Streamflow is expected to decrease in many temperate river basins as global temperature increases. The greatest decreases are expected in areas that are currently arid or semi-arid. Most models project decreases in the southwest United States, while slight increases are projected in the northeast and northwest. There is strong agreement among models that runoff in the Arctic and other high-latitude areas, including Alaska, will increase. The greatest decreases per degree within the United States are projected for the Rio Grande Basin (about 12% per degree) and increases of about 9% per degree are expected in Alaska; see Figure O.2. Thus, warming of a few degrees can be expected to lead to large perturbations to water resources, especially in the southwestern and southern parts of the United States, many of which are already facing water resources challenges due to growing population and environmental issues. {5.3}

Extreme temperatures are expected to increase in a warmer world. For example, for about 3ºC of global warming, 9 out of 10 Northern Hemisphere summers are projected to be “exceptionally warm” in nearly all land areas, and every summer is projected to be “exceptionally warm” in nearly all land areas for about 4ºC, where an “exceptionally warm” summer is defined as one that is warmer than all but about 1 of the 20 summers in the last decades of the 20th century. A complete review of the many studies evaluating changes in extremes in various regions is beyond the scope of the present study. Here we use as an illustrative example the effect of warming on seasonal extremes, based on a simple shift in the distribution of temperatures, using pattern scaling. Some studies have indicated the possibility that the variance of the distribution of temperature will increase, especially in regions that are projected to become drier (e.g., the Mediterranean basin), further enhancing the chances of extreme seasonal temperatures beyond that estimated here (see Figure O.3). {4.5}

Extreme precipitateon (heaviest 15% of daily rainfall) is likely to increase by about 3-10% per degree C as the atmospheric water vapor content increases in a warming climate, with changes likely to be greater