Friday, July 24, 2009

Warming oceans mean less cloud cover

Rising ocean temperatures in the Northeast Pacific have led to fewer low-level clouds in the region, which could lead to even greater warming as more sunlight reaches the ocean, scientists say.

The findings are an example of a positive feedback loop — where the effects of climate change accelerate further change — and provide climate researchers with another piece of the puzzle in their attempt to track the effects of global warming.

University of Miami researcher Amy Clement and her colleagues looked at cloud cover data from satellites and from more subjective ship-based observations compiled over a 25-year period in the region and compared the results with the 18 climate models used by the United Nations Intergovernmental Panel on Climate Change.

A climate model that made higher-than-usual temperature change predictions was the most accurate in matching this feedback effect, she said.

"When the surface of the ocean is warm, these low-level clouds we've been focusing on dissipate and the atmospheric circulation is weaker," said Clement in a podcast interview with the journal Science, which will publish the findings on Friday.

The effect of warming waters on cloud cover has been a matter of some debate for climate researchers, and most models have either excluded cloud cover from predictions or considered a range of possibilities. Low-level clouds are typically composed of water droplets and include the darker clouds that are more likely to block solar radiation and therefore lead to more cooling, so their presence is considered important to climate modelling but difficult to track.

Clement and her colleagues were able to piece together satellite data obtained from International Satellite Cloud Climatology Project and compare it with ship-based observations from the Comprehensive Ocean Atmosphere Data Set. Neither set of data is complete, she said, as satellite data often needs to be adjusted as new satellites replace old ones, while ship-based observations tend to be subjective ratings made by individual sailors reporting cloud cover.

But Clement said the researchers found a surprising correlation in the data sets, something that bodes well for future research of cloud cover.

The study looked only at the Northeast Pacific, but Clement said the atmospheric patterns found there are tied to changes that span the entire Pacific basin, providing a snapshot of a substantial portion of the Earth.

She cautioned, however, that more climate models considering the cloud cover and ocean temperature relationship would need to be developed before researchers would be able to say with any confidence how much it may affect temperature trends.

Robert Burgman from the University of Miami and Joel Norris from Scripps Institution of Oceanography at UC San Diego were the paper's other authors.

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