Azara Blog: Water vapour might be behind climate warming in Europe

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Date published: 2005/11/12

The BBC says:

Water vapour rather than carbon dioxide in the atmosphere is the main reason why Europe's climate is warming, according to a new study.

The scientists say that rising temperatures caused by greenhouse gases are increasing humidity, which in turn amplifies the temperature rise.

This is potentially a positive feedback mechanism which could increase the impact of greenhouse gases such as CO2.

The research is published in the journal Geophysical Research Letters.

The scientists involved have used research networks and weather stations across Europe to measure temperature, humidity and longwave radiation, which plays a key role in the greenhouse effect.

"We observed that between 1995 and 2002, the amount of longwave radiation coming downwards to the Earth in Europe increased significantly, whereas solar radiation did not," said study leader Rolf Philipona, from the World Radiation Center in Davos, Switzerland.

Longwave radiation comes from molecules of gases such as carbon dioxide, methane, and water vapour which have absorbed solar radiation after it has hit the Earth's surface and been reflected back up through the atmosphere.

"We wondered if this effect was simply because of a temperature increase at the surface - you would just get more radiation going up, and so more coming back down," Dr Philipona told the BBC News website.

"But we allowed for this, and for the impact of extra clouds, but still we found an increase."

The researchers calculated that this increase is partly down to higher concentrations of the gases such as carbon dioxide which are often described as causing the "man-made greenhouse effect"; but increased water vapour appears to have a larger effect, accounting for about 70% of the observed temperature rise.

Not all regions of Europe are affected equally.

Between 1995 and 2002, Eastern states appear to have warmed by a rate equivalent to about 2C per decade - considerably faster than their western counterparts.

According to the new study, that may well be down to humidity differences; broadly speaking, humidity has risen fast in the east but not in the west, where evaporation may be limited by the dryness of the Iberian peninsula.

Although rising concentrations of carbon dioxide, methane, nitrous oxides and other gases are almost certainly driving the global rise in temperature observed in recent decades, the natural greenhouse effect - without which the world would be considerably colder - is largely down to atmospheric water vapour.

Because human activities change its concentrations very little, it is generally not mentioned in discussions of modern-day greenhouse warming.

But climate scientists have been aware for decades that mechanisms involving water vapour could amplify temperature increases, and have attempted to model these effects in computer simulations.

More grist for the mill. Of course it is only one study by one group so should be treated with caution.

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