Published online 27 May 2009 | Nature | doi:10.1038/news.2009.518

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American west threatened by more heatwaves than past models have predicted.

More heatwaves in the western United States? Utah Division of Water Resources

Extreme temperatures are expected to become more common in the western United States by 2040 if greenhouse gases continue to rise, researchers say.

Noah Diffenbaugh, a climate scientist at Purdue University in West Lafayette, Indiana, and his colleagues simulated climate change for the United States in decade-long periods from 2000 to 2039 using a climate model that divided the land into areas just 25 kilometres square. It is the first time that the region's temperature extremes have been modelled at such high resolution. The new projections were reported on 26 May at the joint assembly of the American Geophysical Union in Toronto, Canada.

Some regions where high seasonal temperatures had occurred just once during the second half of the twentieth century are projected to experience extreme temperatures many times in a single decade, according to the model. "The once-in-50-years event becomes the five-times-in-ten-year event, and in the western United States it is much higher than that — up to eight times per decade," says Diffenbaugh. Colorado, Arizona, New Mexico and Utah stand to be the most affected.

The new simulation indicates that the western United States will experience more temperature extremes than projected by the global climate models used in the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Many of those models do not account for the topography of the region — which includes the Sierra Nevada Mountains, for example — in a realistic way, says Diffenbaugh.

Unknown effects

An increase in the frequency of temperature extremes could affect crops, river flow and electricity consumption in the western states. For example, regions with the climatic conditions suitable for premium wine grape production might shift, shrink or even disappear with rising temperature extremes1.

"If humans and ecological systems have adapted to the current climate and if they experience temperatures they have never before experienced, it may be a problem for some systems," says Alan Robock, a climate scientist at Rutgers University in New Brunswick, New Jersey. "But Diffenbaugh doesn't say which ones."

Click here for a larger version. N. Diffenbaugh

Earlier work by Diffenbaugh and his colleagues has projected that extreme heat events will at least triple in the Mediterranean during this century, with the greatest extreme temperature increases occurring in France and the Iberian Peninsula (see 'Dangerous Hotspots').

In the new simulation, the increased frequency of extreme events occurs when the global temperature has risen by 1.2 ºC above the 2000 mean. At the Climate Change Congress that took place in Copenhagen in March, climate scientists and policy-makers agreed a goal of keeping the global mean temperature to less than 2 ºC above that of pre-industrial times. As the global mean temperature has increased by about 0.7 ºC since 1850, that would still allow temperatures to rise by around a further 1.3 ºC — enough to trigger the more frequent heat events predicted by Diffenbaugh's model.

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"In terms of the targets that are necessary to avoid dangerous climate change, these results suggest that we may need a re-evaluation of those targets," says Diffenbaugh.

To test the model, Diffenbaugh compared it with temperature extremes observed between 1951 and 1999 — and found that its predictions matched the occurrence of real temperature extremes more closely than the IPCC's models. But the likelihood of these increases in extreme temperatures occurring remains unclear. High-resolution models that are embedded within global models, such as this one, are sensitive to how well the conditions at the boundaries of the two models are matched, says Gavin Schmidt, a climate modeller at NASA's Goddard Institute for Space Studies in New York City, who did not see the new data that Diffenbaugh presented. "Increasing resolution doesn't necessarily increase predictability at the local scale," Schmidt adds.