clear findings offer reminder of climate change severity

Hannah Devlin

Ice cores, tree rings and supercomputer climate models. Each year the world of climate science appears to expand in complexity to the point where bystanders must either place their trust somewhat blindly in the conclusions or wonder whether they’ve been had.
The data from the World Meteorological Organisation is a welcome return to basics. It does not rely on any of the “proxy” measurements that are subject to criticism from sceptics; nor on satellite data, which reflect temperatures in the upper atmosphere; nor on computer predictions.
The 160-year record is based on the simplest, most direct measure of temperature — land and sea-based thermometers.
The figures shown on the graph are given in terms of “temperature anomaly”, which is a measure of how much warmer or cooler the Earth has been in a particular year compared with a reference period of 1961 to 1990. Anomalies are used rather than absolute temperature because getting a realistic estimate for the Earth’s average surface temperature would be unfeasible given that in some regions, for instance the Sahara desert, there are few measurement stations.

Guessing the real temperature for the rest of the desert would be impossible, but guessing that temperatures are going up and down in the same pattern as the surrounding desert is a reasonable assumption.
The first thing the record makes clear is that it is impossible to talk about global warming on a year-by-year basis. The short-term climate record is dominated with seemingly random ups and downs that are explained by separate climate phenomena. The apparent cessation of the warming trend in the latter part of this decade, for instance, is likely to be due to a lull in sunspot activity.
El NiƱo results in a warm peak every three to eight years. The eruption of Mount Pinatubo in the Philippines meant that 1991 was a cooler year than expected. Even if temperatures were on a trajectory to rise by 6C over the next century, the short-term variations mean if you watched it happen one year at a time the trend would be difficult to spot.
Averaged over decades, however, these cycles cancel out, revealing the bigger picture of a rapidly warming planet. Each decade since the 1950s has been warmer than the previous one. At a time when public confidence has been eroded in complex research, this record is a reminder of why the subject should be taken seriously.