AOSC Departmental Seminar
February 5, 2015

Biophysical cooling and warming of forests - evidence from satellite observation and climate models

Yan Li
University of Maryland and Peking University

Forest can affect climate through biophysical and biogeochemical processes. Thus, forest change (deforestation/afforestation) has significant impact on regional and local climate due to changes in albedo, roughness, and evapotranspiration. This study presents evidence from climate models and satellite observation to quantify the biophysical effect of forest on surface temperature.  

Deforestation experiments with a climate model indicate the resulting temperature change has clear latitudinal patterns – from warming in tropics to cooling in extra-tropics. The degree of temperature change also depends on both spatial extent and rate of vegetation change in a non-linear way. The decrease in absorbed shortwave radiation and evapotranspiration (latent heat) are two processes that largely determine the latitudinal and spatial patterns of temperature change. The resulting changes in surface energy reflect the important role of background climate, because solar radiation and precipitation have intrinsic latitude distribution that constrains the biophysical impact. However, we found the deforestation impacts in LUCID (Land-Use and Climate, Identification of Robust Impacts experiments), a model inter-comparison project to address the impact of land cover change, show large variance across models in terms of temperature response and mechanisms.  

Since current climate models cannot accurately reproduce local climate effects due to their coarse spatial resolution and uncertainties, we used satellite data to future analyze the biophysical effects of forests on local climate. Results show tropical forests have a strong cooling effect throughout the year; temperate forests show moderate cooling in summer and moderate warming in winter with net cooling annually; and boreal forests have strong warming in winter and moderate cooling in summer with net warming annually. 

Finally, we also found recent forest changes (gain and loss) during last ten years have detectable impact on local surface temperature trends that are consistent with our empirical estimation.