Foreword

Water and Global Climate Change

Eileen Claussen, Executive Director, Pew Center on Global Climate Change

Availability of an adequate, safe water supply is critical to the health, economy, and environment of any nation and its people. The United States, on average, is well-endowed with water. However, this year's spring floods and summer droughts illlustrate the importance of wide seasonal fluctuations in precipitation. Further, the growing conflicts over environmental and developmental water uses are an indication that water is becoming increasingly scarce.

Current scientific research shows that climate change will have major effects on precipitation, evapotranspiration, and runoff - and ultimately on the nation's water supply. Climate-induced changes in the water cycle likely will affect the magnitude, frequency, and costs of extreme weather events as well as the availability of water to meet growing demand. Recent reports, including the Pew Center report, "The Science of Climate Change," show that climate change is likely to increase the number of days of intense precipitation and the frequency of floods in northern latitudes and snowmelt-driven basins. The frequency and severity of droughts could also increase as a result of a decrease in total rainfall, as well as more frequent dry spells and greater evapotranspiration.

Because of uncertainties about changes in precipitation, many uncertainties exist in predicting specific regional impacts of large-scale changes. Still, some consistent impacts can be identified. In the arid and semiarid western United States, relatively modest changes in precipitation can have proportionally large impacts on water supplies. And in mountainous watersheds, higher temperatures will increase the ratio of rain to snow, accelerate the rate of spring snowmelt, and shorten the overall snowfall season, leading to more rapid, earlier, and greater spring runoff.

"Water and Global Climate Change" is the third in a series examining the potential impacts of climate change on the environment and society. This report identifies impacts not only to the quantity, but also to the quality of the water supply. Changes in stream flows, increased storm surges, and higher water temperatures all could negatively affect the health of the nation's water supply. An increase in the number of days of intense precipitation also could increase the agricultural and urban pollutants washed into streams and lakes. The resulting rise in sea level would contribute to saltwater intrusion into rivers, estuaries, and coastal aquifers.

The authors and the Pew Center are grateful for the input of Drs. John Boland, Kenneth Strzepek, and Barbara Miller, who reviewed previous drafts; and to Joel Smith and Brian Hurd of Stratus Consulting for their oversight of this Environmental Impacts series.