[asa] Another Side to the Climate-Cloud Conundrum Finally Revealed

http://www.sciencemag.org/cgi/content/summary/319/5865/889a

Clouds have always given climate modelers fits. The clouds in their
models are crude at best, and in the real world, researchers struggle
to understand how clouds are responding to--and perhaps magnifying--
greenhouse warming. As a result, cloud behavior is the biggest single
source of uncertainty in climate prediction. But two new studies now
show that much of the worry about clouds' role in the warming has been
misdirected. Clouds' response to global temperature changes may be
much quicker and more direct--and thus easier to study--than experts
have thought.

"It's a little bit of good news," says climate researcher Brian Soden
of the University of Miami in Florida. "People have been working on
[the cloud problem] for 2 decades or more, and we haven't done a lot
to decrease the uncertainty. I'm a little more optimistic now about
making progress on this problem."

Researchers have always considered the cloud problem a matter of
feedbacks. In a positive feedback, increasing greenhouse gases warm
the surface, and the warmer surface then feeds back somehow to
overlying clouds. The nature of the feedback remains mysterious, but
if it's positive, it would decrease global cloud cover. With fewer
clouds reflecting solar energy back into space, more energy would
reach Earth, amplifying the initial warming. But Earth's surface and
especially its oceans are slow to warm, so cloud feedbacks operate
over decades--or so scientists assumed.

Two groups have recently looked at just how quickly model clouds
actually respond to an increase in greenhouse gases. Climate
researchers Jonathan Gregory and Mark Webb, both of the Hadley Centre
for Climate Prediction and Research in Exeter, U.K., report in the
January Journal of Climate (issue 1) that model clouds, at least, can
respond quickly to added carbon dioxide--in months, not decades. In
most of the models examined, the classic cloud feedback driven by
change at the surface played only a minor role. The real action took
place where the clouds themselves were, up in the air. Added carbon
dioxide absorbs more long-wave energy radiating from the surface; the
air holding that carbon dioxide warms, and clouds evaporate, letting
more solar radiation in.

In follow-up work in press in Geophysical Research Letters, climate
researchers Timothy Andrews and Piers Forster, both of the University
of Leeds, U.K., extend and refine the analysis of Gregory and Webb. In
seven models, they doubled carbon dioxide while holding the global
surface temperature constant and watched how atmospheric temperatures
respond. The classic, slow cloud response is only half of previous
estimates, they find, and most of the cloud response is fast.
Scientists "have been looking at the incorrect part of the problem,"
says Forster. Properly accounting for fast response is important when
modeling rising temperatures under the strengthening greenhouse, Webb
and Gregory argue. And because it is fast and therefore has been going
on for decades, notes Gregory, researchers may be able to tease the
newly appreciated cloud response out of observations and improve their
models faster than they have the past few decades.

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Received on Thu Feb 14 23:52:28 2008