Earth's Radiation Budget and Current Changes in the Global Water Cycle

Societies and ecosystems are sensitive to changes in the availability and distribution of fresh water across the globe, in particular relating to rainfall patterns. Anticipating how regional changes in the water cycle will evolve in the future is a great challenge. Yet robust responses to global warming at the largest scales, grounded i n robust physics, are beginning to emerge from the observational record. We are confident that atmospheric moisture increases in a warming world and I will outline why t his is likely to lead to the intensification of heavy rainfall and for wet regions of the globe to become wetter and the driest regions generally drier. Advances have be en made in understanding how radiative forcings and the temperature responses to these forcings control global precipitation amount and influence tropical circulations. Challenges remain in providing useful predictions at the smallest scales, for example river catchments, which depend upon shifts in circulation patterns and regional feedbacks involving the land surface and the ocean.