Greenhouse warming influences the water cycle by a number of mechanisms. Firstly, a warmer atmosphere holds more moisture; therefore changes in temperature are expected to lead to a global scale increase in rainfall. Warmer temperatures are expected to more strongly influence heavy precipitation than mean precipitation. Observed spatial patterns of change in heavy precipitation events seem to show similarity with changes expected due to a greenhouse warming. However, difficulties in detecting such changes are associated with the different scales represented by station-based data and model simulated rainfall extremes. Also, for physical reasons, global annual mean precipitation responds more strongly to forcing by shortwave radiation than by longwave radiation, consistent with some evidence for a detectable response to volcanic eruptions in global land rainfall. A second mechanism is increased drought with increasing evaporation due to higher temperatures. There is evidence for attributable changes in temperature extremes, although the hottest days of the year seem to show relatively small changes. A further mechanism arises from changes in the atmospheric circulation. Global mean sea level pressure shows detectable anthropogenic changes associated with strengthening of the Northern and Southern Annular modes.