Rolling thunderstorm (Cumulonimbus arcus) photographed on 17 July 2004 in Enschede, The Netherlands.

Credit: John Kerstholt

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PARIS, 13 September 2006: Continental Europe's extreme summers of recent years, characterised by heavy floods or killer heatwaves, could be commonplace by the turn of the century, a Swiss study says.

Changes in the complex relationship between air temperature and land moisture, driven by global warming, will cause European summers to suffer from chronic variability by 2100 according to Sonia Seneviratne of the Swiss Federal Institute of Technology, lead author of the study.

Published today in the British journal Nature, the study suggests that the hot, dry climate zone of the Mediterranean rim is destined to creep northwards as temperatures rise. As a result, central and eastern Europe will suffer a "positive feedback mechanism" - a vicious cycle in which higher temperatures cause the soil to evaporate more and vegetation to breathe out more moisture.

In damp regions, more airborne moisture and warm air help to fuel the precipitation cycle, causing more and more rainfall and thus boosting the risk of flooding. But in dry regions, moisture is swiftly driven out of soil and plants, leaving nothing to cool the atmosphere. As a result, the effects of a heatwave worsen.

Complicating this process is the likelihood that as the climate changes, the vegetation that it supports will also change. For instance, in drier areas, forests may be replaced by scrubby, heat-loving plants, and this in turn will affect the moisture-atmosphere exchange.

The work was based on a new computer model which predicted the effect of the land-moisture link on a map of Europe with nearly twice the resolution of previous models. It focuses in particular on the mechanisms that led to more frequent heatwaves, but confirms there will be high variability from year to year, with extremely hot spells or very heavy rainfall.

Devastating floods struck parts of Europe in 2002 and 2005 while in 2003, the continent was gripped by a record heatwave that claimed as many as 35,000 lives. Previous research into climate change in Europe has also sketched the risk of huge variability from one year to the next, but the underlying causes for this were unclear.

A separate study published in Nature said there was no evidence to suggest that changes in the Sun's
brightness over the past millennium had had any significant effect on global warming.

Some scientists - now a tiny minority, but backed by powerful lobby groups in the United States - doubt that the rise in global temperatures since the start of the Industrial Revolution has been caused by greenhouse gases released by burning fossil fuels.

Instead, they point to changes in the amount of heat and light the Earth receives, either because of sunspots or because of tiny shifts in the Earth's orbit.