Editorial LinksRivers and RegionsIssuesDisactersPolicy

May / June 2003

Zambian Water Pictures
Water Resource Info Database

OPINION - Confusing Cause and Effect


Founder : Len Abrams
Water Policy International

    Return to the Climate Change Page    

Although this document pertains to the United States of America, it contains a number of points which are of international importance.

Water: The Potential Consequences of Climate Variability and Change for the Water Resources of the United States

Executive Summary of the Water Sector Report of the National Assessment

September 2000

Download full report (pdf format)
from the Pacific Institute site





Nature of expected climate changes
Major Impacts of Climate Variability and Change on Water Resources
Major Impacts of Climate Variability and Change on Managed Water Systems
Is Climate Change Already Affecting Water Resources?

Effects on Other Sectors

Human Health
Coastal Ecosystems
Other Impacts


Coping and Adaptation
Key Messages for Water Managers, Planners, and Interested Members of the Public
Research Needs

The Report of the Water Sector Assessment Team of the National Assessment of the Potential Consequences of Climate Variability and Change for the U.S. Global Change Research Program
Lead Author
Peter H. Gleick, Pacific Institute for Studies in Development, Environment, and Security

Co-Chairs of Water Sector

D. Briane Adams, U.S. Geological Survey
Peter H. Gleick, Pacific Institute for Studies in Development, Environment, and Security

This Report was supported by the U.S. Department of the Interior though the U.S. Geological Survey (Grant #  98HQAG2118)


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 nationís 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 Ė 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 nationís 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 nationís 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 Ė and potentially dangerous or expensive Ė 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 Changes

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.

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 nationís 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.

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:

Is Climate Change Already Affecting the Nationís 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 Earthís 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:

Effects on Other Sectors

Five separate sectoral reports have been prepared for the National Assessment. In addition to this one on water, work is available on agriculture, human health, coastal ecosystems, and forests (see http://www.nacc.usgcrp.gov). None of the indicated impacts on these sectors is independent of what happens to U.S. water resources and water systems. Yet truly integrated analysis of possible impacts has not yet been done. We urge further work on the combined synergistic effects of climate change on the United States and we offer below a few comments on some critical issues.

Human Health

There 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 NiŮo 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.


Recent 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.


Research 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 Ecosystems

Impacts 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 Impacts

The impacts of climate change on U.S. water resources have the potential to affect international relations at the nationís 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 Adaptation

There are many opportunities to reduce the risks of climate variability and change for U.S. water resources. The nationís 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 Ė in some places and at some times Ė 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 time and information will be available before the onset of large or irreversible climate impacts to permit managers to respond appropriately; and fourth, this approach assumes that no special efforts or plans are required to protect against surprises or uncertainties.

The first situation could require that completely new approaches or technologies be developed. The second could require that efforts above and beyond those currently planned or anticipated be taken early. Complacency on the part of water managers, represented by the third and fourth assumptions, may lead to severe impacts that could have be mitigated or prevented by cost-effective actions taken now.

As a result, we make the following observations and recommendations:

Key Messages for Water Managers, Planners, and Interested Members of the Public

Climate is not static and assumptions made about the future based on the climate of the past may be inappropriate. Assumptions about the probability, frequency, and severity of extreme events used for planning should be carefully re-evaluated.

Climate changes will be imposed on top of current and future non-climate stresses. In some cases, these changes will be larger than those expected from population growth, land-use changes, economic growth, and other non-climate factors.

Certain threshold events may become more probable and non-linear changes and surprises should be anticipated, even if they cannot be predicted.

The time lags between identifying the nature of the problems, understanding them, prescribing remedies, and implementing them are long. Waiting for relative certainty about the nature of climate change before taking actions to reduce climate-change related risks may prove far more costly than taking certain pro-active management and planning steps now. Methods must be used that explicitly incorporate uncertainty into the decision process.

While some kinds of actions should be taken now, expensive and long-lived new infrastructure should be postponed until adequate information on future climate is available. If postponement is not possible, a wider range of climate variability than provided by the historical record should be factored into infrastructure design.

Research Needs

Records of past climate and hydrological conditions are no longer considered to be reliable guides to the future. The design and management of both structural and non-structural water-resource systems should allow for the possible effects of climate change, but little professional guidance is available in this area. Further research by hydrologists, civil engineers, water planners, and water managers is needed to fill this gap, as is broader training of scientists in the universities.


Water Web Management Ltd
1 Dome Hill, Caterham
Surrey CR3 6EE, UK



© 2000/1 Water Policy International Ltd -
Materials may be used with consent and acknowledgement.

| Home | Features | Water Education | Development | News | Documents | Rivers & Regions| Water Issues | Disasters | Water Policy | Editorial | Links |