New NASA Study Points To Extreme Summer Warming In The Future*

Greenbelt MD (SPX) May 12, 2007

A new study by NASA scientists suggests that greenhouse-gas warming may
raise average summer temperatures in the eastern United States nearly
10Fahrenheit by the 2080s. "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, New
York.

The research found that eastern U.S. summer daily high temperatures that
currently average in the low-to-mid-80s (degrees Fahrenheit) will most
likely soar into the low-to-mid-90s during typical summers by the 2080s.
In extreme seasons - when precipitation falls infrequently - July and
August daily high temperatures could average between 100 and
110Fahrenheit in cities such as Chicago, Washington, and Atlanta.

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

The global model, one of the models used in the recently issued climate
report by the Intergovernmental Panel on Climate Change (IPCC), was
utilized in this study to identify future changes in large-scale
atmospheric circulation patterns due to the build up of greenhouse gases.

This information was then 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 two percent a year, the
"business as usual" scenario. These findings are 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.
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 U.S.

"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," noted study
co-author Richard Healy of the Woods Hole Oceanographic Institution,
Woods Hole, Mass. "Less frequent storms result in higher surface and
atmospheric temperatures that then feedback 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. "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, NASA Goddard Institute for Space Studies and Columbia University.

In comparison, the researchers say that 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.

To the authors' knowledge, this is the first study that has documented
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.