Global warming is intensifying the water cycle — the process of precipitation, infiltration, and evaporation. In the future, the wet will get wetter and the dry will become drier.
 | A pedestrian tries to stay dry while walking downtown in a storm in San Diego in January 2008 (Photo: Reuters) |
Every system requires energy: the more energy, the more vigorous the system. The sun powers the life-giving system that is the water cycle and thanks to greenhouse gases, there is more energy, or simply heat, in the system.
As result, the amount of moisture circulating through the atmosphere has increased as well. That’s why the Intergovernmental Panel on Climate Change (IPCC) predicts that global warming will lead to an “intensification” of the water cycle, leading to heavier rainfall in some areas, and more severe drought in other regions.
It is already happening. “In West Africa, there is anecdotal information that the monsoon is starting later, finishing earlier and is characterized by more intensive rain. I have heard the same from Sri Lanka,” says Colin Chartres, director of the International Water Management Institute (IWMI).
With impacts differing all over the globe, a priority for scientists is to figure out how climate change will affect specific river basins and other local water resources.
Precipitation
As a general trend, wet areas are getting wetter, and dry areas are getting drier. The United Nations Environment Programme (UNEP) reports that rainfall could increase 10 to 40 percent in wet regions, mostly in the northern hemisphere, and decrease 10 to 30 percent in dry regions, mostly in the southern hemisphere. Extreme rainfall in the form of more powerful and frequent storms, and extreme drought, will be more frequent, and more intense.
 | Water Cycle (click on the image to enlarge)The major components of the world water cycle (Graphic: Credit: NASA/GSFC) |
One reason for this is an atmospheric circulation pattern called the Hadley cell. At the Equator, warm, moist air rises, cools, condenses, and falls in tropical downpours before moving off towards the Poles. In the subtropics, the dry air descends and sucks up the moisture from the ground, creating the Sahara, the Australian deserts, and the arid American southwest.
Global warming will intensify and extend this cycle, making dry areas even drier and extending the range of these hotspots into traditionally more temperate areas such as the Mediterranean and the American Midwest
Likely areas of dramatic climate-related change are in sub-Saharan Africa, which is increasingly vulnerable to drought. Large coastal cities and big river deltas in Asia fear rising water levels occasioned by intense rainfall river flow, while small island nations in the Pacific and Indian oceans fear being drowned by rising sea levels.
The poor will be the first to feel the impact of changing precipitation patterns. In the meantime, greater rainfall in northern areas could replenish groundwater stocks, although it could also lead to excessive river flows and flooding.
Part 2: The Water Flows Faster - Accelarated Evaporation and Solutions
