Climatic Impact of Global-Scale Deforestation: Radiative versus Nonradiative Processes

Abstract A fully coupled land–ocean–atmosphere GCM is used to explore the biogeophysical impact of large-scale deforestation on surface climate. By analyzing the model sensitivity to global-scale replacement of forests by grassland, it is shown that the surface albedo increase owing to deforestation has a cooling effect of −1.36 K globally. On the other hand, forest removal decreases evapotranspiration efficiency and decreases surface roughness, both leading to a global surface warming of 0.24 and 0.29 K, respectively. The net biogeophysical impact of deforestation results from the competition between these effects. Globally, the albedo effect is dominant because of its wider-scale impact, and the net biogeophysical impact of deforestation is thus a cooling of −1 K. Over land, the balance between the different processes varies with latitude. In temperate and boreal zones of the Northern Hemisphere the albedo effect is stronger and deforestation thus induces a cooling. Conversely, in the tropics the net impact of deforestation is a warming, because evapotranspiration efficiency and surface roughness provide the dominant influence. The authors also explore the importance of the ocean coupling in shaping the climate response to deforestation. First, the temperature over ocean responds to the land cover perturbation. Second, even the temperature change over land is greatly affected by the ocean coupling. By assuming fixed oceanic conditions, the net effect of deforestation, averaged over all land areas, is a warming, whereas taking into account the coupling with the ocean leads, on the contrary, to a net land cooling. Furthermore, it is shown that the main parameter involved in the coupling with the ocean is surface albedo. Indeed, a change in albedo modifies temperature and humidity in the whole troposphere, thus enabling the initially land-confined perturbation to be transferred to the ocean. Finally, the radiative forcing framework is discussed in the context of land cover change impact on climate. The experiments herein illustrate that deforestation triggers two opposite types of forcing mechanisms—radiative forcing (owing to surface albedo change) and nonradiative forcing (owing to change in evapotranspiration efficiency and surface roughness)—that exhibit a similar magnitude globally. However, when applying the radiative forcing concept, nonradiative processes are ignored, which may lead to a misrepresentation of land cover change impact on climate.