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STUDY
FINDS THICKER STORM CLOUDS OVER WARMER TROPICAL WATERS AFFECT
CLIMATE
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Over
warmer ocean waters, tropical storm clouds become thicker,
more extensive and reflect more sunlight back into space than
they do over cooler waters, NASA researchers report.
Using
data from NASA's Tropical Rainfall Measuring Mission (TRMM)
satellite, Anthony Del Genio, a physical scientist at NASA's
Goddard Institute for Space Studies, N.Y., and lead author
of the study, and his co-author, Columbia University's William
Kovari, were able to isolate raining cloud systems and compare
the rain they produce with the water that stays in the clouds
and reflects sunlight. The researchers found that storm clouds
over warmer waters are denser and cover wider areas of the
tropics than those over cooler waters.
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"We
now have a better understanding of tropical storm clouds."
said Del Genio. "Such clouds play a key role in global
climate change, and it is essential to understand this role
thoroughly if we are ever to comprehend humanity's effect
on the Earth's climate."
The
study provides a clearer picture of the behavior of storm
clouds than was previously available. It also runs counter
to a recent theory which claims that storm cloud cover lessens
as tropical ocean temperatures rise, thus making climate more
strongly resistant to the warming that would occur with an
influx of greenhouse gases.
A
key observation was that while warmer storm clouds do release
more of their moisture as rain, as a theory known as "Adaptive
Iris" states, such clouds also have more moisture to
begin with and thus also form bigger clouds. These NASA researchers
believe the theory is inaccurate because it does not take
into account all the factors that come into play when storm
clouds form over warmer tropical regions.
If
the Iris theory were correct, there would be less cloud cover
and less humidity and mostly greater heat loss to space, strongly
countering the warming effects of an influx of greenhouse
gasses. The TRMM satellite data, however, indicate that the
climate is more sensitive to warming caused by increases in
greenhouse gases.
The
Adaptive Iris model claims that tropical clouds cool the Earth,
but to a much greater extent than Del Genio's research indicates.
The Iris theory predicts that clouds in the tropics grow thinner
and less extensive as temperatures rise, thus trapping progressively
less heat.
According
to the hypothesis, increased warmth in the tropics would create
more turbulent storm conditions and cause clouds to quickly
drop their moisture as rain, thus leaving less water in the
clouds and making them both thinner and less extensive. If
this were the case, changes in tropical clouds could potentially
cool the atmosphere as fast as greenhouse gas accumulation
would heat it, making them a natural damper on global warming.
However, such a claim is not supported by these satellite
observations.
Scientists
agree that the atmospheric concentration of greenhouse gases
in the atmosphere will double compared to pre-Industrial Revolution
levels within the next few decades, but there is still widespread
debate over how great an effect this influx of gases will
have on the world's climate. Although doubling the concentration
of greenhouse gases like carbon dioxide would by itself raise
Earth's temperature, the change in atmospheric composition
also may trigger other effects that could amplify or diminish
this temperature change.
"It's
not just the heat the greenhouse gases themselves will trap,"
said Del Genio. "We have other factors to worry about.
Clouds are one of the most influential climate factors, and
one of the most difficult to understand."
All
clouds both trap heat and reflect solar energy. They hold
heat in like a blanket, preventing it from escaping the Earth,
and their white upper surfaces also reflect sunlight back
into space before it can warm the atmosphere. The net effect
can either heat or cool the planet, depending on how thick,
wide and high in altitude the clouds are.
Del
Genio emphasizes that although their work should not be used
to directly predict climate change, it will allow scientists
to develop more accurate climate models that can be used with
greater confidence.
The
research appears in the Sept. 15th issue of the American Meteorological
Society's Journal of Climate.
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