Biophysical Climate Forcings due to Recent Changes in Global Forest Cover

Abstract:

Deforestation impacts climate in two major ways: affecting the atmospheric CO₂ concentration and modulating the land-atmosphere fluxes of energy and water vapor. Given the important role of forests in the global carbon cycle, climate treaties account for land-based mitigation options like afforestation, reforestation and avoided deforestation or forest degradation. On the contrary, predicted climate impacts of biophysical processes, such as the exchange of energy and water vapor, are still uncertain in sign and magnitude, and therefore have not been considered in climate negotiations to date. Direct observations of the biophysical climate effects of forest losses and gains are therefore required to constrain model predictions, reduce the uncertainty of model ensembles, and provide robust recommendations to climate policy.

In this work we report an observation-driven global analysis of the biophysical impacts of forest losses and gains on the local climate, based on a combination of Earth observations of recent changes in forest cover, surface radiometric temperatures and in-situ air temperatures. Our study documents that deforestation causes local changes in skin and air temperature that varies in sign and magnitude according to the climate zone. Results show that forest losses amplify the diurnal temperature variation and increase the mean and maximum air temperature, with the largest signal in arid zones, followed by temperate, tropical and boreal zones. In the decade 2003-2012, variations of forest cover generated a global biophysical warming corresponding to 31% of the biogeochemical signal due to CO₂ emission from land use change. These experimental evidences provide a global and robust quantification of the local climate sensitivities to deforestation and a novel assessment of the mitigation potentials of forests on mean/maximum air temperatures and on the diurnal temperature variations. Overall, the observation-driven, global quantification of the biophysical signal of deforestation provided in this study may support the inclusion of land biophysics in climate negotiations and the definition of novel protocols for the measurement, reporting, and verification of these relevant effects.