Planting Forests May Cool the Planet More Than Thought

From Princeton University.

"Princeton University researchers found that greater formation of clouds over forested areas suggests that reforestation would likely be more effective at cooling Earth’s atmosphere than previously thought. The findings address a concern among scientists that because forests absorb solar radiation, reforesting temperate latitudes could in fact make the planet warmer.

Planting trees and replenishing forests are among the simplest and most appealing natural climate solutions, but the impact of trees on atmospheric temperature is more complex than meets the eye.

One question among scientists is whether reforesting midlatitude locations such as North America or Europe could in fact make the planet hotter. Forests absorb large amounts of solar radiation as a result of having a low albedo, which is the 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. But in temperate climates, 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 a new study from Princeton University researchers found that these concerns may be overlooking a crucial component — clouds. They report in the Proceedings of the National Academy of Sciences that the denser cloud formations associated with forested areas means that reforestation would likely be more effective at cooling Earth’s atmosphere than previously thought.

“The main thing is that nobody has known whether planting trees at midlatitudes is good or bad because of the albedo problem,” said corresponding author Amilcare Porporato, Princeton’s Thomas J. Wu ’94 Professor of Civil and Environmental Engineering and the High Meadows Environmental Institute. “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.”

As anyone who has felt a cloud pass over the sun on a hot day knows, daytime clouds have a cooling — albeit transient — effect on the Earth. In addition to directly blocking the sun, clouds have a high albedo, similar to ice and snow. Clouds, however, are notoriously difficult to study and have been largely discounted from many studies examining the effectiveness of natural climate change mitigation, including reforestation, Porporato said.

To consider reforestation in the context of cloud coverage, Porporato worked with lead author Sara Cerasoli, a Princeton graduate student in civil and environmental engineering, and Jun Ying, an assistant professor at Nanjing University of Information Science and Technology who previously was a postdoctoral fellow in Porporato’s research group. Their work was supported by the Carbon Mitigation Initiative based in HMEI.

Porporato and Yin previously reported that climate models underestimate the cooling effect of the daily cloud cycle. They also reported last year that climate change could result in increased daily cloud coverage in arid regions such as the American Southwest that are currently ideal for solar power production."