Land Use and climate change interactions in tropical South America

Abstract:

Ongoing agricultural expansion in Amazonia and the surrounding areas of Brazil is expected to continue over the next several decades as global food demand increases. The transition of natural forest and savannah ecosystems to pastureland and agricultural crops is predicted to create warmer and drier atmospheric conditions than the native vegetation. Compounding this effect, climate change is likely to lead to reduced transpiration fluxes as plants become more water efficient under higher atmospheric carbon dioxide (CO₂) levels. Here we investigate the expected impacts of predicted future land use on the climate of South America as well as the potential impacts of increasing CO₂. We find that the climate response to land use change generally consistent with expectations from previous global modeling simulations with drier conditions resulting from deforestation, however the direct changes in precipitation are relatively small (on order of a few percent). Local drying from land use change is driven primarily by decreases in evapo-transpiration associated with the loss of forest, and concomitant increases in runoff. Significant changes in convectively available potential energy and convective inhibition during the transition to the wet season indicate that the decrease in surface latent heat flux is indeed leading to a drier atmosphere, however these changes occur around a mean climatological state that is already very favorable for convection, and thus lead to relatively small changes in precipitation. The physiological effects of increasing CO₂ alone also drive a reduction in precipitation, which is compounded by radiation-driven circulation changes. If these land use changes were to occur under a background state of drier conditions, such as those predicted for the future global climate model experiments, this additional atmospheric drying driven by land use change may be sufficient to decrease precipitation more substantially.