Study's Findings Consistent With Earlier Report, Says
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, CA.
In a presentation Friday 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
"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
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.