Potential Effects of Climate Change3
Climate change is likely to have a number of effects on water supplies, which will vary considerably across and within regions. For example, during the past several decades there have been noticeable shifts in the frequency and distribution of precipitation. Dry areas are expected to get drier and wet areas wetter. Scientists project that the subtropics, where one finds most of the world’s deserts, will experience a 5 to 10 percent reduction in precipitation for each degree of global warming. Subpolar and polar regions, on the other hand, are likely to experience more precipitation, especially in the winter.
In addition, warmer temperatures mean more evaporation; warmer air can also hold more water vapor, leading to a measurable increase in the intensity of precipitation in some areas. Observations from many parts of the world indicate that a statistically significant increase in the intensity of heavy rainstorms has occurred. One of the effects of this escalation is an increased risk of flooding. And the intensity is projected to increase even in areas when overall precipitation declines.
These changes in precipitation will have a direct effect on annual streamflow, which is essentially equivalent to runoff, the amount of snow or rain that flows into rivers and streams. This is a key measure of the availability of freshwater. Climate models project that streamflow will decrease in many temperate river basins, especially in arid and semiarid regions. As discussed in the next section, a key question is whether the effects of climate change on water supply, combined with significant human impacts on supply and demand, could lead to tensions and conflict that become concerns for U.S. security.
Water and Conflict
Disputes over water date back millennia; the Water Conflict Chronology List, for example, begins with an account of a Sumerian legend from 3,000 BCE that resembles the Biblical story of Noah. Five hundred years later two Sumerian city-states, Lagash and Umma, provided the first written record of going to war over water; the rulers of Lagash diverted water from boundary canals to deny supplies to neighboring Umma, setting the conflict in motion.4
The idea that water scarcity could be a direct source of violent internal or international conflict has produced a literature on “water wars” from both academic and policy sources (see, for example, Cooley, 1984; Starr,
3 The material in this section is taken from National Research Council (2012b:23–25).