Agricultural expansion dominates climate changes in southeastern Amazonia: the overlooked non-GHG forcing

Tropical deforestation changes the surface energy balance and water cycle, but how much change occurs strongly depends on the land uses that follow deforestation. Here, we quantify how recent (2000-2010) transitions among widespread land uses (i.e., forests, croplands, and pastures) altered the water and energy balance in the Xingu region of southeast Amazonia. Spatial-temporal analyses of multiple satellite data sets revealed that forest-to-crop and forest-to-pasture transitions decreased the net surface radiation (by 18% and 12%, respectively) and latent heat flux (32% and 24%), while increasing sensible heat flux (6% and 9%). Land use transitions during the 2000s reduced contemporaneous evapotranspiration (ET) in the Xingu region by 35 km(3) and warmed the land surface temperature (LST) by 0.3 degrees C. Forest-to-pasture and forest-to-crop transitions accounted for most of the observed ET reduction (25.5 km(3) and 7 km(3), respectively) and LST increase (0.2 degrees C and 0.07 degrees C). Pasture-to-crop transitions reduced ET by an additional 2.5 km(3) and increased LST by 0.03 degrees C. If land use had changed at a similar rate within the region's protected areas, ET would have decreased by another 4.7 km(3) and the surface would have warmed an additional 0.5 degrees C. Forests thus play a key role in regulating regional climate in Amazonia, with protected areas able to attenuate regional climate change caused by land use changes. Our findings show how a major non-GHG forcing, in this case agricultural expansion, has significantly altered regional climate in southeastern Amazonia and how protected forests can mitigate such changes.