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RELEASE NO. 02-009
FEWER CLOUDS FOUND IN TROPICS
NASA scientists discover new evidence of climate change
After examining 22 years of satellite measurements, NASA
researchers find that more sunlight entered the tropics and more
heat escaped to space in the 1990s than in the 1980s. Their
findings indicate less cloud cover blocked incoming radiation and
trapped outgoing heat.
"Since clouds were thought to be the weakest link in predicting
future climate change from greenhouse gases, these new results are
unsettling," said Dr. Bruce Wielicki of NASA Langley Research
Center, Hampton, Va. Wielicki is the lead author of the first of
two papers about this research appearing in the Feb. 1, issue of
"Science."
"It suggests that current climate models may, in fact, be more
uncertain than we had thought," Wielicki added. "Climate change
might be either larger or smaller than the current range of
predictions."
The observations capture changes in the radiation
budget–the balance between Earth’s incoming and outgoing
energy–that controls the planet’s temperature and
climate.
The previously unknown changes in the radiation budget are two
to four times larger than scientists had believed possible. The
reason why and the degree to which it changed are surprising
scientists and create a powerful new test for climate models.
Inspired by this puzzle, a research group at NASA Goddard
Institute for Space Studies (GISS) developed a new method of
comparing the satellite observed changes to other meteorological
data.
"The new method is a conceptual breakthrough in how we analyze
data," said Anthony Del Genio, a scientist at GISS and co-author of
the companion paper.
"What it shows is remarkable," said Wielicki. "The rising and
descending motions of air that covers the entire tropics, known as
the Hadley and Walker circulation cells, appear to increase in
strength from the 1980s to the 1990s. This suggests that the
tropical heat engine increased its speed."
The faster circulation dried out the water vapor that is needed
for cloud formation in the upper regions of the lower atmosphere
over the most northern and southern tropical areas. Less cloudiness
formed allowing more sunlight to enter and more heat to leave the
tropics.
In response, several of the world’s top climate modeling
research groups agreed to take on the challenge of reproducing the
tropical cloud changes. But the climate models failed the test,
predicting smaller than observed variability by factors of two to
four.
"It’s as if the heat engine in the tropics has become less
efficient using more fuel in the 90s than in the 80s," said
Wielicki. "We tracked the changes to a decrease in tropical
cloudiness that allowed more sunlight to reach the Earth’s
surface. But what we want to know is why the clouds would
change."
The results also indicate the tropics are much more variable and
dynamic than previously thought.
"The question is, if this fluctuation is due to global climate
change or to natural variability," said Del Genio. "We think this
is a natural fluctuation, but there is no way to tell yet."
While the current 22-year radiation budget record–the
longest and most accurate ever compiled–is still too short to
pinpoint a cause, the newly discovered change acts as a standard by
which to measure future improvements in cloud modeling.
"A value of this research is it provides a documented change in
climate and a target for climate models to simulate," said Del
Genio.
Images are available on the Internet in high resolution at:
http://asd-www.larc.nasa.gov/ceres/ASDceres.html
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