Some of the most important consequences of a warming climate are linked to the global water cycle, which includes evaporation at the Earth's surface, transport of vapour by winds, condensation of droplets as clouds and eventual precipitation (rain and snow). Runoff through river systems at the surface is also included.

Climate models – detailed mathematical representation of the Earth's atmosphere, oceans and land surface – have long predicted increases in the amount and intensity of precipitation in a warming climate, relating to subtle changes in energy flows between the atmosphere and surface and to well understood increases in the amount of water vapour that a warmer atmosphere can carry. An increase in rainfall in some regions is in stark contrast to decreases in the frequency in rain in other regions, leading to projections of more flooding but also more droughts in the future.

These projections depend on the physics incorporated within climate models. Careful comparisons with observations are vital in assessing how realistic these projections are and also for confirming current changes.

Satellite measurements from the Special Sensor Microwave Imager (SSM/I) are proving invaluable for estimating the observed changes in water vapour and precipitation over the oceans. Combining these data with the output from present-day model simulations, we found an increase in rainfall in the wettest regions of the tropics while total rainfall declines in the already dry regions. Focusing on regions of heaviest daily rainfall, the observations show an increased frequency in downpours during warm months, in agreement with model simulations.

Intriguing is the result that measurements are at the upper limit of the range of simulations made by climate models around the world. Future work will focus on understanding the cause of this discrepancy: does it relate to inadequacies in climate-model physics or in the observational records?