Clouds key to cooling in forested areas

Sept. 15, 2021

Planting trees and replenishing forests are among the simplest and most appealing solutions to combating a warming planet. But some scientists have expressed concerns that reforesting midlatitude locations such as North America or Europe could in fact make the planet hotter.

Forests absorb solar radiation as the result of having low albedo, which is a measure of a surface’s ability to reflect sunlight. In the tropics, low albedo is offset by the higher uptake of carbon dioxide by the dense, year-round vegetation. However, in midlatitude forests, the concern is that the sun’s trapped heat could counteract any cooling effect forests would provide by removing carbon dioxide from the atmosphere.

But new research suggests this fear is unfounded. And the reason can be found in clouds.

Clouds form more frequently over forested areas than over grasslands and other areas with short vegetation. The researchers found that for midlatitude regions, the cooling effect of clouds — in combination with that of carbon sequestration — outweighed the solar radiation that forested areas absorbed.

“We show that if one considers that clouds tend to form more frequently over forested areas, then planting trees over large areas is advantageous and should be done for climate purposes,” said Amilcare Porporato, professor of Princeton’s Civil and Environmental Engineering department.

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