At a given height above sea level, the temperature of the atmosphere is typically warmer over a higher surface than a lower one. This “elevated heating” of the atmosphere above large plateaus, including the Tibetan, Colorado, and Bolivian plateaus, is believed to strongly influence regional climate by driving continental-scale circulation. In contrast, an increase in the albedo, the reflectivity of the land surface, at higher elevations due to changing land cover results in a powerful cooling effect. Despite the relevance for regional climate, however, the competing effects of these two parameters have not been systematically studied.
To investigate this relationship, Hu and Boos analyzed satellite-derived observations of elevation and albedo to confirm that the reflectivity of the land surface typically increases with elevation at nonpolar latitudes (60°N–60°S) and that this relationship exists year-round. The team then modeled the parameters’ combined effects over the Tibetan Plateau, where elevated heating has long been assumed to drive southern Asia’s strong summer monsoon.
The results show that cooling caused by the increase in surface albedo nearly cancels out the plateau’s elevated heating effect. This result, the team argues, explains why recent studies have concluded that the Tibetan Plateau does not act as Asia’s dominant heat source as one would expect from the elevated heating effect. This elevated heating effect, however, does not remain constant as the climate changes; by varying the surface albedo in their model runs, the researchers determined that this effect becomes weaker and can potentially reverse in warmer climates.
In contrast to earlier research, which has largely attributed variations in the South Asian monsoon to changes in surface elevation, this study indicates that differences in surface albedo must be jointly considered. If, for example, forests and lakes covered more of Tibet during the Pliocene than today, the plateau would have had a lower albedo and correspondingly higher atmospheric temperatures above it, even if it sat at its modern elevation. Differences in albedo may therefore be as important as changes in surface elevation for the evolution of regional climate at Earth’s low to middle latitudes. (Geophysical Research Letters, https://doi.org/10.1002/2016GL072441, 2017)
—Terri Cook, Freelance Writer