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 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.
The availability of freshwater to meet the demands of a growing and increasingly affluent population while sustaining a healthy environment has emerged as one of the nation’s primary resource issues. Concerns about water are based in part on uncertainties over the availability of supplies stemming from the vicissitudes of the hydrologic cycle, growing populations, and the prospect that greenhouse gas-induced climate changes will alter the cycle in uncertain ways.
Global climatic changes will have major effects on precipitation, evapotranspiration, and runoff. But estimating the nature, timing, and even the direction of the impacts at the regional and local scales of primary interest to water planners involves many uncertainties. While specific regional impacts will depend on future climate changes as well as uncertain economic, institutional, and structural conditions, some consistent and robust results can be described.
In the relatively arid and semiarid western United States, modest changes in precipitation can have proportionally large impacts on water supplies. 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. Because the temperature projections of climate models are less speculative than the projections of precipitation, temperature-induced shifts in the relative amounts of rain and snow and in the timing of snowmelt in mountainous areas are considered likely. Coping strategies should now be explored.
Where extensive water systems have been built, there are untapped opportunities for rethinking operating and management rules. At the same time, where water systems are already under stress because of limited supplies or water-quality problems, climatic changes may impose different and greater stresses than those already anticipated by water planners.
Climate-induced changes in hydrology will affect the magnitude, frequency, and costs of extreme events, which produce the greatest economic and social costs to humans. Flooding, the nation’s most costly and destructive natural disaster, could become more common and extreme. Recent reports of the Intergovernmental Panel on Climate Change (IPCC) suggest that a greenhouse warming is likely to increase the number of intense precipitation days and flood frequencies in northern latitudes and snowmelt-driven basins. These reports also suggest that the frequency and severity of droughts could increase in some areas as a result of a decrease in total rainfall, more frequent dry spells, and greater evapotranspiration.
Many different general circulation models (GCMs) have been developed and improved over the past decades to understand the implications of increased concentrations of greenhouse gases on the climate. The ongoing National Assessment of the impacts of climate change on the United States is evaluating the implications of two different models – the Hadley and Canadian GCMs. Estimates of the impact of climate change on runoff within the water resource basins and subbasins in the conterminous United States using the outputs of these two general circulation models show similarities and sharp differences. For both models, temperatures and potential evapotranspiration rise significantly by 2100. But the uncertainties about the implications of climate change for water resources are illustrated by the contrasting projections of runoff based on these models. Estimates based on the Hadley model indicate flooding could increase in much of the country, while those based on the Canadian climate model indicate increased water scarcity would pervade much of the country. Both scenarios could result in sharply higher socioeconomic costs. Results based on these GCM outputs as well as more detailed regional studies emphasize two points: the detailed regional impacts of a greenhouse warming on future water supplies are uncertain, and runoff is sensitive to changes in temperature and precipitation.
Climatic changes will affect the demand as well as the supply of water. These changes may influence a wide range of water-system components, including reservoir operations, water quality, hydroelectric generation, and navigation. Irrigation, the largest consumer of U.S. water, is particularly sensitive to climate conditions; demand for irrigation water tends to increase as conditions become hotter and drier. Instream water uses such as hydroelectric power generation, navigation, recreation, and ecosystem maintenance are also sensitive to changes in the quantity, quality, and timing of runoff stemming from greenhouse warming.
Water is becoming increasingly scarce and expensive independent of climate change. Water demands are growing with population, incomes, and an appreciation for the values of instream ecological and recreational uses. Increased withdrawals of water for domestic, industrial, and agricultural uses, however, are limited by high economic costs and by the limited opportunities for increasing withdrawals from rivers or streams without adversely impacting instream uses. Improving the efficiency of our water use is rapidly becoming the primary means of balancing limited water supplies with growing demands. But as more people become dependent on a given water supply, vulnerability to drought can increase. Moreover, the capacity to store water to protect against floods and droughts and deal with the uncertainties of climate change appears to be declining because reservoir storage losses due to sedimentation have exceeded additions through new construction in recent years.
The impacts of climate change on water quality have received less attention than the impacts on quantity, but current research raises several concerns. Potential negative implications of climate change include reductions in dilution flows, increased storm surges, and higher water temperatures. Low flows in many western rivers will lead to increases in salinity levels to downstream water users; higher flows could help reduce some water quality concerns. Warmer water could threaten aquatic life directly as cool-water habitats disappear and indirectly as dissolved oxygen levels decline with higher temperatures. An increase in days with more intense precipitation could increase the agricultural and urban pollutants washed into streams and lakes, further reducing oxygen levels. Heavy rainfall is primarily responsible for soil erosion, leaching of agricultural chemicals, and runoff of urban and livestock wastes and nutrients into water bodies. Sea-level rise would contribute to saltwater intrusion into rivers and coastal aquifers.
The socioeconomic implications of both climate and non-climate impacts on water supply and demand will depend in large part on both the ability to adapt to change and on whether water managers and planners take action. Current laws and policies affecting water use, management, and development are often inefficient and unresponsive to changing conditions. The costs of these inefficiencies will likely rise if water becomes scarcer and supply and demand conditions change. There are four promising opportunities for reducing the costs and conflicts of supplying future water demands and adapting to future climate variability: (1) establishing incentives for using, conserving, and protecting supplies; (2) providing opportunities for transferring water among competing uses in response to changing conditions; (3) influencing how water is managed within and among basins; and (4) re-evaluating the operations of the existing infrastructure to address climate and non-climate changes.
All water-supply systems were designed and are operated on the assumption that future climate will look like past climate. Additional dams, reservoirs, aqueducts, levees, and other structures may eventually be needed to help adapt to climate change. But, when possible, costly and irreversible decisions to build water-related infrastructure should be postponed in anticipation of obtaining better information about the likely consequences and costs of a greenhouse warming. Water managers already have a wide variety of tools available for dealing with risk and uncertainty. One view holds that nothing different needs to be done now to cope with future climate changes as these tools will prove sufficient for dealing with future climate changes. But regional modeling studies suggest that even modest changes in climate can lead to changes in water availability outside the range of historical hydrologic variability. It is unclear whether some climate changes will be so rapid or of such large magnitude as to overwhelm existing systems before current management approaches can react. These uncertainties suggest the wisdom of re-examining design assumptions, operating rules, and contingency planning for a wider range of climate conditions than traditionally used. Maintaining options and building in flexibility are important for designing efficient water programs in the context of climate change.