NASA scientists predict average summer temperatures in the eastern United Stateswill rise as much as five degrees Celsius by the 2080s as a result of global warming from rising amounts of greenhouse gases.

The study's climate model predicts temperatures in the region higher than those predicted by global models used by the Intergovernmental Panel on Climate Change, which said temperatures would rise betweentwoand 3.5 degrees C in the same region by 2100.

This difference in predicted temperature is in large part because the NASA model takes into account the variationof rainfall in the region, said lead author Barry Lynn of NASA's Goddard Institute for Space Studies.

"There is the potential for extremely hot summertime temperatures in the future, especially during summers with less-than-average frequent rainfall," said Lynn.

The study looked specifically at the eastern United States and found areas with summer daily average temperatures currently in the high 20s would most likely rise to averages in the low 30s by 2080 if greenhouse gases continued to rise at current levels.

In extreme seasons with less than average rainfall, average temperatures in urban centres such as Chicago, Washington and Atlanta could rise to the low 40s, the authors contend.

The models used by the IPCC suggested annual mean precipitation is very likely to increase in Canada and the northeast U.S. and to decrease in the southwest U.S. In southern Canada, the report said, precipitation is also likely to increase in winter and spring, but decrease in summer.

But Lynn and his co-authors said the IPCC climate models simulated rainfall too frequently by not looking at local climate patterns or taking into account the effect of rising temperatures on future precipitation.

"Less frequent storms result in higher surface and atmospheric temperatures that then feed back on the atmospheric circulation to further reduce storm frequency and raise surface temperatures even more," said co-author Richard Healy of the Woods Hole Oceanographic Institution.

The authorscontend that previous future models of global climate also predict precipitation that falls too frequently and falls too early in the day. Both of these predictions would lead to increased daytime cloud cover, blocking sunlight that would otherwise cause surface temperatures to rise.

"Since the weather prediction model simulated the frequency and timing of summer precipitation more reliably than the global model, its daily high temperature predictions for the future are also believed to be more accurate," added co-author Leonard Druyan, of the NASA Goddard Institute for Space Studies and Columbia University.

The study is published in the April 2007 issue of the American Meteorological Society's Journal of Climate.