Abstract: A six member ensemble of 20th century simulations with changes to only time-evolving global distributions of black carbon aerosols in a global coupled climate model is analyzed to study the effects of black carbon (BC) aerosols on the Indian monsoon.  The BC aerosols act to increase lower tropospheric heating over south Asia and reduce the amount of solar radiation reaching the surface during the dry season as noted in previous studies.  The increased meridional tropospheric temperature gradient in the pre-monsoon months of March-April-May, particularly between the elevated heat source of the Tibetan Plateau and areas to the south, contributes to enhanced precipitation over India in those months.  With the onset of the monsoon, the reduced surface temperatures in the Bay of Bengal, Arabian Sea, and over India that extend to the Himalayas act to reduce monsoon rainfall over India itself, with some small increases over the Tibetan Plateau.  Precipitation over China generally decreases due to the BC aerosol effects.  There is a weakened latitudinal SST gradient due to BC aerosols as seen in observations, and is present in the multiple forcings experiments with CCSM3 that include natural and anthropogenic forcings (including BC aerosols).  The BC aerosols and consequent weakened latitudinal SST gradient in those experiments are associated with increased precipitation during MAM in northern India and over the Tibetan Plateau, with some decreased precipitation over southwest India, the Bay of Bengal, Burma, Thailand, and Malaysia as seen in observations.  In the summer monsoon season, the BC aerosols appear to have contributed to observed decreasing precipitation trends over parts of India, with increasing trends in the CCSM3 multiple-forcing experiments and observations over Pakistan and the Tibetan Plateau, and decreasing trends over Bangladesh, Burma, and Thailand.