NASA Study Links Severe Storm Increases, Global Warming

MEDIA RELATIONS OFFICE
JET PROPULSION LABORATORY
CALIFORNIA INSTITUTE OF TECHNOLOGY
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
PASADENA, CALIF. 91109 TELEPHONE 818-354-5011
http://www.jpl.nasa.gov

Alan Buis 818-354-0474
Jet Propulsion Laboratory, Pasadena, Calif. December 19, 2008
Alan.buis@jpl.nasa.gov

NEWS RELEASE: 2008-242

NASA Study Links Severe Storm Increases, Global Warming

PASADENA, Calif. -- The frequency of extremely high clouds in Earth's tropics -- the type
associated with severe storms and rainfall -- is increasing as a result of global warming, according to
a study by scientists at NASA's Jet Propulsion Laboratory, Pasadena, Calif.

In a presentation today to the fall meeting of the American Geophysical Union in San Francisco, JPL
Senior Research Scientist Hartmut Aumann outlined the results of a study based on five years of data
from the Atmospheric Infrared Sounder (AIRS) instrument on NASA's Aqua spacecraft. The AIRS
data were used to observe certain types of tropical clouds linked with severe storms, torrential rain
and hail. The instrument typically detects about 6,000 of these clouds each day. Aumann and his team
found a strong correlation between the frequency of these clouds and seasonal variations in the
average sea surface temperature of the tropical oceans.

For every degree Centigrade (1.8 degrees Fahrenheit) increase in average ocean surface temperature,
the team observed a 45-percent increase in the frequency of the very high clouds. At the present rate
of global warming of 0.13 degrees Celsius (0.23 degrees Fahrenheit) per decade, the team inferred
the frequency of these storms can be expected to increase by six percent per decade.

Climate modelers have long speculated that the frequency and intensity of severe storms may or may
not increase with global warming. Aumann said results of the study will help improve their models.

"Clouds and rain have been the weakest link in climate prediction," said Aumann. "The interaction
between the daytime warming of the sea surface under clear-sky conditions and increases in the
formation of low clouds, high clouds and, ultimately, rain is very complicated. The high clouds in our
observations—typically at altitudes of 20 kilometers (12 miles) and higher—present the greatest
difficulties for current climate models, which aren't able to resolve cloud structures smaller than
about 250 kilometers (155 miles) in size."

Aumann said the results of his study, published recently in Geophysical Research Letters, are
consistent with another NASA-funded study by Frank Wentz and colleagues in 2005. That study
found an increase in the global rain rate of 1.5 percent per decade over 18 years, a value that is about
five times higher than the value estimated by climate models that were used in the 2007 report of the
Intergovernmental Panel on Climate Change.

JPL manages the AIRS project for NASA's Science Mission Directorate, Washington. For more
information on AIRS, visit http://airs.jpl.nasa.gov/ .

JPL is managed for NASA by the California Institute of Technology in Pasadena.

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