Fresh water figures prominently in the machineryof the Earth system and is key to understandingthe full scope of global change. Greenhousewarming with a potentially accelerated hydro-logic cycle is already a well-articulated scienceissue, with strong policy implications.A broadarray of other anthropogenic factors—wide-spread land cover change,engineering of riverchannels, irrigation and other consumptivelosses, aquatic habitat disappearance,and pol-lution—also influences the water system indirect and important ways.A rich history ofsite-specific research demonstrates the clearimpact of such factors on local environments.Evidence now shows that humans are rapidlyintervening in the basic character of the watercycle over much broader domains.The collectivesignificance of these many transformations onboth the Earth system and human society remainsfundamentally unknown [Framing Committee ofthe GWSP, 2004].

The Notion of a Global Water System

Diminutive by ocean standards and repre-senting but a small fraction of the planet’shydrosphere (< 3% of total volume [Shiklomanovand Rodda, 2003]), fresh water nonethelessserves as an essential building block of theEarth system. Fresh water is intertwined withenergy exchange,atmospheric teleconnections,and feedbacks linking the climate system.Water movement constitutes the largest flowof any material through the biosphere, andserves as the primary vehicle for erosion anddissolution of the continents.The importanceof fresh water, which strongly regulatesproductivity and supports ecosystems andbiodiversity,is evident throughout the biosphere.

Fresh water is also critical to human society.It underpins global food production by pro-viding the fundamental resource upon whichirrigation, livestock production, fisheries, and

aquaculture depend. Domestic, industrial,hydropower, and recreational water use is crucial to a large and growing population thataspires to long-term improvements in well-being.Providing basic sanitation and clean drinkingwater services remains a major public healthchallenge. More than 1 billion people arewithout access to clean drinking water, 2.5 billion are without sanitation, and over 5,000people, mostly children, die each day fromwater-related diarrheal diseases [World WaterAssessment Programme, 2003].

Key manifestations of variability in the ter-restrial water cycle continue to shape humanhistory and are a costly source of vulnerability.In the United States, annual drought damageaverages $6 billion, with the 1988 droughtalone costing over $60 billion in 2002 dollars[Ross and Lott,2003].Annual damages fromflooding and other extreme weather involving theglobal water cycle are even more costly. Initialestimates in press reports put losses from the2004 hurricane season in the tens of billions ofdollars.

In the context of water’s many roles in theEarth system, the concept of a Global WaterSystem (GWS) provides a useful organizingframework.The GWS is defined by a series ofinteracting components (Figure 1): (1) waterin all its forms,as part of the physical hydrologiccycle; (2) biological systems, as integral trans-formers of water and constituent fluxes thatdetermine biogeochemical cycling and waterquality; and (3) human beings and their insti-tutions, as agents of environmental change,and as entities that experience and respond toongoing transformations of the GWS.

A systematic assessment of how each ofthese components and their interactionsdefine the evolving state of the GWS is a fundamental challenge confronting the Earthand human-dimensions science communities.

The Global Water System Project

A new international research effort consti-tuted as an Earth System Science Partnership(ESSP) project of the Global EnvironmentalChange Programmes (DIVERSITAS,InternationalGeosphere-Biosphere Programme [IGBP],International Human Dimensions Programmeon Global Environmental Change [IHDP],

andWorld Climate Research Programme [WCRP])has been launched to study these complexissues.The primary aim of the Global WaterSystem Project (GWSP) is to promote improvedunderstanding of fresh water in the Earth system through integrated study of its interac-tions, feedbacks, and thresholds.The GWSPscience agenda emerged from a broad consensusof the water science and assessment community,with more than 200 contributors to interdisci-plinary planning meetings starting in 2002, sci-ence planning documents, and a recent OpenScience Conference (October 2003; Portsmouth,New Hampshire).A peer-reviewed frameworkand implementation plan consolidates thesedeliberations [Framing Committee of the GWSP,2004].This article presents the scientificrationale for the GWSP, the project’s keyresearch questions, and an emerging agendafor the decade-long effort.

In the crowded landscape of acronyms representing projects,programs,and institutionsthat deal with water, a strong justification mustaccompany any new international initiative.Several characteristics distinguish the GWSPfrom other international water-related programs.The GWSP is designed to be the following:

(1) Science driven but policy-informing.GWSPconsiders fundamental questions about waterand global change. Owing to the central roleof water in human society, the questions bearhigh relevance to environmental managementand sustainable development.

(2) Global in its perspective. GWSP will helpdetermine the importance of pandemic localchanges to the hydrologic cycle on the behaviorof the Global Water System as a whole.Whilenew world water models and databases areenvisioned, the project will draw heavily froma rich history of case studies and regionalanalysis.

(3) Integrative and interdisciplinary. From itsinception, GWSP has sought to unite socio-economic,physical,and ecological perspectives.

(4) Multitemporal.GWSP focuses on a centurytime frame starting in the mid-20th century, atime of rapid change and growing humaninfluence on many of the planet’s physicaland biogeochemical cycles. For broader con-text,it will draw on historical and paleo perspec-tives and scenario-based visions of the future.

Global Change and the Global Water System

The freshwater cycle is under rapid transfor-mation (Figure 1). Climate change has clearramifications for global hydrology, with majorconcerns surrounding the links of progressive

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Humans Transforming the Global Water System

BY C.VÖRÖSMARTY, D. LETTENMAIER, C. LEVEQUE,M.MEYBECK,C.PAHL-WOSTL,J.ALCAMO,W.COSGROVE,H.GRASSL,H.HOFF,P. KABAT,F.LANSIGAN,R.LAWFORD,R.NAIMAN,AS MEMBERS OF THE FRAMING COMMITTEE

OF THE GLOBAL WATER SYSTEM PROJECT

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greenhouse warming to extreme weather andreduced reliability of water resources.

But several other factors,until recently largelyignored, are proving to be globally significantas well (Table 1).These involve a great varietyof direct anthropogenic activities, many oper-ating at highly local scales.Tabulations madepossible by improvements in remote sensing,GIS, data assimilation, and synthesis show thatmany impacts are now detectable over conti-nental-to-global domains as well [Framing Committee of the GWSP, 2004].

A recent synthesis [Meybeck and Vörösmarty,2004] goes further, suggesting that the globalimpact of direct human intervention in the terrestrial water cycle (through land coverchange,urbanization,industrialization, and waterresources development) is likely to surpassthat of recent or anticipated climate change,atleast over decadal time scales.

Water resource engineering provides a goodexample of the scope and rapid transformationof the GWS.Discharge in many rivers has beenheavily altered (Figure 2), with the aim of sta-bilizing or redirecting flows to optimize watersupply. Such hydraulic manipulation includesmajor surface water diversions and groundwaterabstraction for irrigation (70% of global use[Shiklomanov and Rodda,2003]),impoundment(45,000 large dams [World Commission onDams,2000]),channel dredging, river “training,”and wetland drainage.

Many basins have been dramatically trans-formed, with some of the world’s largest riversshowing a complete or nearly complete lossof perennial discharge to the ocean (e.g., theColorado,Yellow, and Nile Rivers). Global man-ifestations include a doubling-to-tripling of theresidence time of continental runoff in other-wise free-flowing rivers, a 600–700% increasein fresh water stored in channels, and a 30%decrease in global suspended sediment deliv-ery to the oceans [Vörösmarty et al., 2003].Dam construction has resulted in a worldwidepattern of habitat fragmentation that threatensthe biodiversity, structure, and function ofaquatic ecosystems [Revenga et al., 2000].

Given that the vast majority of such changeshave occurred over the last half-century, byany measure of global change, these arearguably among the most rapid and substantial.Understanding the global consequence of thisdiverse array of changes will require coordi-nated, interdisciplinary study.

GWSP Science Questions

With continuing population growth and risingstandards of living, transformations to the GWSwill be inevitable.An important issue surroundswhether these now-worldwide changes aresimply a collection of many local alterationsor if they elicit synergisms that modify overallbehavior of the GWS.The issue is central toour understanding of global change and waterresources, as humans are likely to expandtheir control of this important resource wellinto the future [Shiklomanov and Rodda,2003].

The central tenet of the GWSP is thathuman-induced changes to the water system

are now global in extent, yet we lack an ade-quate understanding of how the overall systemworks, how it responds to change, and howsociety can best adapt to rapidly-evolving andpotentially new system states. The GWSP isorganized to address this assertion in asystematic and unified manner. A specialissue of Aquatic Sciences [Pahl-Wostl et al.,2002] highlights our collective capacity forpursuing this broad objective, which mustunite social science, water resource manage-ment, and biogeophysical perspectives inorder to achieve success.

From this vantage point, the GWS can beviewed as a unified system, with equal atten-tion needing to be paid to its physical,chemical,biological, and anthropogenic components.

The GWSP also needs to define how humansare transforming this water system beyondgreenhouse warming alone, and to determineif the accumulated impact of a much broaderset of human-derived changes has moved theGWS outside the range of natural variability toa nonanalogue state.Like other components ofthe Earth system, the GWS could also have sig-nificant linkages, feedbacks,and thresholdsthat are yet to be discovered.A modifiedhydrologic cycle could lead to abrupt change

and surprises, such as a potential shutdown ofNorth Atlantic deep water formation and oceancirculation arising from changes in Eurasianriver discharge, or the emergence of anoxicdead zones near the mouths of rivers heavilypolluted by upstream agriculture and urban-ization [Framing Committee of the GWSP, 2004].

The GWSP is supported by three framingquestions, which form its thematic structure:

Question 1:What are the magnitudes ofanthropogenic and environmental changes inthe GWS,and what are the key mechanisms bywhich they are induced? This question is ded-icated to first quantifying the principal indica-tors of change to the global hydrologic cycleand then assigning causality. In general, themechanisms by which changes have occurredin the physical components of the GWS arebetter understood than in its biogeochemicaland biological dimensions.A major challengeis to identify the unique role of human versusnatural sources of variability and change.

Question 2:What are the main linkages andfeedbacks within the Earth system arising froma changing GWS? Knowledge from Question 1will formally be linked to synthesis studiesaimed at uncovering interactions among GWScomponents, their sensitivities, and responses

Fig.1.The Global Water System under natural conditions is a complex amalgam of pools anddynamics, linked through complex interactions defined by the physics,biogeochemistry, and biology of the planet. In populated regions of the world, these systems have been transformeddramatically through direct and indirect human interventions in the water cycle.Understandingthese complex transformations requires an integrated, interdisciplinary perspective that considershumans as an important and interactive part of the Global Water System.See Table 1 for descriptionof numerical entries.

to change.A major goal of the GWSP is toimprove our understanding of the co-evolutionof human-technology-environment systems acrossscales, and to identify major teleconnections,nonlinearities, and thresholds.

Question 3: How resilient and adaptable isthe GWS to change, and what are sustainablemanagement strategies? If the hypothesizedimportance of humans on the global waterstage is confirmed,water resource management,which traditionally has been cast at more localor regional scales, will move into the domainof global climate change, chlorofluorocarbon(CFC) regulation,and other pressing internationalpolicy concerns.The GWSP will catalyze a dia-logue around this issue by focusing on (1) thecapacity of the GWS to maintain its importantservices to humans in the face of global change(resilience) and (2) its propensity to evolve(adaptability).The allied issue of societal andecosystem vulnerability will also be addressed.

New Opportunities,New Challenges

With the advent of high-resolution Earth systems science models and data sets, theGWSP community is poised to assemble a fullyglobal view of the GWS,or at least of its majorcomponents. High technology observing sys-tems,global and regional simulations,numericalweather prediction models, and assimilationschemes produce digital products that areoften global in domain and near-real time, arespatially and temporally consistent, and pro-vide a consistent, political “boundary-free”view of the terrestrial water cycle. Compilingrelevant subsets of these data into an integratedcompendium constitutes a major opportunityas well as a challenge.In this context,continuedthreats to the wide availability of field-basedhydrographic records necessary for globalmodel development and validation will limitprogress [Framing Committee of the GWSP,2004].

A coherent view of global water resourceswill be impossible without a common frame-work to bridge the conceptual and practicalgaps separating the social and natural sciences.Such gaps arise from differences in nomencla-ture,quantitative and descriptive approaches,and the scope and scale of typical disciplinarystudies.The global scale is also a major chal-lenge for ecologists and social scientists,whosestudies are typically executed at the localscale. A systematic assessment of requireddata sets, a shared lexicon, and a formal program to harmonize this information arecritically needed. Entraining the engineeringcommunity to identify and assess major trendsin water technology is another important goal.

While the state of water resources at localand even national scales is often a highly visi-ble policy concern, the international scope ofthe issue is only now being conveyed to deci-sion-makers. Recent milestones like the threeWorld Water Forums, the U.N.World Summiton Sustainable Development and its emphasison water, and the World Water DevelopmentReport [World Water Assessment Programme,2003] have elevated the profile of the globalwater situation.Ambitious United Nations Mil-lennium Development Goals to halve by 2015

the proportion of people lacking clean drinkingwater and sanitation require sound scientificinformation upon which to monitor progressand reformulate policy interventions, as doesthe challenge of feeding several billion morepeople over the coming decades, keepingpeace across the more than 250 internationalriver basins,and protecting aquatic biodiversity[World Water Assessment Programme,2003].GWSPscience therefore has an important role toplay in helping to maintain momentum onthese and other important policy fronts.

The GWSP is seen as the first step in estab-lishing a new understanding of humans andtheir place in a changing hydrosphere.Theconsolidation of otherwise independent studiesof water,a move toward a fully global perspective,and interdisciplinarity are the project’s centralfeatures. Studies of linkages, including thenonlinearities and feedbacks that resonatethrough the biogeophysical and social dimen-sions of the water system represent a newchallenge.

The GWSP implementation plan [FramingCommittee of the GWSP, 2004] describes anarray of supporting activities and practicaloutputs, including workshops,science meetings,

an integrated database, modeling strategies,apolicy dialogue,capacity building,and outreach.Inputs from the water sciences,management,and policy communities are welcome. Contactsand additional background information canbe obtained from the GWSP International Project Office in Bonn,Germany (www.gwsp.org).

References

Framing Committee of the GWSP (2004),The GlobalWater System Project: Science Framework andImplementation Activities, Earth System Sci. Partner-ship Proj.,Global Water Syst.Proj.,Bonn,Germany.

Meybeck,M.,and C.J.Vörösmarty (Eds.)(2004),Theintegrity of river and drainage basin systems:Challenges from environmental change, inVegeta-tion,Water,Humans and the Climate:A New Perspectiveon an Interactive System, edited by P. Kabat et al.,part D,pp.297-479, Springer-Verlag, New York.

Pahl-Wostl,C.,H.Hoff,M.Meybeck,and S.Sorooshian(2002),The role of global change research foraquatic sciences, Aquat. Sci., 64, iv-vi.

Revenga, C., J. Brunner, N. Henninger, K. Kassem, andR.Payne (2000),Pilot Analysis of Global Ecosystems:Freshwater Systems,World Resour. Inst.,Washington,D.C.

Ross,T. F. and J. N. Lott (2003),A climatology of 1980-2003 extreme weather and climate events, Tech.Rep.2003-1, Nat. Clim. Data Cent.,Asheville, N.C.

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Fig.2.Hydraulic engineering to ensure reliable water supply is now a worldwide phenomenonand central feature of the Global Water System. (top) In the United States and industrialized partsof the world, river regulation expanded rapidly during the 20th century,with much of the develop-ing world awaiting such growth. (middle) Globally, engineered impoundments store on the orderof 20–25% of continental runoff.(bottom) Water management has greatly modified natural flowregimes in many rivers,and such transformations have been nearly instantaneous from a globalchange perspective.While water engineering conveys important benefits, there are many unintendedconsequences that cascade through the physics,biology, chemistry, and socioeconomics of theGlobal Water System.Sources: (top) National Inventory of Dams; (middle) modified from Vörösmarty et al. [2003]; (bottom) data from UNESCO and Global Runoff Data Center (Koblenz,Germany) archives.

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Shiklomanov, I.A., and J. Rodda (2003), World WaterResources at the Beginning of the 21st Century,U.N. Educ. Sci. and Cult. Organ., Paris.

Vörösmarty, C. J., M. Meybeck, B. Fekete, K. Sharma,P. Green, and J. Syvitski (2003),Anthropogenic sedi-ment retention: Major global impact fromregistered river impoundments, Global Planet.Change,39, 169–190.

World Commission on Dams (2000), Dams andDevelopment:A New Framework for Decision-Making,Earthscan, London.

World Water Assessment Programme (2003), Waterfor People,Water for Life: First U.N.World WaterDevelopment Report,U.N.Educ.Sci.and Cult.Organ.,Paris.

Author Information

C.Vörösmarty, D. Lettenmaier, C. Leveque, M. Mey-beck, C. Pahl-Wostl, J.Alcamo,W. Cosgrove, H.Grassl,H.Hoff,P. Kabat,F.Lansigan,R.Lawford, R. Naiman, asmembers of The Framing Committee of the GlobalWater System Project

For additional information, contact the GWSPInternational Project Office, Bonn, Germany; E-mail:gwsp.ipo@uni-bonn.de.