Return of Trees to Eastern U.S. Kept Region Cool as Planet Warmed

Chattahoochee-Oconee National Forest.

Chattahoochee-Oconee National Forest. U.S. Forest Service

Over the 20th century, the U.S. as a whole warmed by 1.2 degrees F (0.7 degrees C), but across much the East, temperatures dropped by 0.5 degrees F (0.3 degrees C). A new study posits that the restoration of lost forest countered warming, keeping the region cool.

“This widespread history of reforestation, a huge shift in land cover, hasn’t been widely studied for how it could’ve contributed to the anomalous lack of warming in the eastern U.S., which climate scientists call a ‘warming hole,’” said lead author Mallory Barnes, of Indiana University. “That’s why we initially set out to do this work.”

The arrival of Europeans to North America brought widespread deforestation, especially in the eastern U.S., where vast areas of forest were cleared for timber or farmland. In the 20th century, this trend reversed, as trees regrew on abandoned farmland and the government worked to replant lost forest. In total, the eastern U.S. saw the restoration of some 60,000 square miles of woods, an area roughly the size of Illinois.

Not only did these trees provide shade, they also drew up water from the ground and released it through their leaves, helping to cool the air. By the end of the 20th century, weather stations in reforested areas were up to 1.8 degrees F (1.0 degrees C) cooler than those in other areas, according to the new study, published in Earth’s Future.

Still, the return of trees can only partially account for the drop in temperature. Other possible explanations include the growth of irrigation, a source of water vapor, and the uptick in particulate pollution, which reflects sunlight, thereby cooling the air. While the new study can’t fully explain the cooling, said Barnes, “we propose that reforestation is an important part of the story.”


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