Impact of atmospheric aerosol from biomass burning on Amazon dry-season drought

It is becoming increasingly apparent that the future of the Amazon rainforest is under threat from both climate change and agricultural practices such as deforestation and biomass burning. Atmospheric aerosols are likely to play an important role in the interaction between deforestation, fire and drought, but until now, observations of aerosol optical depth (AOD) in this region have been limited to time series of less than 7 years for satellite retrievals, or to single-site measurements. Here we use a 13-year time series of Along Track Scanning Radiometer derived AOD measurements to examine the role of aerosols in biosphere-climate interactions over the Amazon. The seasonal cycle of AOD shows peaks in March and September. The September peak is caused by local dry-season biomass burning. The March peak has not been identified previously but is coincident with more remote fires located in northern South America. A decreasing trend in dry-season AOD between 1995 and 2000 and a subsequent increase from 2000 to 2004 can be explained by deforestation practices driven by economic forces, whereas even higher AOD levels in 2005 were probably caused more by the exceptional drought of that year. Throughout the time series, dry-season AODs are inversely correlated with dry-season precipitation, suggesting a positive feedback between aerosols and drought that may contribute to enhanced drought under climate change.