Release No: 01-3

THE EASTERN U.S. KEEPS ITS COOL WHILE THE WORLD WARMS

Much of the Earth has warmed over the last half-century, but the
eastern half of the United States has shown a cooling trend.
NASA-funded research indicates cooler temperatures in the eastern
U.S. are caused by an increase in sun-shielding clouds produced by
warmer ocean temperatures in the Pacific.

Walter A. Robinson of the University of Illinois at
Urbana-Champaign, James Hansen of NASA Goddard Institute for Space
Studies, and Reto Reudy of Science Systems and Applications, Inc.
will present these findings in a paper entitled "Where's the Heat?
Insights From GCM Experiments into the Lack of Eastern U.S. Warming"
at the American Meteorological Society Annual Meeting in Albuquerque,
N.M. on January 15.

Eastern U.S. temperatures have displayed a cooling trend of 0.1°
Celsius per decade, while global temperatures warmed by that same
amount from 1950 to 1997. The researchers used a computer climate
model to see if this regional cooling could be caused by changes in
sea surface temperature. Robinson said that in the GISS model,
"Warmer sea surface temperatures in the tropical Pacific cause
greater cloud cover over the eastern United States. This increased
cloud cover is directly responsible for the cooling." The brightness
of a cloud causes a large percentage of incoming solar radiation to
be reflected back into space, thus keeping the atmosphere cooler than
if the cloud wasn't there.

Using the climate simulations, Robinson found the amount of
water vapor in the Gulf of Mexico follows closely the water vapor
released by the warm sea surface temperatures in the Pacific Ocean.
Water vapor from the Pacific moves east to the Gulf of Mexico and is
then carried over the eastern U.S. by the clockwise circulation
around an Atlantic subtropical high pressure system. When the water
vapor arrives over the U.S. it condenses and generates more cloud
cover, allowing less solar radiation to reach and warm the Earth's
surface.

Robinson's research utilized the GISS (Goddard Institute for
Space Studies) "general circulation model," which simulates the
circulation of the atmosphere around the world and used sea surface
temperatures from around the globe.

In order to create a focus on sea-surface temperatures in the
model runs, three components that can contribute to warming or
climate forcing, were "fixed." These are aerosols (particles in the
atmosphere), solar irradiance or brightness, and greenhouse gases
(such as carbon dioxide). Because these factors were fixed, they can
be ruled out as the cause of cooling in the model, leaving only sea
surface temperatures as a variable.

The GISS model used ocean temperature data over a 47-year span,
from 1950 to 1997 and looked at global sea surface temperatures in
different areas. The model used temperatures from 20 degrees north to
20 degrees south, and from each of those endpoints to each pole. The
only time the model showed significant cooling in the eastern United
States was when the tropical Pacific waters warmed.

For more information about the American Meteorological Society's
81st Annual meeting, please visit the web site:
http://www.ametsoc.org/AMS/meet/81annual/