During El Ni˜no years, ?res in tropical forests and peatlands in equatorial Asia create large regional smoke clouds. We characterized the sensitivity of these clouds to regional drought, and we investigated their effects on climate by using an atmospheric general circulation model. Satellite observations during 2000–2006 indicated that El Ni˜no-induced regional drought led to increases in ?re emissions and, consequently, increases in aerosol optical depths over Sumatra, Borneo and the surrounding ocean. Next, we used the Community Atmosphere Model (CAM) to investigate how climate responded to this forcing. We conducted two 30 year simulations in which monthly ?re emissions were prescribed for either a high (El Ni˜no, 1997) or low (La Ni˜na, 2000) ?re year using a satellite-derived time series of ?re emissions. Our simulations included the direct and semi-direct effects of aerosols on the radiation budget within the model. We assessed the radiative and climate effects of anthropogenic ?re by analyzing the differences between the high and low ?re simulations. Fire aerosols reduced net shortwave radiation at the surface during August– October by 19.1±12.9Wm-2 (10%) in a region that encompassed most of Sumatra and Borneo (90?E–120?E, 5?S– 5?N). The reductions in net shortwave radiation cooled sea surface temperatures (SSTs) and land surface temperatures by 0.5±0.3 and 0.4±0.2?C during these months. Tropospheric heating from black carbon (BC) absorption averaged 20.5±9.3Wm-2 and was balanced by a reduction in latent heating. The combination of decreased SSTs and increased atmospheric heating reduced regional precipitation by 0.9±0.6mmd-1 (10%). The vulnerability of ecosystems to ?re was enhanced because the decreases in precipitation exceeded those for evapotranspiration. Together, the satellite and modeling results imply a possible positive feedback loop in which anthropogenic burning in the region intensi?es drought stress during El Ni˜no.