A century of gradual reforestation across the American East and Southeast has kept the region cooler than it otherwise would have become, a new study shows.

The pioneering study of progress shows how the last 25 years of accelerated reforestation around the world might significantly pay off in the second half of the 21st century.

Using a variety of calculative methods and estimations based on satellite and temperature data from weather stations, the authors determined that forests in the eastern United States cool the land surface by 1.8 – 3.6°F annually compared to nearby grasslands and croplands, with the strongest effect seen in summer, when cooling amounts to 3.6 – 9°F.

The younger the forest, the more this cooling effect was detected, with forest trees between 20 and 40 years old offering the coolest temperatures underneath.

"The reforestation has been remarkable and we have shown this has translated into the surrounding air temperature," Mallory Barnes, an environmental scientist at Indiana University who led the research, told The Guardian.

"Moving forward, we need to think about tree planting not just as a way to absorb carbon dioxide but also the cooling effects in adapting for climate change, to help cities be resilient against these very hot temperatures."

The cooling of the land surface affected the air near ground level as well, with a stepwise reduction in heat linked to reductions in near-surface air temps.

"Analyses of historical land cover and air temperature trends showed that the cooling benefits of reforestation extend across the landscape," the authors write. "Locations surrounded by reforestation were up to 1.8°F cooler than neighboring locations that did not undergo land cover change, and areas dominated by regrowing forests were associated with cooling temperature trends in much of the Eastern United States."

By the 1930s, forest cover loss in the eastern states like the Carolinas and Mississippi had stopped, as the descendants of European settlers moved in greater and greater numbers into cities and marginal agricultural land was abandoned.

The Civilian Conservation Corps undertook large replanting efforts of forests that had been cleared, and this is believed to be what is causing the lower average temperatures observed in the study data.

However, the authors note that other causes, like more sophisticated crop irrigation and increases in airborne pollutants that block incoming sunlight, may have also contributed to the lowering of temperatures over time. They also note that tree planting might not always produce this effect, such as in the boreal zone where increases in trees are linked with increases in humidity that way raise average temperatures.

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