A Century of Reforestation Reduced Anthropogenic Warming in the Eastern United States

Restoring and preserving the world's forests are promising natural pathways to mitigate some aspects of climate change. In addition to regulating atmospheric carbon dioxide concentrations, forests modify surface and near-surface air temperatures through biophysical processes. In the eastern United States (EUS), widespread reforestation during the 20th century coincided with an anomalous lack of warming, raising questions about reforestation's contribution to local cooling and climate mitigation. Using new cross-scale approaches and multiple independent sources of data, we uncovered links between reforestation and the response of both surface and air temperature in the EUS. Ground- and satellite-based observations showed that EUS forests cool the land surface by 1-2 degrees C annually compared to nearby grasslands and croplands, with the strongest cooling effect during midday in the growing season, when cooling is 2-5 degrees C. Young forests (20-40 years) have the strongest cooling effect on surface temperature. Surface cooling extends to the near-surface air, with forests reducing midday air temperature by up to 1 degrees C compared to nearby non-forests. Analyses of historical land cover and air temperature trends showed that the cooling benefits of reforestation extend across the landscape. Locations surrounded by reforestation were up to 1 degrees C 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 EUS. Our work indicates reforestation contributed to the historically slow pace of warming in the EUS, underscoring reforestation's potential as a local climate adaptation strategy in temperate regions. A century of eastern US reforestation has had a cooling effect that helps to explain a lack of regional warming in the 20th century, which stands in contrast to warming trends across the rest of North America during the same period. Our study shows that forests across much of the eastern United States have a substantial adaptive cooling benefit for surface temperature, and for the first time, we demonstrate that this benefit also extends to near-surface air temperature. Therefore, reforestation in temperate zones could provide a complementary set of benefits: mitigating climate change by removing carbon dioxide from the atmosphere, while also helping with adaptation to rising temperatures by cooling surface and air temperatures over large areas. Reforestation in the eastern United States (EUS) contributes to cooling the land surface and near-surface air temperature The biophysical impacts of reforestation help explain the anomalous lack of 20th-century warming in the EUS Reforestation in temperate regions can provide biophysical climate adaptation benefits by cooling surface and air temperatures