Study suggests evaporation from trees cools global climate

In addition to providing shade and oxygen, a Lawrence Berkeley National Laboratory researcher’s study published Sept. 14 in the journal Environmental Research Letters proposes that trees could help cool the planet through evaporation.

According to the study, evaporated water from trees adds to low-level clouds, which reflect solar radiation back to space instead of allowing heat to reach the earth’s surface.

George Ban-Weiss, an atmospheric science researcher at the lab, conducted the study along with researchers from the Carnegie Institution for Science at Stanford University and the Indian Institute of Science.

“Past studies have looked at the broad climate effects of deforestation and afforestation,” Ban-Weiss said. “We wanted to look at which specific factors cause those climate changes. We knew that adding vegetation to urban areas had cooling effects on cities, but it wasn’t clear what happened around the globe.”

Despite the cooling effect observed in the study, the water vapor generated from evaporation contributes to greenhouse gases. But the cooling impact of the clouds far surpasses the greenhouse effect created, according to Ken Caldeira, staff scientist at the climate science laboratory at the Carnegie Institution and co-author of the study.

According to Ban-Weiss, the most important implication of the study is that even though planting trees has positive and negative effects on individual ecosystems, the net result is a cooler global climate.

“We used to worry that people adding trees might create warming somewhere else,” he said. “Now, people considering adding trees can rest easy knowing that they’re cooling their cities and the rest of the world as well.”

Long Cao, senior research associate at the Carnegie Institution, said that the study’s conclusion is not definitive because the study was conducted under ideal conditions, which may not be representative of the actual global climate.

“We are unable to precisely determine the global scale now because of all of the different factors that contribute to climate change,” Cao said. “Our findings are based on climate changing at a consistent rate.”

Caldeira said that the next step in their research will be to look at regional and seasonal data because, regardless of the global cooling effect, evaporation from trees will not help cool the climate in every ecosystem. In some instances, adding trees and increasing evaporation will just result in warmer temperatures.  Additional research will be required to determine how evaporation in these areas affects global climate, he said.

“If you look at the desert where there aren’t any low-level clouds, the water vapor would just make it warmer because of the greenhouse effect,” he said.  “Your results may vary.”