Throughout the past century, surface temperature over the majority of the earth’s surface has increased rapidly. In the southeastern United States, an anomalous lack of warming in the 20th century, deemed the southeast "warming hole", coincided with widespread reforestation in the region. Reforestation can result in net warming or a net cooling, depending on the balance of competing mechanisms on energy balance components. Recent studies underscore the climate mitigation potential of widespread reforestation in terms of carbon dioxide uptake, however, at least in the temperate zone, the potential cooling impacts of reforestation via changes in surface energy balance remain largely unknown.We hypothesized that the biophysical impacts of reforestation are responsible for observed regional near-surface cooling in the southeast United States.
Results/Conclusions
Here, we demonstrate that biophysical processes that link forest cover and local temperature can help explain the lack of expected warming in the southeast United States. Using a combination of remotely sensed land surface temperature and gridded air temperature data, we found greater daytime surface cooling in forests compared to short-statured vegetation types (i.e. grasslands, croplands, and pasturelands) during the growing season. Our synthesis of NEON and Ameriflux eddy covariance sites across the Southeast US revealed that daytime surface temperatures in forests are consistently cooler compared to background air temperature than short-statured vegetation types in both the growing and dormant seasons. Analysis of fine-resolution satellite imagery across forest:crop boundaries uncovered a smooth temperature transition from cooler forests to warmer croplands, indicating mixing between the air and the surface. An analysis of historical temperature records from over 200 USHCN weather stations showed that cooling air temperature trends in forested sites coincided with the maturation of forests following widespread agricultural abandonment and tree plantings from New Deal work programs in the 1940s. Our results highlight the climate mitigation potential of forests through changes in energy balance that lead to surface and regional cooling.