Conceptual framework for changes of extremes of the hydrological cycle with climate change

A physically based conceptual framework is put forward that explains why an increase in heavy precipitation events should be a primary manifestation o f the climate change that accompanies increases in greenhouse gases in the atmosphere. Increased concentrations of greenhouse gases in the atmosphere increase downwelling infrared radiation, and this global heating at the sur face not only acts to increase temperatures but also increases evaporation which enhances the atmospheric moisture content. Consequently all weather s ystems, ranging from individual clouds and thunderstorms to extratropical c yclones, which feed on the available moisture through storm-scale moisture convergence, are likely to produce correspondingly enhanced precipitation r ates. Increases in heavy rainfall at the expense of more moderate rainfall are the consequence along with increased runoff and risk of flooding. Howev er, because of constraints in the surface energy budget, there are also imp lications for the frequency and/or efficiency of precipitation. It follows that increased attention should be given to trends in atmospheric moisture content, and datasets on hourly precipitation rates and frequency need to b e developed and analyzed as well as total accumulation.