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15 Sep 2011: Evaporation from Trees
Helps Cool the Global Climate, Study Says

Water that evaporates from trees and forests not only has a significant local cooling effect, but also plays a role in cooling the global climate, according to a new study by scientists at the Carnegie Institution's Global Ecology department. Reporting in the journal Environmental Research Letters, the scientists found that evaporation from trees cooled the global climate by causing clouds to form low in the atmosphere, which reflects the sun’s rays back into space. Scientists have long known that evaporation had a local cooling effect, but have been unsure of the global effect of evaporation from trees, since water vapor also acts as a greenhouse gas in the atmosphere. But Carnegie scientists devised a climate model on the impact of tree evaporation showing that the overall global impact of evaporation is to stimulate the formation of more low-level clouds. This finding has important implications for land-use decision-making and underscores the importance of preserving forests and planting trees, the researchers said. “This shows us that the evaporation of water from trees and lakes in urban parks, like New York’s Central Park, not only help keep our cities cool, but also helps keep the whole planet cool,” said Carnegie atmospheric scientist Ken Caldeira.

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