An innovative approach is developed and presented to assess historical climate variations and to quantify future climate change for the entire Brahmaputra basin. Historical trends in temperature and precipitation are analysed from 1900 to 2002 for the Tibetan plateau (TP), the Himalayan belt and the floodplains (FP) using a global 100 year monthly high resolution dataset. Temperature patterns are consistent with global warming and out of the 10% warmest years from 1900 to 2002 six occurred between 1995 and 2002. No clear trends in precipitation were found and annual precipitation in the basin is mainly determined by the strength of the monsoon. Regression analysis is used to further explain monsoon precipitation. A significant inverse relation is found between air temperature differences between the FP and the TP and the strength of the monsoon, whereas the El Niño Southern Oscillation teleconnection does not have a prominent role in explaining variation in monsoon precipitation. Simulation results of six general circulation models are statistically downscaled to the spatial resolution of the observed dataset for two future storylines. The analysis predicts accelerated seasonal increases in both temperature and precipitation from 2000 to 2100. The largest changes occur on the TP and the smallest on the FP. Multiple regression analysis shows a sharp increase in the occurrence of average and extreme downstream discharges for both storylines. The strongest increases are projected for the monsoon season and the largest threat of climate change lies in the associated flooding in the densely populated FP. Copyright © 2007 Royal Meteorological Society