A study by National Aeronautics and Space Administration (NASA) scientists suggests that greenhouse gas (GHG) emissions could raise average summer temperatures in the eastern part of the United States nearly 10 degrees Fahrenheit by the 2080s.

The research found that eastern U.S. summer daily high temperatures that currently average in the low to mid 80s Fahrenheit will most likely soar into the low to mid 90s during typical summers by the 2080s. In extreme seasons, for example, if a drought occurred, July and August daily high temperatures could average between 100 and 110 degrees in Washington, DC, Atlanta and as far west as Chicago.

"There is the potential for extremely hot summertime temperatures in the future, especially during summers with less-than-average frequent rainfall," said lead author Barry Lynn of NASA's Goddard Institute for Space Studies and Columbia University in New York City.

To reach their conclusions, the researchers analyzed nearly 30 years of observational temperature and precipitation data and used computer model simulations that considered soil, atmospheric and oceanic conditions and projected changes in greenhouse gases. The simulations were produced using a weather prediction model coupled to a global model developed by NASA's Goddard Institute.

The global model, one of the models used in the recently issued climate report by the Intergovernmental Panel on Climate Change (IPCC), was used to identify future changes in large-scale atmospheric circulation patterns because of a build-up of GHG. The information then was fed into the weather prediction model to forecast summer-to-summer temperature variability in the eastern United States during the 2080s.

The weather model showed that extreme summertime surface temperatures developed when carbon dioxide emissions were assumed to continue to increase about 2% a year, the "business as usual" scenario. The findings were too recent to be included in the latest IPCC report.

"The weather prediction model used in this research is advantageous because it assesses details about future climate at a smaller geographic scale than global models, providing reliable simulations not only on the amounts of summer precipitation, but also on its frequency and timing," the authors stated. "This is an important capability for predicting summer temperatures because observed daily temperatures are usually higher on rainless days and when precipitation falls less frequently than normal."

Observational climate data also showed that sea surface temperatures in the Pacific Ocean have a significant influence on summer air temperatures in the eastern part of the country.

"Relatively cool waters in the eastern Pacific often result in stubborn summer high-pressure systems over the eastern states that block storms, reducing the frequency of precipitation below normal," said study co-author Richard Healy of Woods Hole Oceanographic Institution in Massachusetts. "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."

The global model simulated rainfall too frequently, so that its surface temperatures were not appropriately sensitive to interannual changes in Pacific sea surface temperatures, he said.

"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," said co-author Leonard Druyan of the NASA Goddard Institute and Columbia.

In comparison, the researchers said a number of the global models used in previous studies of future climate change predict too frequent precipitation that often falls too early in the day. As a result, they tend to underestimate the amount of future warming by reflecting solar radiation back to space before it can warm the surface and by simulating excessive evaporation from the wet ground.

According to NASA, the latest study is the first to document the impact of precipitation simulation imperfections on model predictions of surface air temperature.

"Using high-resolution weather prediction models, we were able to show how greenhouse gases enhance feedbacks between precipitation, radiation and atmospheric circulations that will likely lead to extreme temperatures in our not so distant future," said Lynn.

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

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