The water resources of the United States of America, like the water anywhere on the planet, are an integral part of the global hydrologic cycle. Precipitation originates as evaporation from land and the oceans. Soil moisture is used by plants, which return more moisture to the atmosphere. Water that does not evaporate or transpire or seep into aquifers runs off to form the nation’s streams and rivers. Snow stored in winter in the mountains provides water for rivers and deltas in the spring and summer. Storms bring extra moisture; droughts arise from protracted periods of low rainfall – all as part of our natural climate.Over the past century, the United States has built a vast and complex infrastructure to provide clean water for drinking and for industry, dispose of wastes, facilitate transportation, generate electricity, irrigate crops, and reduce the risks of floods and droughts. This infrastructure has brought tremendous benefits, albeit at a substantial economic and environmental cost. To the average citizen, the nation’s dams, aqueducts, reservoirs, treatment plants, and pipes are largely invisible and taken for granted. Yet they help insulate us from wet and dry years and moderate other aspects of our naturally variable climate. Indeed they have permitted us to almost forget about our complex dependences on climate. We can no longer ignore these close connections.The scientific evidence that humans are changing the climate is increasingly compelling. Complex impacts affecting every sector of society, including, especially, the nation’s water resources, now seem unavoidable.This report summarizes the conclusions of the substantial body of literature on the implications of both existing climate variability and future climate change for U.S. water resources. We have identified nearly 1,000 relevant peer-reviewed studies, and that number grows larger every day. As a result, this report must be considered just a snapshot in time, a summary of what we think we know, do not know, and would like to know at the beginning of the 21st century. In the coming years, we hope and expect that our understanding of the impacts of climate changes for U.S. water resources will improve, as will our understanding of the ability of existing and new technologies, policies, economic tools, and institutions to help us mitigate and adapt to those impacts.Many uncertainties remain; indeed, we expect that uncertainties will always remain. The nature and intensity of future greenhouse gas emissions depend upon future decisions of governments and individuals, the speed of deployment of alternative energy systems, population sizes and affluence, and many more factors. The models that simulate the role of these gases in our atmosphere are imperfect. There are significant limitations in the ability of climate models to incorporate and reproduce important aspects of the hydrologic cycle. Many fundamental hydrologic processes, such as the formation and distribution of clouds and precipitation, occur on a spatial scale smaller than most climate models are able to resolve. Regional data on water availability and use are often poor. Tools for quantifying many impacts are imperfect, at best. We thus know much less about how the water cycle will change than we would like in order to make appropriate decisions about how to plan, manage, and operate water systems. At the same time, not everything is uncertain. The research done to date tells us many things, both positive and negative, about how hydrology and U.S. water resources could be affected by climate variability and changes. We have learned important things about the vulnerability and sensitivity of water systems and management rules, and we are exploring the strengths and weaknesses of technologies and policies that might help us cope with adverse impacts and take advantage of possible beneficial effects.In many cases and in many locations, there is compelling scientific evidence that climate changes will pose serious challenges to our water systems. The good news is that where climate changes are minor or where other factors dominate, the impacts on U.S. water resources may be low. In some regions and for some issues, climate changes may even reduce the risks and stresses imposed by growing populations, industrialization, and land-use changes. The bad news is that a growing body of evidence suggests that certain aspects of our water resources are very sensitive to both climate and to how we choose to manage our complex water systems. Changes in management of these systems requires understanding what changes would be most effective and then applying the will and direction of those responsible. Coping or mitigating other kinds of impacts, even if possible, may prove very costly in dollars, environmental health, and even human lives.We also note that most impacts studies have been done using information from global climate models that evaluate the effects of increases in greenhouse gas concentrations up to particular levels. At this point in time, there is no reason to believe that increasing concentrations will stop at these levels. Greater and greater impacts would be expected to result from ever increasing levels of climate change.It is vital that uncertainties not be used to delay or avoid taking certain kinds of action now. Prudent planning requires that a strong national climate and water research program be maintained, that decisions about future water planning and management be flexible, and that the risks and benefits of climate change be incorporated into all long-term water planning. Rigid, expensive, and irreversible actions in climate-sensitive areas can increase vulnerability and long-term costs. Water managers and policymakers must start considering climate change as a factor in all decisions about water investments and the operation of existing facilities and systems.A continued reliance solely on current engineering practice may lead us to make incorrect – and potentially dangerous or expensive – decisions. The United States has hundreds of billions of dollars invested in dams, reservoirs, aqueducts, water-treatment facilities, and other concrete structures. These systems were designed and for the most part are operated assuming that future climatic and hydrologic conditions will look like past conditions. We now know this is no longer true. Accordingly, two of the most important coping strategies must be to try to understand what the consequences of climate change will be for water resources and to begin planning for and adapting to those changes.More than two decades of research into the implications of climate change for water resources have improved our understanding of possible impacts and points of vulnerability. Many critical issues and some clear and consistent results have been identified. Taken together, the current state-of-the-science suggests a wide range of concerns that should be addressed by national and local water managers and planners, climatologists, hydrologists, policymakers, and the public. Many climate changes are expected. We summarize below some of those with the greatest implications for the hydrologic cycle and U.S. water resources, using a consistent set of terms to denote levels of confidence. Sidebar ES-1 lists the common terms of uncertainty used here.The Nature of Expected Climate ChangesGlobal-average and U.S.-average surface temperatures will continue to increase above recent historical levels unless there are substantial changes in both U.S. and international energy and land-use patterns (very high confidence). As greenhouse-gas emissions continue into the future, the size of these temperature increases will become larger over time.The regional and seasonal pattern of temperature increases across the U.S. will vary (very high confidence). Researchers have low confidence in estimates of detailed regional departures from these larger-scale changes.As atmospheric greenhouse-gas concentrations continue to rise, global average precipitation will increase (very high confidence).There will be changes in the timing and regional patterns of precipitation (very high confidence). Researchers have low confidence in projections for specific regions because different models produce different detailed regional results.Temperature increases in mountainous areas with seasonal snowpack will lead to increases in the ratio of rain to snow and decreases in the length of the snow storage season (very high confidence). It is likely that reductions in snowfall and earlier snowmelt and runoff would increase the probability of flooding early in the year and reduce the runoff of water during late spring and summer. Basins in the western United States are particularly vulnerable to such shifts.Average precipitation will increase in higher latitudes, particularly in winter (high confidence). Models are inconsistent in other estimates of how the seasonality of precipitation will change.Increases in annual average runoff in the high latitudes caused by higher precipitation are likely to occur (high confidence).Research results suggest that flood frequencies in some areas are likely to change. In northern latitudes and snowmelt-driven basins, research results suggest with medium confidence that flood frequencies will increase, although the amount of increase for any given climate scenario is uncertain and impacts will vary among basins.Research results suggest that drought frequencies in some areas are likely to change. The net risks to society from such changes have not been evaluated and specific projections of where such changes will occur are speculative. Models project that the frequency and severity of droughts in some areas could increase as a result of regional decreases in total rainfall, more frequent dry spells, and higher evaporation. Models suggest with equal confidence that the frequency and severity of droughts in some regions would decrease as a result of regional increases in total rainfall and less frequent dry spells.Higher sea levels associated with thermal expansion of the oceans and increased melting of glaciers will push salt water further inland in rivers, deltas, and coastal aquifers (very high confidence). It is well understood that such advances would adversely affect the quality and quantity of freshwater supplies in many coastal areas.Water-quality problems will worsen where rising temperatures are the predominant climate change (high confidence). Where there are changes in flow, complex positive and negative changes in water quality will occur. Specific regional projections are not well-established at this time because of uncertainties in how regional flows will change.Increased atmospheric carbon dioxide will affect the use of water by vegetation (high confidence), but hydrologists have low confidence in the net effects of this and other competing influences. Increasing CO2 concentrations in some circumstances can reduce the rate of transpiration from certain plants. This in turn would tend to increase runoff since less water is returned directly to the atmosphere by such vegetation, allowing a greater share of precipitation to reach streams or aquifers. Rising CO2 concentrations can also increase plant growth, leading to a larger area of transpiring tissue and a corresponding increase in transpiration.While we note that some climate scenarios can produce conditions that might reduce stresses on certain ecosystems, experience with ecosystem dynamics strongly suggests that perturbing ecosystems in any direction away from the conditions under which they developed and thrive will have adverse impacts on the health of that system. Aquatic ecosystems can be highly sensitive to hydroclimatic factors, particularly water temperature, water quality, the probability of extreme events, and flow volumes, rates, and timing. Determining the impacts on particular species or ecosystems will require additional region-specific, and ecosystem-specific, research.Ecologists have high confidence that climate warming will produce a shift in species distributions northward, with extinctions and extirpations of temperate or cold-water species at lower latitudes, and range expansion of warm-water and cool-water species into higher latitudes.The southern boundary of continuous permafrost is projected to shift north by 500 kilometers over the next 50 years due to warming projected by GCMs. A 5o C warming in Alaska would eventually melt virtually all of the subarctic permafrost in Alaska (medium confidence), which would affect more wetland area than currently found in the rest of the United States.A growing number of studies suggest that climate changes will increases the frequency and intensity of the heaviest precipitation events, but there is little agreement on detailed regional changes in storminess that might occur in a warmed world. Contradictory results from models support the need for more research, especially to address the mismatch between the resolution of models and the scales at which extreme events can occur. This issue should be regularly revisited in later assessments.What are the Major Impacts of Climate Variability and Change on U.S. Water Resources?The current state-of-the-science suggests that plausible climate changes, projected by general circulation models, raise a wide range of concerns that should be addressed by national and local water managers and planners, climatologists, hydrologists, policymakers, and the public.Detailed estimates of changes in runoff due to climate change have been produced for the United States using regional hydrologic models of many specific river basins. In spite of many remaining uncertainties, model suggest that some significant changes in the timing and amount of runoff will result from plausible changes in climatic variables (high confidence).With few exceptions, we have low confidence that we can determine specific changes for specific regions. In the arid and semi-arid western United States, it is well established that relatively modest changes in precipitation can have proportionally large impacts on runoff.Research indicates that U.S. watersheds with a substantial snowpack in winter will experience major changes in the timing and intensity of runoff as average temperatures rise (very high confidence). Reductions in spring and summer runoff, increases in winter runoff, and earlier peak runoff are all common responses to rising temperatures. The ability of existing systems and operating rules to manage these changes has not been adequately assessed.Research to date suggests that there is a risk of increased flooding in parts of the U.S. that experience large increases in precipitation (medium confidence). The Intergovernmental Panel on Climate Change concluded in 1996, and we concur, that: “the flood related consequences of climate change may be as serious and widely distributed as the adverse impacts of droughts” and “there is more evidence now that flooding is likely to become a larger problem in many temperate regions, requiring adaptations not only to droughts and chronic water shortages, but also to floods and associated damages, raising concerns about dam and levee failure.”Non-linear or threshold events are likely to occur, but are difficult to project. Examples include a fall in lake level that cuts off outflows or separates a lake into two separate parts, an increase in flood intensity that passes specific damage thresholds, and exceedance of water-quality limits.Relative sea-level rise adversely affects groundwater aquifers and freshwater coastal ecosystems (high confidence). Rising sea level causes an increase in the intrusion of salt water into coastal aquifers. Shallow island aquifers (such as those found in Hawaii, Nantucket, Martha’s Vineyard, and along the southeastern seaboard) together with coastal aquifers supporting large amounts of human use (such as those in Long Island, New York, and central coastal California) are at greatest risk. Other impacts of sea-level rise are likely to include changes in salinity distribution in estuaries, altered coastal circulation patterns, destruction of transportation infrastructure in low-lying areas, and increased pressure on coastal levee systems.Climate changes have the potential to alter water quality significantly by changing temperatures, flows, runoff rates and timing, and the ability of watersheds to assimilate wastes and pollutants. Global and regional increases in air temperature, and the associated increases in water temperature, are likely to lead to adverse changes in water quality, even in the absence of changes in precipitation. Changes in precipitation can lead to both positive and negative impacts on water quality. The net effect on water quality for rivers, lakes, and groundwater in the future depends not just on how climate might change but also on a wide range of other human actions.Lakes are known to be sensitive to a wide array of changes in climate. Even small changes in climate can produce large changes in lake levels and salinity. As air temperatures increase, fewer lakes and streams in high-latitude areas will freeze to the bottom and the number of ice-free days will increase, leading to increases in nutrient cycling and productivity. Other effects of increased temperature on lakes could include higher thermal stress for cold-water fish, improved habitat for warm-water fish, increased productivity and lower dissolved oxygen, and degraded water quality.The direct effects of climate change on freshwater ecosystems will be complex, depending on the nature of the change, the system affected, and the nature and scope of intentional interventions by humans. Work across the United States suggests a wide range of serious concerns for ecosystems, with changes in vegetation patterns, possible extinction of endemic fish species already close to their thermal limits, declining area of wetlands with reductions in waterfowl populations, concerns about stream health, and major habitat loss.Researchers express concern for the limited ability of natural ecosystems to adapt or cope with climate changes that occur over a short time frame. This limited ability to adapt to rapid changes may lead to irreversible impacts, such as extinctions. While some research has been done on these issues, far more is needed.Little work has been done on the impacts of climate change for specific groundwater basins, or for general groundwater recharge characteristics or water quality. Some studies suggest that some regional groundwater storage volumes are very sensitive to even modest changes in available recharge.What are the Major Impacts of Climate Variability and Change on Managed U.S. Water Systems?Climate change will affect the availability of water in the United States, as well as its quality, distribution, and form. Climate change will also affect the complex infrastructure and systems in place to manage the nation’s water and existing climate variability. There is a growing literature about how different climate changes may affect the infrastructure and complex systems built to manage U.S. water resources (http://www.pacinst.org/CCBib.html). Research has been conducted on potential impacts over a wide range of water-system characteristics, including reservoir operations, hydroelectric generation, navigation, and other concerns. At the same time, significant knowledge gaps remain and far more research is needed. Priorities and directions for future work should come from water managers and planners as well as from the more traditional academic and scientific research community.Dynamic management srategies can be effective in mitigating the adverse impacts of climate change, but such policies need to be implemented before such changes occur to maximize their effectiveness.Climate change will play a role in power production from conventional fossil fuel and nuclear power plants by raising cooling water temperatures and reducing plant efficiencies (medium confidence). In some circumstances, higher water temperatures will constrain plant operations.Water-borne shipping and navigation are sensitive to changes in flows, water depth, ice formation, and other climatic factors. A warming would increase the potential length of the shipping season on some northern lakes and rivers that typically freeze in winter. Decreases in river flows could reduce the periods when navigation is possible, increase transportation costs, or increase the conflicts over water allocated for other purposes. Changes in storm frequency or intensity would affect Great Lakes navigation. Research done to date suggests that the net effects of climate change may be to increase shipping and navigation costs in the Great Lakes region (medium confidence).Research in specific watersheds has shown that some major U.S. river basins are so heavily developed, with such complicated overlapping management layers, that their ability to adapt to changes in climate may be compromised.All of the physical and ecological impacts of climate change will entail social and economic costs and benefits. On top of the uncertainties in evaluating both climate change and potential impacts, evaluating the economic implications of the diverse impacts is fraught with additional difficulties, and few efforts to quantify them have been made. Ultimately, however, comprehensive efforts to evaluate costs will be necessary in order to assist policymakers and the public in understanding the implications of both taking and not taking actions to reduce or adapt to the impacts of climate change.The socioeconomic impacts of a greenhouse warming look very different depending on which climate projections are used, and on the methods and assumptions adopted by the researchers. The results published to date are a valuable guide for future assessments but policymakers should have low confidence in specific quantitative estimates. Some results are described below:Even given the uncertainties, research indicates that the possible economic impacts of reductions in flow could be very large and that some U.S. water systems are highly sensitive to climate. Under some climate scenarios, the additional costs imposed by climate changes are considerably larger than the additional costs imposed by future population growth, industrial changes, and changing agricultural water demands.The contrasting hydrologic implications of the two climate models used for the National Assessment indicate that the direction and size of many socioeconomic impacts are uncertain and likely to vary among regions.The upper end of the costs on U.S. water resources imposed by climate changes described in some studies is on the order of 0.5 percent of the nation’s total gross domestic product.There are many opportunities to adapt to changing hydrologic conditions, and the net costs are sensitive to the institutions that determine how water is managed and allocated among users.Is Climate Change Already Affecting the Nation’s Water Resources?There is a very high degree of confidence in the scientific community that unchecked increases in atmospheric greenhouse-gas concentrations will eventually lead to changes in the Earth’s climate, including the variability of that climate. Despite gaps in data, inadequate and uneven climate and hydrologic monitoring, short collection periods, and biases in instrumental records, there is an increasing amount of evidence that indicates some changes are already occurring.The evidence that humans are changing the water cycle of the United States is increasingly compelling. Some of the observed changes with the most relevance for U.S. hydrology and water resources are summarized here:The United States has, on average, warmed by two-thirds of a degree C since 1900 (very high confidence).Permafrost in the Alaskan arctic is beginning to thaw (very high confidence).Mean sea level has risen between 10 and 20 centimeters since the 1890s (very high confidence).Mountain glaciers are melting at rates unprecedented in recorded history (high confidence).Arctic ice thickness has declined significantly from levels recorded in the mid-20th century (high confidence). A comparison of these trends with model estimates reveals that the observed decreases are similar to model projections and that both trends are much larger than would be expected from natural climate variability.Vegetation is blooming earlier in spring and summer and continuing to photosynthesize longer in the fall (medium confidence).Snow and ice cover are decreasing and melting earlier, on average, while total annual snowfall in the far northern latitudes is increasing (medium confidence). Field surveys show that snow cover over the Northern Hemisphere land surface since 1988 has been consistently below averages over the last quarter century, with an annual mean decrease in snow cover of about 10 percent over North America. These changes have been linked to the observed increases in temperature.There is evidence of historical trends of both increasing and decreasing precipitation in different parts of North America since 1900. Average precipitation over the contiguous U.S. has increased by about 10 percent since 1910. The intensity of precipitation has increased for very heavy and extreme precipitation days. There is medium confidence in average U.S. results but low confidence in any particular regional changes.The timing of runoff in snowmelt-dominated rivers in the western U.S. appears to be changing (medium confidence), with a decrease in spring runoff and an increase in winter runoff. The causes of these changes are not completely understood.Human HealthThere are direct and indirect links between water availability and quality and human health. Changes in climate will affect the viability of disease vectors like mosquitoes that carry malaria or dengue fever. The transport of water-borne pathogens such as Cryptosporidium is known to be affected by changes in precipitation and runoff intensities and by land-use practices. The distribution of Vibrio cholerae, the bacteria responsible for cholera, is affected by climate, including El Niño frequency and intensity, temperature, and ocean salinity.No clear evidence is available yet to conclude how climate change will ultimately affect these factors or to suggest any climate-related change in the incidences of these kinds of diseases in the United States, but we urge more research and careful monitoring of water-related disease vectors and data.AgricultureRecent studies of U.S. agriculture suggest that overall production of food may not be seriously threatened by climate changes as currently projected by GCMs. Indeed, in the climate scenarios evaluated for the National Assessment the net economic effects of changes in agriculture were generally positive, although there were substantial regional differences and some regions suffered production declines. The overall results showed a decline in water demand for irrigation, largely because of the differential effects of climate change on productivity of irrigated versus non-irrigated crops, and the assumed positive effects on plants of higher levels of CO2. At the same time, there are serious caveats that accompany the research done to date, including some related to water availability and quality. Reliable information on changes in storm frequency and intensity is not yet available. Integration of indirect effects of climate change on hydrology and water into agroclimatic models has not yet been widely done, particularly effects of pests, soil conditions, disease vectors, and socioeconomic factors. Even less work has evaluated the impacts of changes in climate variability for agriculture. Integrating these and other links between water and food should remain a high priority for researchers.ForestsResearch suggests that climate change can lead to dramatic long-term changes in forest health and distribution. These factors depend partly on how precipitation and runoff patterns will change. But changes in forest conditions will, in turn, have locally and regionally important effects on runoff, soil erosion, soil salinization, groundwater quality, and more. These effects have not been adequately assessed.Coastal EcosystemsImpacts of climate change on water resources will have a wide range of consequences for coastal ecosystems. Ecosystem health will be affected by changes in the quality and quantity of freshwater runoff into coastal wetlands, higher water temperatures, extreme runoff rates or altered timing, and the ability of watersheds to assimilate wastes and pollutants. The net effect on coastal systems depends not just on how climate might change but also on a wide range of other human actions, including construction and operation of dams that trap sediments and nutrients, water withdrawal rates and volumes, disposal of wastes, and more.Higher average or a greater range of flows of water could reduce pollutant concentrations or increase erosion of land surfaces and stream channels, leading to more sediment and greater chemical and nutrient loads in rivers and coastal deltas. Lower average flows could reduce dissolved oxygen concentrations, reduce the dilution of pollutants, reduce erosion, and increase zones with high temperatures. For almost every source or water body, land use and agricultural practices have a significant impact on water quality. Changes in these practices, together with technical and regulatory actions to protect water quality, can be critical to future water conditions.Other ImpactsThe impacts of climate change on U.S. water resources have the potential to affect international relations at the nation’s northern and southern borders, where shared watersheds can lead to local and international political disputes. International agreements covering these shared waters do not include provisions for explicitly addressing the risks of climate-induced changes in water availability or quality.A change in flood risks is one of the potential effects of climate change with the greatest implications for human well-being. Few studies have looked explicitly at the implications of climate change for flood frequency, in large part because of the lack of detailed regional precipitation information from climate models and because of the substantial influence of both human settlement patterns and water-management choices on overall flood risk.Climate change is just one of a number of factors influencing the hydrological system and water resources of the United States. Population growth, changes in land use, restructuring of the industrial sector, and demands for ecosystem protection and restoration are all occurring simultaneously. Current laws and policies affecting water use, management, and development are often contradictory, inefficient, or unresponsive to changing conditions. In the absence of explicit efforts to address these issues, the societal costs of water problems are likely to rise as competition for water grows and supply and demand conditions change.Coping and AdaptationThere are many opportunities to reduce the risks of climate variability and change for U.S. water resources. The nation’s water systems are highly developed and water managers have a long history of adapting to changes in supply and demand. Past efforts have been focused on minimizing the risks of natural variability and maximizing system reliability. Many of the approaches for effectively dealing with climate change are little different than the approaches already available to manage risks associated with existing variability. Tools for reducing these risks have traditionally included supply-side options such as new dams, reservoirs, and pipelines, and more recently, demand-management options, such as improving efficiency, modifying demand, altering water-use processes, and changing land-use patterns in floodplains. This work is going on largely independently of the issue of climate change, but it will have important implications for the ultimate severity of climate impacts.Sole reliance on traditional management responses is a mistake: first, climate changes are likely to produce – in some places and at some times – hydrologic conditions and extremes of a different nature than current systems were designed to manage; second, climate changes may produce similar kinds of variability but outside of the range for which current infrastructure was designed and built; third, relying solely on traditional methods assumes that sufficient
