Current climate models suggest that global warming will result in more frequent extreme hydrological events (floods and droughts). These results, however, must be tempered with the fact that current climate models do not realistically represent many of the processes important to the formation of clouds and precipitation at various time and space scales. For instance, the diurnal cycle of precipitation is poorly represented in most climate models. The proper representation of precipitation is a major challenge to global climate models, which typically only resolve processes at 200- to 400-km scales, and is a focus of current scientific research. This chapter addresses the current understanding of the likely climate impact on precipitation, as well as some of the key challenges facing climate modelers with regard to improving future projections of precipitation.
Heated by sunlight and atmospheric radiation, water evaporates from the ocean and land surfaces, moves along with winds in the atmosphere, condenses to form clouds, and falls back to the Earth's surface through rain and snow, some of which flows back to oceans through rivers, thereby completing this global water cycle (Figure 16.1).
Daily newspaper headlines of floods and droughts reflect the critical importance of the water cycle, in particular, precipitation in human affairs.