REVI EWSUMMARYSUSTAINABILITYSystems integration forglobal sustainabilityJianguo Liu,* Harold Mooney, Vanessa Hull, Steven J. Davis, Joanne Gaskell,Thomas Hertel, Jane Lubchenco, Karen C. Seto, Peter Gleick,Claire Kremen, Shuxin LiBACKGROUND: Many key global sustain-ability challenges are closely intertwined (ex-amples areprovidedinthefigure). Thesechallengesincludeairpollution, biodiver-sity loss, climate change, energy and food sec-urity, diseasespread, speciesinvasion, andwater shortages and pollution. They are inter-connectedacross three dimensions (orga-nizational levels, space, andtime) but areoftenseparatelystudiedand managed.Sys-tems integrationholistic approaches to in-tegrating various components of coupledhuman and natural systems (for example, social-ecological systems andhuman-environmentsystems)acrossall dimensionsisnecessa-rytoaddresscomplexinterconnectionsandidentify effective solutions to sustainabilitychallenges.ADVANCES: One major advance has beenrecognizing Earth as a large, coupled humanand natural systemconsisting of many smallercoupled systems linked through flows of in-formation, matter, andenergyandevolv-ingthroughtimeasasetofinterconnectedcomplexadaptivesystems. Anumberofin-fluential integrated frameworks (such as eco-systemservices, environmental footprints,human-nature nexus, planetaryboundaries,andtelecoupling) andtoolsforsystemsin-tegrationhavebeendevelopedandtestedthroughinterdisciplinaryandtransdiscipli-naryinquiries. Systemsintegration has led to fun-damental discoveries andsustainability actions thatare not possible by usingconventional disciplinary,reductionist, and compart-mentalizedapproaches. Theseincludefind-ings on emergent properties and complexity;interconnections among multiple key issues(such as air, climate, energy, food, land, andwater); assessment of multiple, often conflict-ing, objectives;andsynergisticinteractionsin which, for example, economic efficiency canbe enhanced while environmental impacts aremitigated. Inaddition, systemsintegrationallows for clarification and reassignment ofenvironmental responsibilities (for example,amongproducers, consumers, andtraders);mediationoftrade-offsandenhancementofsynergies; reduction of conflicts; and design ofharmonious conservationand developmentpolicies and practices.OUTLOOK: Although some studies have rec-ognizedspillovereffects(effectsspillingoverfrominteractions amongother systems) orspatial externalities, there is a need to simul-taneously consider socioeconomic and envi-ronmental effects rather thanconsideringthemseparately. Furthermore, identifyingcauses, agents, andflowsbehindthespill-over effects can help us to understand betterand hence managetheeffects acrossmulti-ple systems and scales. Integrating spilloversystems with sending and receiving systemsthrough network analysis and other advancedanalytical methodscanuncoverhiddenin-terrelationships andleadtoimportant in-sights. Human-naturefeedbacks, includingspatial feedbacks (such as those among send-ing, receiving, and spillover systems), are thecoreelementsofcoupledsystemsandthusarelikelytoplayimportantrolesinglobalsustainability. Systems integration for glob-al sustainabilityis poisedfor morerapiddevelopment, andtransformativechangesaimedatconnectingdisciplinarysilosareneeded to sustain an increasingly telecoupledworld.RESEARCHSCIENCE sciencemag.org 27FEBRUARY2015VOL347ISSUE6225 963ON OUR WEB SITERead the full articleat http://dx.doi.org/10.1126/science.1258832..................................................The list of author affiliations is available in the full article online.*Corresponding author. E-mail: liuji@msu.eduCite this article as J. Liu et al., Science 347, 1258832 (2015).DOI: 10.1126/science.1258832Integratinghuman and naturalsystemsIllustrative representation of systems integration. Among Brazil, China, the Caribbean, and theSaharaDesert inAfrica, therearecomplexhuman-natureinteractionsacrossspace, time, andorganizational levels. Deforestation in Brazil due to soybean production provides food for people andlivestock in China. Food trade between Brazil and China also contributes to changes in the global foodmarket, which affects other areas around the world, including the Caribbean and Africa, that alsoengage in trade with China and Brazil. Dust particles fromthe Sahara Desert in Africaaggravated byagricultural practicestravel via the air to the Caribbean, where they contribute to the decline in coralreefs and soil fertility and increase asthma rates.These in turn affect China and Brazil, which have bothinvestedheavilyinCaribbeantourism, infrastructure, andtransportation. Nutrient-richdustfromAfrica alsoreaches Brazil, whereit improves forest productivity. [Photocredits clockwise fromright topphoto: Caitlin Jacobs, Brandon Prince, Rhett Butler, and David Burdick, used with permission] on February 26, 2015www.sciencemag.orgDownloaded from on February 26, 2015www.sciencemag.orgDownloaded from on February 26, 2015www.sciencemag.orgDownloaded from on February 26, 2015www.sciencemag.orgDownloaded from on February 26, 2015www.sciencemag.orgDownloaded from on February 26, 2015www.sciencemag.orgDownloaded from on February 26, 2015www.sciencemag.orgDownloaded from on February 26, 2015www.sciencemag.orgDownloaded from on February 26, 2015www.sciencemag.orgDownloaded from on February 26, 2015www.sciencemag.orgDownloaded from REVI EWSUSTAINABILITYSystems integration forglobal sustainabilityJianguo Liu,1* Harold Mooney,2Vanessa Hull,1Steven J. Davis,3Joanne Gaskell,4Thomas Hertel,5Jane Lubchenco,6Karen C. Seto,7Peter Gleick,8Claire Kremen,9Shuxin Li1Global sustainability challenges, from maintaining biodiversity to providing clean air andwater, are closely interconnected yet often separately studied and managed. Systemsintegrationholistic approaches to integrating various components of coupled humanand natural systemsis critical to understand socioeconomic and environmentalinterconnections and to create sustainability solutions. Recent advances include thedevelopment and quantification of integrated frameworks that incorporate ecosystemservices, environmental footprints, planetary boundaries, human-nature nexuses, andtelecoupling. Although systems integration has led to fundamental discoveries andpractical applications, further efforts are needed to incorporate more human and naturalcomponents simultaneously, quantify spillover systems and feedbacks, integrate multiplespatial and temporal scales, develop new tools, and translate findings into policy andpractice. Such efforts can help address important knowledge gaps, link seeminglyunconnected challenges, and inform policy and management decisions.The goal of achieving global sustainabilityis to meet societys current needs by usingEarths natural resources without compro-mising the needs of future generations (1).Yet, many disparate research and manage-ment efforts are uncoordinated and unintention-ally counterproductive toward global sustainabilitybecause a reductionist focus on individual com-ponents of anintegratedglobal systemcanoverlookcritical interactions across system components.Although our planet is a single system compris-ing complex interactions between humans andnature, research and management typically iso-late systemcomponents (such as air, biodiversity,energy, food, land, water, andpeople). Asare-sult, the compounding environmental impacts ofhuman activities have too often been missed be-cause they gobeyondthe organizational level, space,and time of focus. For example, large amountsof affordable and reliable energy are available infossil fuels, but concomitant emissions of carbondioxide (CO2) will alter global climate and affectother human and natural systemsa trade-off thatcurrentpolicieshavenotadequatelyaddressed(2). Likewise, attention to growing more food onland may inadvertentlyresult in excess use offertilizersandinturneutrophicationofdown-streamcoastalwatersthatcompromisesfoodproduction from the ocean. Progressing towardglobal sustainability requires a systems approachto integrate various socioeconomic and environ-mental components that interact across organi-zational levels, space, and time (35).Systemsintegrationgeneratesmanybenefitscompared with isolated studies, including under-standing of interconnectivity and complexity(Table 1). Here, we review recent advances in de-veloping and quantifying frameworks for systemsintegrationofcoupledhumanandnaturalsys-tems; illustrate successful applications, focusingonunexpectedimpactsofbiofuelsandhiddenrolesofvirtual wateranddiscussfuturedirec-tions for using systems integration toward globalsustainability.Framework developmentand quantificationThedevelopmentandquantificationof frame-works arecritical steps inintegratinghumanandnatural systems(69). Forinstance, inter-actions between sectors and stakeholders in thehuman system or between biotic and abiotic fac-torsinthenaturalsystematdifferentorganiza-tional levels (for example, government agenciesfromlocal tonational levels, andfoodtrophiclevelsfromproducerstoconsumersinecosys-tems) lead to emergent properties that individualcomponentsdonothave(10). All coupledsys-tems evolve over time as complex adaptive systems(11). Their interactions, emergence, evolution,and adaptation also vary with spatial scales (12).Accordingly, integrationalong organizational, spa-tial, and temporal dimensions is needed to avoidsustainability solutions in one systemthat causedeleteriouseffectsinothersystems. Suchin-tegration can also enhance positive and reducenegative socioeconomic and environmental effectsacross multiple systems at various organizationallevels over time (Table 1).Integration requiresblending and distillingof ideas, concepts, and theories from multiplenaturalandsocialsciencedisciplinesaswellasengineering and medical sciences (4, 13), varioustoolsandapproaches(suchassimulation, re-motesensing, andlifecycleassessment), anddifferenttypesandsourcesofbiophysicalandsocioeconomic data (14). For example, integratedassessment models such as those used by the In-tergovernmental Panel on Climate Change (IPCC)analyze information from diverse fields to under-stand complex environmental problems (such asacid rain, climate change, energy shortages, andwater scarcity) (15, 16). The Global Trade Analy-sis Project has recently evolved from a databasefor analyzing global trade-related economic issuesto aplatform forintegratingtradewith globalland use and associated greenhouse gas (GHG)emissions (17). The Global Biosphere ManagementModelanalyzesandplansland useamong sec-tors (agriculture, forestry, and bioenergy) acrossthe globe in an integrated way (18). Below, we il-lustrate the development and quantification ofsome important integration frameworks that haveled to substantial advances.Ecosystems services, environmentalfootprints, and planetary boundariesHuman and natural systems interact in a multi-tudeofways. Several integrationframeworksbring multiple aspects of human-nature interac-tionstogether(Fig. 1). Quantifyingtheservicesthatecosystemsprovide(Fig. 1A) forsocietalneeds (such as clean water, nutrient cycling, andrecreation) (6) helps assign value to natural com-ponents for humans. Recent advances considera variety of ecosystem services simultaneously inorder to evaluate trade-offs and synergies amongthem(19). Environmental footprint (20) andplanetary boundary (21) frameworks attempt toquantify the negative effects that human activitieshaveonnatural systems. Theenvironmentalfootprints framework quantifies resources (suchas natural capital) consumed and wastes gener-atedbyhumans(Fig. 1B)(20).Recentmanifes-tations of the concept go beyond the previouslydevelopedecological footprintsframeworkbyincluding more diverse types of footprints [forexample,water, carbon, and material footprints(20)]. Planetary boundaries are threshold levelsfor key Earth system components and processes(suchasstratosphericozone, globalfreshwater,and nitrogen cycling) beyond which humanitycannot safely be sustained (21) (Fig. 1C).Quantifyingtheaboveframeworksreliesonsystems integration. For instance, organizationalintegrationinenvironmentalfootprintanalysisdemonstrates howdifferent human activities con-tribute to human impacts at local to global levels(20). Spatial integrationis illustrated inintegratedRESEARCHSCIENCE sciencemag.org 27FEBRUARY2015VOL347ISSUE6225 1258832-11Center for Systems Integration and Sustainability, Departmentof Fisheries and Wildlife, Michigan State University, EastLansing, MI, USA.2Department of Biology, Stanford University,Stanford, CA, USA. 3Department of Earth System Science,University of California, Irvine, CA, USA. 4World Bank,Washington, DC, USA.5Department of Agricultural Economics,Purdue University, West Lafayette, IN, USA.6Department ofIntegrative Biology, Oregon State University, Corvallis, OR, USA.7School of Forestry and Environmental Studies, Yale University,New Haven, CT, USA.8The Pacific Institute, Oakland, CA, USA.9Department of Environmental Science, Policy andManagement, University of California, Berkeley, CA, USA.*Corresponding author. E-mail: liuji@msu.edu1258832-2 27FEBRUARY2015VOL347ISSUE6225 sciencemag.org SCIENCETable 1. Example benefits of systems integration.Benefit Illustrative studyRevealing mechanisms ofecological degradation inprotected areasSocioeconomic factors (such as forest harvesting, fuelwood collection, and increases inhousehold numbers) are responsible for ecological degradation in protected areas forgiant pandas (which are supposed to be protected from human activities) (100).Understanding complexity Agricultural intensification schemes may promote further agricultural expansion over thelong term; responses varied across space and were nonlinearly related to agriculturalinputs (101).Improving economic efficiency Integrated assessment modeling shows specific cost estimates for delaying climate changemitigation with respect to geophysical, technological, social, and political factors (102).Reducing environmentalimpacts in distant placesIntegrated cross-boundary management suggests ways of decreasing the spread of pollutionand spillover of climate-change effects to distant places around the globe (15).Addressing multipleissues simultaneouslyThe climate changehealthfood security nexus demonstrates ways that managementmeasures can improve all three key issues at the same time (103).Assessing the feasibility ofmultiple and conflicting goalsIntegrated coastal zone management allows for multiorganizational management forcompeting interests such as recreation, fisheries, and biodiversity conservation (104).Developing priorities forresearch and sustainability actionIntegrated modeling of global water, agriculture, and climate change pinpoints areasvulnerable to future water scarcity and puts forth actionable strategies for mitigation (16).Identifying complementaryconservation and managementstrategiesCoupling global energy security policy with climate change and air pollution policies(the air-climate-energy nexus) would decrease oil consumption compared to implementingenergy policy alone (46).Enhancing synergiesamong factorsCross-site integration of natural resource management approaches in response todisturbances shows opportunities for reframing ecosystem management to enhancecollaboration among institutions (such as NGOs, government agencies, researchorganizations, businesses) (105).Anticipating feedbacks A lag between fire control management and the response of the forests to such changesaffects the eagerness of landowners to continue implementing control measures (106).Detecting latencies The latent effect of mosquito ditch construction on fish populations only emerged duringnew pressures from residential development and recreational fisheries (107).Maximizing economic gainsand minimizing environmental costsIntegrated soil-crop management system could maximize grain yields, while minimizingapplications of fertilizers and GHG emissions (108).Fig. 1. Examplesofecosystemservices,environmentalfootprints,andplanetary boundaries. (A) Ecosystemservices. (B) Environmental footprints.(C) Planetary boundaries. Outward arrows in (A to C) indicate increases in thevalues, inward arrows indicate decreases, and dashed lines indicate no data.In(B)and(C),theinnergreenshadingrepresentsmaximumsustainablefootprints (20) and safe operating space for nine planetary system variables(21), respectively. Red wedges refer to the estimated current positions for thevariables. Mostof theecosystemservicesin(A)decreasedbetweenthe1950stoearly2000s(94).In(B),atleastthreetypesoffootprints(eco-logical, carbon, and material) have exceeded maximum sustainable footprints(20).Bluewaterfootprint was1690billionm3/year(19851999)(81),graywater footprint increased during 19702000 (95), and green water footprint is6700 billion m3/year (without reference point) (20). Question marks indicatethattheinformationisuncertain.Carbonfootprintincreasedduring19602009 (96). With every 10% increase in gross domestic product, the averagenational material footprint increases by 6% (97). For (C), all planetary systemvariables have increased in values between preindustry and 2000s, and threeboundaries have been crossed (21).RESEARCH | REVIEWlandscape planning for ecosystemservices, whichallows for coordination across space. For example,it can promote afforestation and reforestation inuplandareas above irrigatedagricultural systems,thus reducing erosion, protecting waterways, min-imizing flooding, providing drinking water, andfacilitating sustainable agricultural production(22).Temporalintegrationiscrucialtoquantifythe planetary boundaries framework, as short-term fluctuations in key Earth system processesare scaled up to predict long-term trends, manyof which cannot be accuratelypredictedwith-out a systems approach (21). Temporal inte-grationcanalsoreveal legacyeffectsofpriorhuman-nature couplings. For example, carbonfootprints are driven in large part by past landuse (23). Acondition termed carbon lock-in hasbeen used to describe systems that have evolvedover long time frames to become dependent onfossil fuels (24, 25). The fossil energy system iscomprised of long-lived infrastructure such aspower plants, which represent an investment infutureCO2emissions. Retiringthisinfrastruc-turebeforetheendofitseconomicorphysicallifetime would entail substantial costs. As of 2013,itisestimatedtheglobalcommittedemissionsrelated to existing fossil infrastructure are rough-ly 700 billion tons of CO2 (26).Human-nature nexusesIn contrast to the conventional decision-makingthat takes place within separate disciplines orsectors, the human-nature nexus framework rec-ognizestheinterdependencybetweentwoormoreissues(ornodes)andaddressesthemto-gether. For example, the energy-foodnexus consid-ers both the effects of energy on food production,processing, transporting, and consumption, andthe effects of food (such as corn) production onthe generation of energy (such as ethanol) (27).The nexus framework can help anticipate other-wise unforeseen consequences, evaluate trade-offs,produce co-benefits, and allowthe different andoftencompetinginterests toseeka commonground(28) and co-optimization (29). The vast majorityof the 229 human-nature nexus studies recordedintheWebof Science(asof 16August 2014)analyzed two-node nexuses (80%), with only 16and 4%of the nexus studies including three andfour nodes, respectively. Although the concept offood-energy nexus firstappeared in 1982,therewas no paper recorded in the Web of Science formany years. The interest in the nexus frameworkhas reemerged recently and has grown rapidlysince 2010. Several two-node nexuses have receivedspecial focus, including energy-water nexus, food-water nexus, energy-foodnexus, air-climate nexus,health-water nexus, and energy-national securitynexus. Amongthe more commonly examinedthree-and four-node nexuses are economy-environment-land nexus and climate-energy-food-water nexus.Adding more nodes to a nexus framework leadstomorecomplexitybutalsocapturesgreaterreality. Forinstance, theclimate-energy-foodnexus considers not only the interrelationshipsbetween energy and food, but also the relation-ships between energy and climate (for example,energy use emits CO2, and climate change affectsenergy demand such as heating and cooling) andinterrelationships between food and climate (forexample, climate changes affect food production,and CO2 is emitted throughout the food produc-tion, processing, transporting, and consumption).Thenodesinthenexusframeworkareme-diated by and influence many organizational lev-els. For example, energy production and use areshaped by international markets and policies atdifferent government organizations and at thesame time influence many trophic levels of ani-mals, plants, and microorganisms (30). Buildingontheincreasingrecognitionofconceptualin-terconnections among various nodes, efforts areunderwaytoquantifytheirrelationships,suchas via hydro-economic modeling (31), structuraland nonstructural economic models (32), and lifecycle assessments (33). Scenario analysis is partic-ularly promising for teasing out roles of differentorganizationsfunctioning atdifferentscales(34, 35). For example, the Agrimonde model hasbeen used to examine intersections between foodand numerous other sectors (including energy andwater) worldwide under different growth and con-sumptionscenarios(35) andillustratestheef-fects of diverse individual governments on globalcross-sector dynamics.Temporal integration is also a key element ofthe nexus framework. For example, recent long-termquantitativeintegrationstudiesontheeconomy-energy nexus showthatreductions inenergyusecanhavenegativeimpactsongrossdomesticproduct(GDP)intheshort-termbutlittle detectable effect over the long term (36).Alternatively, one model predicted that a smallincrease in foreign trade in Indonesia will leadto substantial increases inlong-term percapitaCO2 emissions, although its contribution to CO2emissions is negligible in the short term (37).TelecouplingMany studies on sustainability have been place-based even iftheylookat coupled systems [forexample,theenergy-waternexusintheUnitedStates (38)]. However, economic production andresource use in different regions or countries mayleadtovery different consequences. Furthermore,there are increasing distant interactions aroundthe world so that local events have consequencesglobally (39). For example, each year several hun-dred million tons of dust from Africa (especiallytheSaharadesert)travel viatheairacrosstheAtlantic Oceanto distant places suchas theCaribbean, whereitcausessevereimpacts, in-cluding decline in coral reefs, increase in asthma,disease spread, and loss of soil fertility (40, 41).Greenhousegasesemittedintotheatmospherefroma point source become mixedand transportedglobally, affectingsocietiesandecosystemsfardistant from the point sources of origin. Manyof the changes to the biotic composition of localplaces can also affect society regionally and glob-ally through ever-increasing global trade as wellas the often dramatic impact of invasive speciesanddiseasetransmission. Inotherwords, pat-terns and processes at one place may enhance orcompromisesustainabilityinotherplaces(42).Humanactions[suchasproductionofbiofuels(43)]inoneplacemaycreateunintendedcon-sequenceselsewhere[suchascarbonleakage(44), biodiversity losses (45, 46), and pollution(47)]. Although external factors originating fromother systems are sometimes considered in sus-tainability research and practices, they are typi-callytreatedasone-waydriversofchangesinthe systemof interest, with little attention to thefeedbacksbetweenthesystemofinterestandother systems (6, 42).The framework of telecoupling (socioeconom-ic and environmental interactions over distances)has been developed to tie distant places together(42). It is a natural extension of the frameworksof coupled human and natural systems and builton disciplinary frameworks such as climate tele-connections(distantinteractionsbetweencli-mate systems), urban land teleconnections (landchanges that are linked to underlying urbaniza-tion dynamics) (7), and economic globalization(distant interactions between human systems).So far, the telecoupling framework has been ap-pliedtoanumberofimportantissuesacrossspatial scales, such as global land-use and land-change science (39, 48, 49), international landdeals (39), species invasion (39, 42), payments forecosystemservicesprograms(50), andtradeoffood (42) and forest products (9).The framework is particularly effective for un-derstandingsocioeconomicandenvironmentalinteractions at international scales. For example,the flow of coal from Australia (sending system)to a number of countries and regions (receivingsystems; for example, Japan, the EuropeanUnion,and Brazil) reflects abundant Australian coal sup-pliesandthedemandforcoalinreceivingsys-tems(Fig. 2). Thecoal tradeisfacilitatedbymany agents in receiving and sending systems(suchasgovernmentagenciesthatmakecoaltrade policies) and international organizations(such as shipping companies). Many other coun-tries (suchas those inAfrica) are spillover systemssystemsthatmaybeaffectedbythecoalflowsbecause of financial flows between sending andreceivingsystemsaswellastheCO2emissionsproduced when the coal is burned. Global effortsto address this and similar feedbacks, such as theREDD+ programto mitigate CO2emissions fromdeforestation (51) and the Green Climate Fundto facilitate low-emission projects in developingnations (52), should focus on all countries.The telecoupling framework can also be use-ful at regional or national scales. For example,the 20 million residents in Chinas capital city ofBeijing(receivingsystem) receivecleanwaterfromtheMiyunReservoirwatershed(sendingsystem),morethan100kmaway fromthecity(50). The framework explicitly links agents, causes,andeffectsinthesendingandreceivingsys-tems. For instance, the quantity and quality ofthe water flows are made possible through thePaddy Land-to-Dry Land program, an ecosystemservices payment programthat Beijing establishedwith the farmers in the watershed who convertedrice cultivation in paddy land to corn productionSCIENCE sciencemag.org 27FEBRUARY2015VOL347ISSUE6225 1258832-3RESEARCH | REVIEWin dry land to provide clean water for Beijingin exchange for cash payments (53). Throughsystematicanalysis, theframeworkalsohelpsidentify research and governance gaps, such asspillover systemsregions that are affected bywater and cash flows between the watershed andBeijing but have received little attention fromresearchers and government agencies (50).Thetelecouplingframeworkalsoemphasizestemporal dynamics. A lack of temporal integra-tion may miss key dynamics and create a misun-derstandingof infrequentbutdrasticchanges,such as disasters, wars, outbreaks of deadly dis-eases such as Ebola (54), regime shifts, and pro-found policy changes. For instance, in the WolongNature Reserve of China designated for conserv-ingtheendangeredgiantpandas,thedevastat-ing earthquake in 2008 substantially altered thetelecouplings between Wolong and outside sys-tems [for example, collapse of tourismand agricul-tural trade (55)]. Studies omitting the earthquakeimpacts could misrepresent the mechanisms be-hind the system dynamics (such as increases inlandslides and relocation of households).Applications of systems integrationSystems integration has been applied success-fullytomanysustainabilityissues. IntegratedCoastal Zone Management (56), Marine SpatialPlanning (57), and Ecosystem-Based Management(58) all integrate multiple dimensions for naturalresourcemanagement. Althoughsomeofthesepractices have existed for several decades, therehave beencontinued efforts for more integration,new advances, and novel insights. For example,Ecosystem-Based Management has expanded totackle issues not traditionally thought of withinnatural resource management, such as food secu-rity(59), politics(60), anddisease(61). Theex-amples of biofuels and virtual water below alsoillustrate the importance of systems integrationin detail. We chose to focus on these two exam-ples because they are emerging and contentiousglobalphenomenathatrepresentchallengingsustainability issues, and they have unexpectedand hidden socioeconomic and environmentaleffectsthatwereimpossibleto revealwithoutsystems integration.Unexpected impacts of biofuelsTheenvironmentalandsocioeconomicimpactsof biofuelshavebeenamongthemost hotlycontested policy issues over the past decade. TheUnited States, European Union, and nearly threedozenothercountriesinAfrica, Asia, andtheAmericashavedevelopedbiofuel mandatesortargets (62). This enthusiasm was buoyed by theprospect of displacinghigh-pricedoil imports,generating rural incomes, and contributing toclimatechangemitigation. Asof2006, itwassuggested that biofuels could be both economi-callyandenvironmentallybeneficial. However,withtheimplementationsofthese policiesandsystemsintegrationresearch, serious concernshavearisenabouttheirgeospatialimpactsandthe temporal viability of these mandates.Biofuels are a prime topic for systems integra-tionresearchbecausebiofuelproductionandconsumption as well as their impacts vary acrosstime, space, andorganizationallevels. Forin-stance, the carbon fluxes after conversion of newcroplands depend not only on below- and above-ground carbon at present, but also on the legacyeffects stemming fromhistorical land use such aslandclearingaswellassubsequentcultivationandcroppingpractices(63). TheUnitedStatesand Brazil were responsible for 90%of the globalbiofuel production of 105 billion liters in 2011, butseveral other countries with new mandates suchas China, Canada, and Argentina are increasingthe spatial extent of production (64). Assessingorganizational impacts on biofuel production en-compasses analysis that integrates across institu-tions. Forexample, whenandwhereadditionalcroplandis convertedfor biofuel productionde-pends critically on local, national, and internationalinstitutions, as well as the global supply chain.The systems integrationframeworks discussedabovehave directrelevancetobiofuels. Forex-ample, theenvironmentalfootprintframeworkhasbeenappliedtoassesstheimpactsofbio-fuels. It isestimatedthat theglobal footprintfrom biofuels was 0.72 billion gha in 2010 andexpectedtoriseby73%in2019(consistingoflanduse, carbon, embodiedenergy, materialsand waste, transport, and water) (65). Fromtheperspectiveofecosystemservices, biofuelshaveboth positive (for example, energy) and negative(for example, loss of food and freshwater services)impacts. Biofuel productionalsoaffectsseveralplanetary boundaries, including climate change,land-use change (proportion of cropland), nutri-ent cycles (increased use of phosphorus and nitro-gen), and biodiversity loss (66). For instance, it isestimated that corn-based ethanol nearly doublesgreenhouseemissionsacrosstheworldovera30-year period because of land-use change (43).Biofuelshavealsobeenstudiedunder thehuman-naturenexusframeworkinparticular,theenergy-foodnexusandenergy-food-waternexus. Rising demand for ethanol feedstocks bid1258832-4 27FEBRUARY2015VOL347ISSUE6225 sciencemag.org SCIENCEFig. 2. Illustrative example of sending, receiving, and spillover systems,as well as flows under the telecoupling framework. In thecaseoftradein Australian coal in 2004 [measured in megatons (Mt) of CO2 emissions],Australia is a sending system (in blue), the main receiving systems are ingreen (destinations of the coal are Japan, South Korea, Taiwan, Malaysia,India, the European Union, and Brazil), and the spillover systems are in lightred(all othercountriesandregionsareaffectedbyCO2emissionsfromusing the coal produced in Australia and consumed in the receivingsystems). Thearrowsshowthemagnitudeof theflows. Countriesthatreceivelessthan10MtofemissionsofAustraliancoal arenotincluded.FlowstoEuropeareaggregatedtoincludeall 28memberstatesof theEuropean Union[data from(44)]. Financial flows take place betweensending and receiving systems but may also affect financial conditions inspillover systems indirectly [data from (98)].RESEARCH | REVIEWup food price (67), which has major implicationsfor food security (68). And, questions have beenraised about the adverse interplay between bio-fuelmandatesandincreasedinterannualvaria-bility in crop production anticipated under futureclimate change (69). Water also comes into play,as limits on the future availability of water forirrigatedagriculturewill shiftthelocationofcropland conversion owing to biofuel expansiontoward regions with carbon-rich rainfed agricul-ture. Overall, accountingforhydrological con-straintsboostsestimatedGHGemissionsfromland use by 25% (70).Telecoupled processes such as internationaltradeandflowsof information(forexample,global market prices) cause biofuel programs inone part of the world to translate spatially intolandconversioninotherregions(indirectlanduse). They have already contributed to croplandexpansionintheUnitedStates andoverseasand to cascading and spillover effects over longdistances(Fig. 3). National biofuel programs,which looked environmentally beneficial at firstblush, mightinfactleadtoincreasedenviron-mental damage when viewed over time and attheglobalscale (42,71).Unlikeearly analyses(42) that assumed that higher prices effectivelyinfluenced all agents in the market equally, sub-sequent research has revealed that some agricul-tural suppliers(suchastheUnitedStatesandArgentina) are more closely telecoupled than oth-ers (72). The spatial pattern and extent of landconversionstemmingfrombiofuelsarealsoaf-fected by geophysical characteristics such as poten-tial productivity of the newly converted lands.Hidden roles of virtual waterAlthoughmanysustainabilitystudies havefo-cused on flows of real material and energy suchas biofuels, there has been increasing interest inthe flows of virtual material and energy, suchas virtual water, virtual energy, virtual land,and virtual nutrients (73). Virtual resources arethoseresourcesusedforproductionandincor-porated into goods and services in the same waythat related pollution and impacts are embodied(orhidden) intheseproducts. Inthecaseofwater, for example, it is used to grow crops, raiselivestock and grow their food, and produce mar-ketablegoods. Virtualwateristradedamongcountries as goods are traded. Globally, the vol-ume of virtual water trade and the number oflinks (pairs of trading countries) have bothdoubledfrom1986 to 2010(74). Withroughly 27 trillionm3of water traded virtually worldwide in 2010 (74),virtual water trade fromwater-rich countries hashelped mitigate water shortages in water-poorcountries(75). Theconceptofvirtualresourceshas helped analysts think more clearly about therealrisksofresourcescarcityandtherolethattrade plays in mitigating or worsening those risks(76, 77). Targeted trade policies may help to fur-ther prevent water scarcity by encouraging morewater-efficient trade links (78).Virtual water is a good target for systems in-tegrationresearchbecausetheissuesinvolvedare dynamic across time, space, andorganizationallevels. Global water scarcity issues are inherent-ly temporally sensitive, with cumulative effectsSCIENCE sciencemag.org 27FEBRUARY2015VOL347ISSUE6225 1258832-5Fig. 3. Cascadingandspillovereffectsofbiofuel productiononlandconversionandCO2emissions, as revealedbysystems integration.MeetingtheRenewableFuel Standardmandate[toproduce50Gigaliters(GL) of additional ethanol on top of the 2001 production level] in the UnitedStates reduces the use of petroleumbut requires additional corn area (99).Theexpansion in U.S. corn area leads to reduction in harvested area of oilseeds andother crops in the United States. This boosts world prices for these crops andencourages more production of oilseeds and corn in the rest of the world. Theexpansion of cropland in the United States and the rest of the world leads tomore emissions of CO2and the conversion of pasture and forest lands aroundthe world. This land conversion also releases CO2, offsetting the reduction ofCO2 emissions from using less fossil fuels and more biofuels (assuming a 2/3ethanol/petroleum energy conversion rate). Estimates are on an annual basisover a 30-year production period for the biofuel facilities and in approximateamounts derivedfrom(99). Arrows indicate the directionof influences.Symbols and + refer to decrease and increase, respectively, in land area,ethanol, fossil fuel, or CO2. Tg, teragrams; Mha, million ha. [Graphics are usedwith permissions from Fotolia.com]RESEARCH | REVIEWstemming from legacies ofoveruse of water in-teracting withnewdrivers suchas climate change.Estimates suggest that global virtual water trademay decrease with climate change because of thedifficulty ofgrowing crops in warmer, driercli-mates [water savings from reduction in growingrice, soybeans, and wheat may amount to up to1.5trillionm3in2030(78)]. Also, therewasaprofound shift in the spatial distribution of hu-manpopulations(thus,waterdemand)relativeto water distribution over the past few decades.In 1986, 68% of the worlds population was inwater-exporting countries, but by 2010, the dis-tributionwasalmostcompletelyreversed, with60%of the global population in water-importingcountries (74). One of the greatest organizationalconcerns related to virtual water is that a fewcountries control the majority of the global trade,which leaves the market vulnerable to the deci-sions made by a fewkey players (74). In addition,there is unequal distribution of resources withincountries and a tendency for local agrarian com-munities to be marginalized owing to trade dic-tated by country-level agencies (79).Theecosystemservicesframeworkhascon-tributed to virtual water research in many ways.For example, Canada is a major exporter of vir-tual water worldwide (95 Gm3/year); exportingvirtual water affects the ecosystem services pro-vided by the nations boreal forests, which makeupnearly60%of Canadasterritory(80). Pro-duction of commodities through processes suchas hydroelectric power generation, oil extraction,crop irrigation, and livestock-rearing contributesto virtual water exports, which in turn threatenfreshwaterresourcesthatareakeypartoftheboreal forests. Boreal freshwater comprises 80 to90%of Canadas lakes and25%of theentireEarths wetlands (80). Removal of water compro-mises the estimated annual gain of $703 billionin ecosystem services that the boreal forests pro-vide, including carbon storage, flood control andwaterfiltering, biodiversityconservation, andpest control (80).The environmental footprints framework hasinformed virtual water research by depicting thewater resourcesusedfor productionof goodsandservices(waterfootprints). Forexample,2,320Gm3/yearof thetotal global waterfoot-print of 9087 Gm3/year comes fromvirtual watertrade (81). There is a close relationship betweenvirtual water and planetary boundaries becauseone of the nine key planetary boundaries iden-tified is the limit to global freshwater use (21). Arelatedconceptpeakwaterhelps to illustratehowcloseglobalfreshwaterbodiesaretothisthreshold. Global water consumption has alreadyreached a peak and begun to decline in manyareas because of limited remaining water (13).Furthermore, scarcity in global freshwater is inlarge part linked to the virtual water embeddedin agricultural productionand trade (81). Thehuman-nature nexus framework is also usefulfor virtual water research. For example, a studyon water-food nexus indicates that 76%of virtualwater trade is attributed to crops or crop-derivedproducts (81).Virtualwatertradevariesspatiallyandisanimportant telecoupling process. The main virtualwater exporters (sending systems) are water-richregions in North and South America and Australia,whereas Mexico, Japan, China, and water-poorregionsinEuropearethemainimporters(re-ceivingsystems)(Fig.4)(75).Sendingandre-ceiving systems involved in virtual water tradehave dynamic roles. Asia recently switched itsvirtualwaterimportsfromNorthAmericatoSouth America (82). On the other hand, NorthAmericahasengagedinanincreaseddiversifi-cation of intraregional water trade while tradingwithdistantcountriesinAsia(82).Chinahasundergone a dramatic increase in virtual waterimportssince2000, viaproductssuchassoy-beans from Brazil (nearly doubling from 2001 to2007andamountingto13%ofthetotalglobalworld water trade) (82). The spatial shift in theuse of soybean products in Brazil from domesticto international has led to water savings in othercountries, but at thecost of deforestationinBrazilian Amazon (82). Within-country virtualwater transfer is also common. For example, vir-tual water flow through grain trade from NorthChina to South China goes in the opposite direc-tion of real water transfer through large projects,such as the South-to-North Water Transfer Pro-ject, that aimtoalleviatewatershortagesinNorth China.Future directionsDespite the substantial progress in systems inte-grationillustratedabove, manyimportant challengesremain. For example, the integrated frameworkshave been studied largely in isolation, althoughthey are interconnected throughhumanactivities(for example, using more ecosystemservices maylead to larger environmental footprints). Achiev-ing a greater degree of integration would involveanalyzingandmanagingcoupledhumanandnatural systems over longer time periods, largerspatial extents (for example, macrosystems andultimatelytheentireplanet), andacrossmorediverseorganizationsatdifferentlevels. Below,we suggest several ways to advance systems in-tegration with the intent of improving its theo-retical foundations, expanding its tool box, andprovidingbroadimplicationsformanagementand policy.Incorporate more human and naturalcomponents simultaneouslyAlthough some previous studies have consideredmultiple components of coupled humanand natu-ralsystems, manycomponentsareeithernot1258832-6 27FEBRUARY2015VOL347ISSUE6225 sciencemag.org SCIENCEFig. 4. Balance and flows of virtual water related to trade of agricultural and industrial products during 19962005. Net exporters (sending systems)are in green, and net importers (receiving systems) are in red. The arrows indicate the relative sizes of large gross virtual water flows between sending andreceiving systems (> 15 Gm3/year). Countries without arrows are potential spillover systems of the large virtual water flows. Data are from (81).RESEARCH | REVIEWconsideredortreatedasexogenousvariables,leading to biases and even incorrect conclusions.For example, a food-waternexusstudy withoutconsidering GHGemissions during groundwaterextraction for irrigation of crops in China under-estimatedGHGemissionsbyasmuchas33.1MtCO2e (83). One way to correct this problem istoconvertmorevariablesfromexogenoustoendogenousinternalizeall importantrelevantvariablessothat theirdynamicsandfeedbackeffectsareexplicitlystudied. Forinstance, con-sidering multiple telecoupling processes (suchas species invasion, trade, disease spread, andtechnology transfers) at the same time can helplink many seemingly disconnected, distant inter-actions.Unifyingevolutionaryapproachessuchas seen with complex adaptive systems (84) mayprovideproductivewaystointegratedisparateideasandunderstandtemporal dynamicsandsustainability of coupled systems.Identify and quantify spillover systemsPrevious research on issues such as trade oftenfocusedonsendingandreceivingsystems(forexample, trade partners), with little attention tospillover systems (for example, nontrade partners)other systems affected by the interactions betweensendingandreceivingsystems. Althoughsomeprevious studies have recognized some spillovereffects [such as spatial externalities (85, 86)], theywere often on either socioeconomic or environ-mental effects, rather thanall effectssimulta-neously. Furthermore, they rarely consider othercomponents of spillover systems (causes, agents,andflows) asarticulatedinthetelecouplingframework (41). Identifying and quantifying oth-ercomponentsofspilloversystemsrelatedtospillover effects may help understand the mech-anisms behind the spillover effects and developmore effective management strategies. Connect-ing spillover systems with sending and receivingsystems through network analysis (87) may gen-erate fruitful outcomes, such as the appreciationof dynamicinterrelationshipsamongdifferentsystems.Explicitly account for feedbacksHuman-nature feedbacks are a core componentofcoupledsystems. Forinstance,animportantnegative feedback in Wolong Nature Reserve forgiant pandas in China occurred when deforesta-tionandpandahabitat degradationbylocalhouseholds prompted the government to devel-opandimplementnewconservationprogramsthatprovidesubsidiesforlocalhouseholdstomonitor forests and thus reduce deforestationand improve panda habitat (88). This feedbackhashelpedforest andhabitat recoverywhileincreasingincomeforlocalhouseholds. Moreinnovative measures such as this are needed toidentify and use feedbacks as mechanisms forsustainability.Integrate multiple temporal andspatial scalesHumanandnatural processesandpatternsatmultiplescalesmaybedifferent, andtheycaninteract with each other. For example, food pro-duction at the local scale may create jobs at thelocal scale but may not affect overall job creationattheglobalscale.Manyurbansustainabilityeffortsfocusonlocallyspecificsolutionsthatmay not be scalable (7). Thus, considering mul-tiplespatialscalesatthesametimecanhelpidentifyallimportantfactors, theirinterdepen-dence,and their effects and nonlinear relation-ships. Temporally, short-term studies should becombinedwithlong-termstudiesinordertomaximize the strengths of each approach. Forinstance, short-term studies may capture morenuanced immediate changes in system behavior,but long-term studies may account for temporaldynamics, time lags, cumulative effects, legacy ef-fects, and other phenomena (such as rare events)thatcannotbeseenovershorterterms. Moresystematic incorporation of human dimensionsto long-term studies such as the Long-termEcol-ogical Research sites and the National EcologicalObservatory Network is needed.Develop and use new toolsMore effective integration requires developingand using powerful tools to overcome difficultbarriers (for example, mathematical and com-putational challenges, quantification of impactsat one scale on other scales, and relationshipsamong patterns and processes across scales andacross borders) and to predict emergence of un-expectedthreatsforsustainabilitypolicyandmanagement. Examples include spatially explicitlife cycle assessment, supply chain analysis, andmultilevel modeling. Agent-basedmodelsareparticularly promising tools because they takeinteractions(suchashumanadaptationtoen-vironmental changes) at different scales into ac-countandmodelcoupledsystemsascomplexadaptive systems. Agent-based models create vir-tual worlds that mimic the real world, in contrasttotraditionalempiricalstatisticalmodels(suchas econometric models)that are fitted to pastdata and fail when the future differs from thepast, and dynamic stochastic general equilibriummodels that presume a perfect world and ignoredisturbances or crises (89). Many agent-basedmodels have beendevelopedinvarious disciplinesto provide insights on complexities and informa-tion for policymaking in issues such as economicdevelopment and management of common spaces(90, 91). However, newmodelsareneededtoaccount for telecoupled systems. Also, increasingcomputational power will allowagent-basedmod-els to include more agents in larger areas andultimately all important agents across the world.As more high-resolution data become available,it is necessary to develop and use big data tools(such as distributed databases, massively parallelprocessing, andcloudcomputing) foreffectiveand efficient searching, retrieving, analysis, andintegration (92).Translate findings into policy and practiceSystems integration can provide more unbiasedinformation for policy and practice to help clarifyresponsibilities, mediate trade-offs, reduce con-flicts, and anticipate future trends. It is necessaryto foster coordination among multiple nationaland international policies andminimize situationsin which different policies offset one another be-cause of conflicting goals and counterproductiveimplementation. Unfortunately, institutions andregulations have traditionally focused on singleissues and often do not have the mandate or in-frastructuretoaddresstheorganizational con-nectionsanddetrimentalspillovers.TheWorldTrade Organization, for example, has the princi-palmandateofpromotingglobal trade.Oneofthe spillover effects of this mission is the globaltransmission of invasive species, but the meanstoaddressinvasivespeciesareweakcomparedwith the forces driving the global market (93).Adopting the telecoupling framework can helpassign responsibilities of addressing spillover ef-fects(suchasCO2emissionsandspeciesinva-sion) to consumers and producers (for example,via regulation at the source of extraction or con-sumption)aswell asotherssuchastradersofgoodsandproductsacrossspace. Last, govern-ments need to incorporate long-termstudies intotheir policies to account for the complex dynam-ics of coupled systems (such as time lags). Moreapplicationsofsystemsintegrationframeworksand methods suchas those discussedinthis papercan accelerate understanding and solving globalsustainability challenges.REFERENCESANDNOTES1. World Commission On Environment and Development,Our Common Future (Oxford Univ. Press, Oxford, 1987).2. M. A. Hanjra, M. E. Qureshi, Global water crisis and futurefood security in an era of climate change. Food Policy 35,365377 (2010). doi: 10.1016/j.foodpol.2010.05.0063. J. Liu et al., Complexity of coupled human and naturalsystems. Science 317, 15131516 (2007). doi: 10.1126/science.1144004; pmid: 178724364. H. A. Mooney, A. Duraiappah, A. Larigauderie, Evolution ofnatural and social science interactions in global changeresearch programs. Proc. Natl. Acad. Sci. U.S.A. 110(suppl. 1), 36653672 (2013). doi: 10.1073/pnas.1107484110;pmid: 232972375. W. Steffen et al., Global Change and the Earth System:A Planet Under Pressure. IGBP Science 4 (Springer Verlag,Heidelberg, Germany, 2004).6. Millennium Ecosystem Assessment, Ecosystems & HumanWell-being: Synthesis (Island Press, Washington, DC, 2005).7. K. C. Seto et al., Urban land teleconnections andsustainability. Proc. Natl. Acad. Sci. U.S.A. 109, 76877692(2012). doi: 10.1073/pnas.1117622109; pmid: 225501748. E. Ostrom, A general framework for analyzing sustainabilityof social-ecological systems. Science 325, 419422 (2009).doi: 10.1126/science.1172133; pmid: 196288579. J. Liu, Forest sustainability in China and implications for atelecoupled world. Asia Pacific Pol. Stud. 1, 230250 (2014).10. J. Norberg, G. S. Cumming, Complexity Theory for aSustainable Future (Columbia Univ. Press, New York, 2013).11. S. A. Levin, Ecosystems and the biosphere as complexadaptive systems. Ecosystems (N.Y.) 1, 431436 (1998).doi: 10.1007/s10021990003712. E. F. Lambin, Conditions for sustainability of humanenvironment systems: Information, motivation, andcapacity. Glob. Environ. Change 15, 177180 (2005).doi: 10.1016/j.gloenvcha.2005.06.00213. P. H. Gleick, M. Palaniappan, Peak water limits to freshwaterwithdrawal and use. Proc. Natl. Acad. Sci. U.S.A. 107, 1115511162(2010). doi: 10.1073/pnas.1004812107; pmid: 2049808214. J. Cools et al., Coupling a hydrological water qualitymodel and an economic optimization model to set upa cost-effective emission reduction scenario for nitrogen.Environ. Model. Softw. 26, 4451 (2011). doi: 10.1016/j.envsoft.2010.04.017SCIENCE sciencemag.org 27FEBRUARY2015VOL347ISSUE6225 1258832-7RESEARCH | REVIEW15. S. Reis et al., From acid rain to climate change. Science338, 11531154 (2012). doi: 10.1126/science.1226514;pmid: 2319751716. M. Hejazi et al., Long-term global water projections using sixsocioeconomic scenarios in an integrated assessmentmodeling framework. Technol. Forecast. Soc. 81, 205226(2014). doi: 10.1016/j.techfore.2013.05.00617. GTAP, Global Trade Analysis Project, www.gtap.agecon.purdue.edu (2014).18. IIASA, Global Biosphere Management Model (GLOBIOM);www.globiom.org (2014).19. E. M. Bennett, G. D. Peterson, L. J. Gordon, Understandingrelationships among multiple ecosystem services. Ecol. Lett.12, 13941404 (2009). doi: 10.1111/j.1461-0248.2009.01387.x;pmid: 1984572520. A. Y. Hoekstra, T. O. Wiedmann, Humanitys unsustainableenvironmental footprint. Science 344, 11141117 (2014).doi: 10.1126/science.1248365; pmid: 2490415521. J. Rockstrm et al., A safe operating space for humanity. Nature461, 472475 (2009). doi: 10.1038/461472a; pmid: 1977943322. C. Kremen, A. Miles, Ecosystem services in biologicallydiversified versus conventional farming systems: Benefits,externalities, and trade-offs. Ecol. Soc. 17, 40 (2012).23. J. Fargione, J. Hill, D. Tilman, S. Polasky, P. Hawthorne, Landclearing and the biofuel carbon debt. Science 319, 12351238(2008). doi: 10.1126/science.1152747; pmid: 1825886224. P. J. Vergragt, N. Markusson, H. Karlsson, Carbon captureand storage, bio-energy with carbon capture and storage, andthe escape from the fossil-fuel lock-in. Glob. Environ. Change 21,282292 (2011). doi: 10.1016/j.gloenvcha.2011.01.02025. G. C. Unruh, Understanding carbon lock-in. Energy Policy 28,817830 (2000). doi: 10.1016/S0301-4215(00)00070-726. M. R. Raupach et al., Sharing a quota on cumulative carbonemissions. Nature Clim. Change 4, 873 (2014). doi: 10.1038/nclimate238427. R. L. Naylor, The Evolving Sphere of Food Security(Oxford Univ. Press, Oxford, 2014).28. J. I. Agboola, A. K. Braimoh, Strategic partnershipfor sustainable management of aquatic resources.Water Resour. Manage. 23, 27612775 (2009). doi: 10.1007/s11269-009-9407-429. A. Santhosh, A. M. Farid, K. Youcef-Toumi, The impact ofstorage facility capacity and ramping capabilities on thesupply side economic dispatch of the energywater nexus.Energy 66, 363377 (2014). doi: 10.1016/j.energy.2014.01.03130. S. C. Doney et al., Climate change impacts on marineecosystems. Annu. Rev. Mar. Sci. 4, 1137 (2012).doi: 10.1146/annurev-marine-041911-11161131. J. Granit, M. Fogde, S. Holger Hoff, J. Joyce, Unpacking theWater-Energy-Food Nexus: Tools for Assessment andCooperation Along a Continuum (Stockholm InternationalWater Institute, Stockholm, Sweden, 2013), pp. 4550.32. G. Oladosu, S. Msangi, Biofuel-food market interactions: Areview of modeling approaches and findings. Agriculture 3,5371 (2013). doi: 10.3390/agriculture301005333. S. Nair, B. George, H. M. Malano, M. Arora, B. Nawarathna,Waterenergygreenhouse gas nexus of urban watersystems: Review of concepts, state-of-art and methods.Resour. Conserv. Recycling 89, 110 (2014). doi: 10.1016/j.resconrec.2014.05.00734. P. Criqui, S. Mima, European climateenergy security nexus:A model based scenario analysis. Energy Policy 41, 827842(2012). doi: 10.1016/j.enpol.2011.11.06135. B. Hubert, M. Rosegrant, M. A. J. S. van Boekel, R. Ortiz, Thefuture of food: Scenarios for 2050. Crop Sci. 50 (suppl. 1),S33S-50 (2010). doi: 10.2135/cropsci2009.09.053036. R. Coers, M. Sanders, The energyGDP nexus; Addressing anold question with new methods. Energy Econ. 36, 708715(2013). doi: 10.1016/j.eneco.2012.11.01537. B. Saboori, J. B. Sulaiman, S. Mohd, An empirical analysis of theenvironmental Kuznets curve for CO2 emissions in Indonesia: Therole of energy consumption and foreign trade. Int. J. Econ. Fin. 4,243 (2012).38. A. S. Stillwell, D. C. Hoppock, M. E. Webber, Energy recoveryfrom wastewater treatment plants in the United States:A case study of the energy-water nexus. Sustainability 2,945962 (2010). doi: 10.3390/su204094539. J. Liu et al., in Rethinking Global Land Use in an Urban Era,K. Seto, A. Reenberg, Eds. (MIT Press, Cambridge, MA, 2014),pp. 119139.40. J. M. Prospero, O. L. Mayol-Bracero, Understanding thetransport and impact of African dust on the Caribbean basin.Bull. Am. Meteorol. Soc. 94, 13291337 (2013). doi: 10.1175/BAMS-D-12-00142.141. D. W. Griffin, C. A. Kellogg, V. H. Garrison, E. A. Shinn,The global transport of dust. Am. Sci. 90, 228 (2002).doi: 10.1511/2002.3.22842. J. Liu et al., Framing sustainability in a telecoupled world.Ecol. Soc. 18, art26 (2013). doi: 10.5751/ES-05873-18022643. T. Searchinger et al., Use of U.S. croplands for biofuelsincreases greenhouse gases through emissions from land-use change. Science 319, 12381240 (2008). doi: 10.1126/science.1151861; pmid: 1825886044. S. J. Davis, G. P. Peters, K. Caldeira, The supply chain of CO2emissions. Proc. Natl. Acad. Sci. U.S.A. 108, 1855418559(2011). doi: 10.1073/pnas.1107409108; pmid: 2200631445. M. Lenzen et al., International trade drives biodiversitythreats in developing nations. Nature 486, 109112 (2012).doi: 10.1038/nature11145; pmid: 2267829046. S. L. Pimm et al., The biodiversity ofspecies andtheirratesofextinction, distribution, andprotection. Science344, 1246752 (2014). doi: 10.1126/science.1246752;pmid: 2487650147. J. Bollen, S. Hers, B. Van der Zwaan, An integratedassessment of climate change, air pollution, and energysecurity policy. Energy Policy 38, 40214030 (2010).doi: 10.1016/j.enpol.2010.03.02648. H. Eakin et al., in Rethinking Global Land Use in an Urban Era,K. Seto, A. Reenberg, Eds. (MIT Press, Cambridge, MA, 2014),pp. 141161.49. B. Wicke, in Rethinking Global Land Use in an Urban Era,K. Seto, A. Reenberg, Eds. (MIT Press, Cambridge, MA, 2014),pp. 163181.50. J. Liu, W. Yang, Integrated assessments of payments forecosystem services programs. Proc. Natl. Acad. Sci. U.S.A.110, 1629716298 (2013). doi: 10.1073/pnas.1316036110;pmid: 2407264851. M. C. Thompson, M. Baruah, E. R. Carr, Seeing REDD+ as aproject of environmental governance. Environ. Sci. Pol. 14,100110 (2011).52. F. I. Khan, D. S. Schinn, Triple transformation. Nature Clim.Change 3, 692 (2013). doi: 10.1038/nclimate196553. H. Zheng et al., Benefits, costs, and livelihood implicationsof a regional payment for ecosystem service program.Proc. Natl. Acad. Sci. U.S.A. 110, 1668116686 (2013).doi: 10.1073/pnas.1312324110; pmid: 2400316054. L. O. Gostin, D. Lucey, A. Phelan, The Ebola epidemic:A global health emergency. JAMA 312, 10951096 (2014).doi: 10.1001/jama.2014.11176; pmid: 2511104455. A. Via et al., Effects of natural disasters on conservationpolicies: The case of the 2008 Wenchuan earthquake, China.Ambio 40, 274284 (2011). doi: 10.1007/s13280-010-0098-0;pmid: 2164445656. M. E. Portman, L. S. Esteves, X. Q. Le, A. Z. Khan,Improving integration for integrated coastal zonemanagement: An eight country study. Sci. Total Environ.439, 194201 (2012). doi: 10.1016/j.scitotenv.2012.09.016;pmid: 2306392557. C. White, B.S. Halpern, C.V. Kappel, Ecosystem servicetradeoff analysis reveals the value of marine spatial planningfor multiple ocean uses. Proc. Natl. Acad. Sci. U.S.A. 109,46964701 (2012).58. R. C. Szaro, W. T. Sexton, C. R. Malone, The emergence ofecosystem management as a tool for meeting peoples needsand sustaining ecosystems. Landsc. Urban Plan. 40, 17(1998). doi: 10.1016/S0169-2046(97)00093-559. R. T. Munang, I. Thiaw, M. Rivington, Ecosystemmanagement: Tomorrows approach to enhancing foodsecurity under a changing climate. Sustainability 3, 937954(2011). doi: 10.3390/su307093760. T. Hahn, Self-organized governance networks for ecosystemmanagement: Who is accountable? Ecol. Soc. 16, 18 (2011).61. S. E. Shackleton, C. M. Shackleton, Linking poverty,HIV/AIDS and climate change to human and ecosystemvulnerability in southern Africa: Consequences forlivelihoods and sustainable ecosystem management. Int. J.Sustain. Dev. World Ecol. 19, 275286 (2012). doi: 10.1080/13504509.2011.64103962. B. Digest, Biofuels Mandates around the World 2012;www.biofuelsdigest.com/bdigest/2012/11/22/biofuels-mandates-around-the-world-2012 (2014).63. S. J. Davis, J. A. Burney, J. Pongratz, K. Caldeira, Methods forattributing land-use emissions to products. Carbon Management5, 233245 (2014). doi: 10.1080/17583004.2014.91386764. Worldwatch Institute, Biofuels make a comeback despitetough economy (2011); available at www.worldwatch.org/biofuels-make-comeback-despite-tough-economy.65. G. P. Hammond, S. M. Seth, Carbon and environmentalfootprinting of global biofuel production. Appl. Energy 112,547559 (2013). doi: 10.1016/j.apenergy.2013.01.00966. V. Galaz et al.,Planetary boundariesExploring thechallenges for global environmental governance. Curr. Opin.Environ. Sustain. 4, 8087 (2012). doi: 10.1016/j.cosust.2012.01.00667. P. Westhoff, The Economics of Food (Financial Times Press,NJ, 2010).68. M. Ivanic, W. Martin, Implications of higher global food pricesfor poverty in low-income countries. Agric. Econ. 39, 405416(2008). doi: 10.1111/j.1574-0862.2008.00347.x69. N. S. Diffenbaugh, T. W. Hertel, M. Scherer, M. Verma,Response of corn markets to climate volatility underalternative energy futures. Nat. Clim. Chang. 2, 514518(2012). pmid: 2324346870. F. Taheripour, T. W. Hertel, J. Liu, The role of irrigation indetermining the global land use impacts of biofuels. EnergySustain. Soc. 3, 1 (2013).71. J. M. Melillo et al., Indirect emissions from biofuels: Howimportant? Science 326, 13971399 (2009). doi: 10.1126/science.1180251; pmid: 1993310172. N. B. Villoria, T. W. Hertel, Geography matters: Internationaltrade patterns and the indirect land use effects of biofuels.Am. J. Agric. Econ. 93, 919935 (2011). doi: 10.1093/ajae/aar02573. J. N. Galloway et al., International trade in meat: The tip ofthe pork chop. Ambio 36, 622629 (2007). doi: 10.1579/0044-7447(2007)36[622:ITIMTT]2.0.CO;2; pmid: 1824067574. J. A. Carr, P. DOdorico, F. Laio, L. Ridolfi, Recent history andgeography of virtual water trade. PLOS ONE 8, e55825(2013). doi: 10.1371/journal.pone.0055825; pmid: 2345748175. S. Suweis, A. Rinaldo, A. Maritan, P. DOdorico, Water-controlledwealth of nations. Proc. Natl. Acad. Sci. U.S.A. 110, 42304233(2013). doi: 10.1073/pnas.1222452110; pmid: 2335970976. J. A. Allan, 'Virtual Water': A Long Term Solution for WaterShort Middle Eastern Economies? (School of Oriental andAfrican Studies, University of London, 1997).77. A. Y. Hoekstra, P. Hung, Virtual water trade: A quantificationof virtual water flows between nations in relation tointernational crop trade, Value of Water Research ReportSeries 11 (IHE Delft, Delft, Netherlands, 2002).78. M. Konar, Z. Hussein, N. Hanasaki, D. Mauzerall,I. Rodriguez-Iturbe, Virtual water trade flows and savingsunder climate change. Hydrol. Earth Syst. Sci. 17, 32193234(2013). doi: 10.5194/hess-17-3219-201379. R. Boelens, J. Vos, The danger of naturalizing water policyconcepts: Water productivity and efficiency discourses fromfield irrigation to virtual water trade. Agric. Water Manage.108, 1626 (2012). doi: 10.1016/j.agwat.2011.06.01380. D. Schindler, P. Lee, Comprehensive conservation planning toprotect biodiversity and ecosystem services in Canadianboreal regions under a warming climate and increasingexploitation. Biol. Conserv. 143, 15711586 (2010).doi: 10.1016/j.biocon.2010.04.00381. A. Y. Hoekstra, M. M. Mekonnen, The water footprint ofhumanity. Proc. Natl. Acad. Sci. U.S.A. 109, 32323237(2012). doi: 10.1073/pnas.1109936109; pmid: 2233189082. C. Dalin, M. Konar, N. Hanasaki, A. Rinaldo, I. Rodriguez-Iturbe,Evolution of the global virtual water trade network. Proc. Natl.Acad. Sci. U.S.A. 109, 59895994 (2012). doi: 10.1073/pnas.1203176109; pmid: 2247436383. J. Wang et al., Chinas waterenergy nexus: Greenhouse-gasemissions fromgroundwater use for agriculture. Environ. Res. Lett.7, 014035 (2012). doi: 10.1088/1748-9326/7/1/01403584. S. A. Levin, J. Lubchenco, Resilience, robustness, and marineecosystem-based management. Bioscience 58, 27 (2008).doi: 10.1641/B58010785. B. S. Halpern, S. E. Lester, J. B. Kellner, Spillover from marinereserves and the replenishment of fished stocks. Environ.Conserv. 36, 268276 (2009). doi: 10.1017/S037689291000003286. K. S. Andam, P. J. Ferraro, A. Pfaff, G. A. Sanchez-Azofeifa,J. A. Robalino, Measuring the effectiveness of protected areanetworks in reducing deforestation. Proc. Natl. Acad. Sci. U.S.A.105, 1608916094 (2008). doi: 10.1073/pnas.0800437105;pmid: 1885441487. B. D. Anderson, S. Vongpanitlerd, Network Analysis andSynthesis: A Modern Systems Theory Approach (CourierDover Publications, Mineola, NY, 2006).88. W. Yang et al., Nonlinear effects of group size on collectiveaction and resource outcomes. Proc. Natl. Acad. Sci. U.S.A.110, 1091610921 (2013). doi: 10.1073/pnas.1301733110;pmid: 237762221258832-8 27FEBRUARY2015VOL347ISSUE6225 sciencemag.org SCIENCERESEARCH | REVIEW89. J. D. Farmer, D. Foley, The economy needs agent-basedmodelling. Nature 460, 685686 (2009). doi: 10.1038/460685a; pmid: 1966189690. M. A. Janssen, E. Ostrom, Empirically based, agent-basedmodels. Ecol. Soc. 11, 37 (2006).91. E. Ostrom, V. Ostrom, Choice, Rules and Collective Action:The Ostroms on the Study of Institutions and Governance(ECPR Press, Colchester, UK, 2014).92. D. Agrawal, S. Das, A. El Abbadi, in Proceedings of the 14thInternational Conference on Extending Database Technology.(ACM, New York, 2011), pp. 530533.93. M. Margolis, J. F. Shogren, Disguised protectionism, globaltrade rules and alien invasive species. Environ. Resour. Econ.51, 105118 (2012). doi: 10.1007/s10640-011-9490-x94. S. R. Carpenter et al., Science for managing ecosystemservices: Beyond the Millennium Ecosystem Assessment.Proc. Natl. Acad. Sci. U.S.A. 106, 13051312 (2009).doi: 10.1073/pnas.0808772106; pmid: 1917928095. C. Liu, C. Kroeze, A. Y. Hoekstra, W. Gerbens-Leenes, Pastand future trends in grey water footprints of anthropogenicnitrogen and phosphorus inputs to major world rivers. Ecol. Indic.18, 4249 (2012). doi: 10.1016/j.ecolind.2011.10.00596. CO2Now.org; co2now.org/Current-CO2/CO2-Now/global-carbon-emissions.html (2014).97. T. O. Wiedmann et al., The material footprint of nations. Proc.Natl. Acad. Sci. U.S.A. 10.1073/pnas.1220362110 (2013).98. F. Bingham, B. Perkins, Trade Competitiveness & AdvocacyBranch, Australias coal and iron ore exports20012011(2012).99. T. W. Hertel et al., Effects of US maize ethanol on globalland use and greenhouse gas emissions: Estimatingmarket-mediated responses. Bioscience 60, 223231 (2010).doi: 10.1525/bio.2010.60.3.8100. J. Liu et al., Ecological degradation in protected areas:The case of Wolong Nature Reserve for giant pandas.Science 292, 98101 (2001). doi: 10.1126/science.1058104;pmid: 11292872101. J. Phelps, L. R. Carrasco, E. L. Webb, L. P. Koh,U. Pascual, Agricultural intensification escalates futureconservation costs. Proc. Natl. Acad. Sci. U.S.A. 110,76017606 (2013). doi: 10.1073/pnas.1220070110;pmid: 23589860102. J. Rogelj, D. L. McCollum, A. Reisinger, M. Meinshausen,K. Riahi, Probabilistic cost estimates for climate changemitigation. Nature 493, 7983 (2013). doi: 10.1038/nature11787; pmid: 23282364103. D. Shindell et al., Simultaneously mitigating near-termclimate change and improving human health and foodsecurity. Science 335, 183189 (2012). doi: 10.1126/science.1210026; pmid: 22246768104. J. Reis, T. Stojanovic, H. Smith, Relevance of systemsapproaches for implementing Integrated Coastal ZoneManagement principles in Europe. Mar. Policy 43, 312(2014). doi: 10.1016/j.marpol.2013.03.013105. R. Biggs, F. R. Westley, S. R. Carpenter, Navigating the backloop: Fostering social innovation and transformation inecosystem management. Ecol. Soc. 15, 9 (2010).106. T. A. Spies et al., Examining fire-prone forest landscapes ascoupled human and natural systems. Ecol. Soc. 19, art9(2014). doi: 10.5751/ES-06584-190309107. T. C. Coverdale, N. C. Herrmann, A. H. Altieri, M. D. Bertness,Latent impacts: The role of historical human activity incoastal habitat loss. Front. Ecol. Environ 11, 6974 (2013).doi: 10.1890/120130108. X. Chen et al., Producing more grain with lowerenvironmental costs. Nature 514, 486489 (2014).doi: 10.1038/nature13609; pmid: 25186728ACKNOWLEDGMENTSWe thank the U.S. National Science Foundation, the InternationalNetwork of Research on Coupled Human and Natural Systems,Michigan State University, and Michigan AgBioResearch forfinancial support. We are grateful to J. Broderick, W. McConnell,J. McCoy, S. Nichols, and W. Yang for assistance and to twoanonymous reviewers for helpful comments.10.1126/science.1258832SCIENCE sciencemag.org 27FEBRUARY2015VOL347ISSUE6225 1258832-9RESEARCH | REVIEWDOI: 10.1126/science.1258832, (2015); 347 Science et al. 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