lunes, 23 de julio de 2007

Influence of global warming seen in changing rains

* 15:35 23 July 2007
* NewScientist.com news service
* David Chandler

The pattern of rainfall around the world is being changed by greenhouse-gas emissions from human activities, researchers have shown for the first time.

Tropical regions north of the equator, including such areas as the Sahel in Africa which borders the Sahara desert, have already begun to get even drier and will continue to do so, the data show. Regions in the far north, including Canada, Northern Europe and Russia, will get wetter, as will the southern tropics.

Detecting the effects of climate change on rainfall patterns has proved much more elusive than temperature changes because of the much greater natural variability of precipitation.

The key was to take results from 92 computer simulations, using 14 different global circulation models, and to compare the average of these with actual rainfall data over wide bands of latitude around the world.

The results show a clear agreement with the observed trends in global rainfall data over the past century. In fact, although they agree in direction, the observed changes were much stronger than the predictions.

"Over the 20th century, we now detect the signal [in rainfall changes] that is predicted by climate models," says Francis Zwiers, one of the research team. "If you're able to reproduce the past, you also have greater confidence for predictions of the future."
Dry zone

Zwiers, of Environment Canada in Toronto, says that the pattern shows a substantial drying of the region from the equator up to 30° north. This band encompasses all of north Africa, as well as India, southeast Asia, Mexico and northern South America. Some of these regions, such as the Sahara and Sahel in Africa, are already among the world's driest.

Regions further to the south, including the rainforest regions of central Africa and South America, have begun to get increased rainfall and will continue to get wetter.

The findings are important, Zwiers says, because "as humans, our activities are much more constrained by limits of water than by temperature. In places where agriculture is marginal, it will become more marginal in the future".
Precipitation extremes

Richard Seager, a climatologist at the Lamont Doherty Earth Observatory in New York, US, says this detection of 20th century rainfall changes seems "barely discernible from the noise right now", but he agrees that the projected trends for the coming century shown by the combined climate models are a highly convincing prediction.

Seager's own research has shown that, in addition to the trends shown by Zwiers' team, there will also be a significant drying of areas in the northern subtropics, including the US southwest and the Mediterranean.

But aside from the overall trends, Zwiers says an important message from the combined models is that they consistently show that, for all regions, there will be a significant increase in extremes of precipitation – both floods and droughts. Thus, even desert areas that will undergo serious drying could simultaneously suffer greater risks of flash flooding.

"More or less uniformly across all the models, these extreme events will become more intense just about everywhere," he says.

Journal reference: Nature (DOI: 10.1038/nature06025)


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