23-July-2009 14:00 ET
It's a question Amy Clement and colleagues have been investigating in detail. Why? Well clouds might slow down climate change or they could speed it up. It depends what happens to them as greenhouse gases warm the Earth. There are two possibilities.:
Or it could get less cloudy. That would let more energy from the sun through, which would make the warming worse, which would produce even less clouds .... and round and round in a positive feedback that we don't want at all.
Now Clement and Robert Burgman from the University of Miami's Rosenstiel School of Marine and Atmospheric Science, together with Joel Norris from Scripps Institution of Oceanography at UC San Diego, have looked in detail at the question of the clouds. Their research is published in the July 24 issue of Science.
And the answer they've found is not good news.
Clement and her colleagues have been studying observations of clouds in the northeast Pacific, made over the past 50 years. This is one of the best-studied areas of low-level clouds in the world.
But that doesn't mean they had an easy time of it. Why? Well there are two different ways to gather information about clouds - satellite instruments and the human eye - and both have their problems, say the scientists.
With the human eye it's the "subjective nature of the measurements and sparse sampling for large regions of the ocean."
With the satellite measurements it's "spurious trends related to instrument drift and calibration" - as well as the fact that data are available only for the past 25 years.
So the scientists had to do a great deal of digging in the data, a whole heap of statistical analysis and a big bunch of checking out ideas and exploring measurements to look for real patterns, if there were any.
The outcome of all this was a big surprise. The totally different sets of measurements were telling the same story. "The agreement we found between the surface-based observations and the satellite data was almost shocking," says Clement, the University of Miami professor of meteorology and physical oceanography, and winner of the American Geophysical Union's 2007 Macelwane Award for her work on climate change.
"These are subtle changes that take place over decades. It is extremely encouraging that a satellite passing miles above the earth would document the same thing as sailors looking up at a cloudy sky from the deck of a ship."
And what is this "same thing"?
That warming oceans produce less clouds above them.
Most climate models don't produce this observed cloud behaviour. Only the Hadley Centre model at the UK Met Office was able to reproduce the observations, the researchers found.
"We have a long way to go in getting the models right, but the Hadley Centre model results point us in the right direction," says co-author Burgman, a research scientist at the University of Miami.
Taken together, the observations and the Hadley Centre results provide evidence that there will be fewer low-level clouds, which shield Earth from the sun's radiation, as the climate gets warmer.
This will allow the oceans to heat up, which will then cause fewer clouds, which will let more energy through, which will allow the oceans to heat up even more. Positive feedback. Increased global warming. The clouds are not going to save us
"This is somewhat of a vicious cycle, potentially exacerbating global warming," said Clement. "But these findings provide a new way of looking at cloud changes.This can help improve the simulation of clouds in climate models, which will lead to more accurate projections of future climate changes. "