Deforestation Drives Cooling at Mid- to High Latitudes
Deforestation in mid- to high latitudes is hypothesized to have the potential to cool the Earth’s surface by altering biophysical processes. When continental-scale land clearing is included in climate models, cooling is triggered by increases in surface albedo and is reinforced by a land albedo-sea ice feedback. This feedback is a key component of the model predictions; without it other processes overwhelm the albedo effect to generate warming. Ongoing activities, such as land management for climate mitigation, are occurring at local scales (hectares) presumably too small to generate the feedback. It is not known if the intrinsic biophysical mechanism on its own can consistently change surface temperatures. The effect of deforestation on climate has also not been demonstrated over large areas from direct observations. Now, DOE researchers show that surface air temperature is lower in open land than in nearby forested land. The effect is 0.85°±0.44K (mean ± one standard deviation) north of 45°N (essentially north of the U.S.-Canadian border) and 0.21°±0.53K southwards. Below 35°N (south of Tennessee, all of Texas and New Mexico, and southern California), there is weak evidence that deforestation leads to warming. Results are based on temperature comparisons at forested eddy covariance towers in the United States and Canada and, as a proxy for small areas of cleared land, nearby surface weather stations. Night-time temperature changes unrelated to changes in surface albedo are also an important contributor to the overall cooling effect. The observed latitudinal dependence is consistent with theoretical expectations of changes in energy loss from convection and radiation across latitudes in both the daytime and night-time phase of the diurnal cycle, the latter of which remains uncertain in climate models.
Lee, X., M. L. Goulden, D. Y. Hollinger, A. Barr, T. A. Black, G. Bohrer, R. Bracho, B. Drake, A. Goldstein, L. Gu, G. Katul, T. Kolb, B. E. Law, H. Margolis, T. Meyers, R. Monson, W. Munger, R. Oren, K. T. Paw, A. D. Richardson, H. P. Schmid, R. Stabler, S. Wofsy, and L. Zhao. 2011. “Observed Increase in Local Cooling Effect of Deforestation at Higher Latitudes,” Nature 479, 384-87. DOI: 10.1038/nature10588.