DOI: 10.1126/science.1111772 , 570 (2005); 309 Science et al ...

R EVIEWin ecosystem services, including many thatare important to agriculture.Freshwater ResourcesLand use can disrupt the surface waterbalance and the partitioning of precipitationinto evapotranspiration, runoff, and groundwaterflow. Surface runoff and river dischargegenerally increase when natural vegetation(especially forest) is cleared (25, 26). Forinstance, the Tocantins River basin in Brazilshowed a È25% increase in river dischargebetween 1960 and 1995, coincident with expandingagriculture but no major change inprecipitation (26).Water demands associated with land-usepractices, especially irrigation, directly affectfreshwater supplies through water withdrawalsand diversions. Global waterwithdrawals now totalÈ3900 km 3 yr j1 ,orÈ10%of the total global renewableresource, and the consumptiveuse of water (notreturned to the watershed) isestimated to be È1800 to2300 km 3 yr j1 (22, 27)(fig.S3A). Agriculture alone accountsfor È85% of globalconsumptive use (22). As aresult, many large rivers, especiallyin semiarid regions,have greatly reduced flows,and some routinely dry up(21, 28). In addition, theextraction of groundwaterreserves is almost universallyunsustainable and hasresulted in declining watertables in many regions(21, 28) (fig.S2,BandC).Water quality is oftendegraded by land use. Intensiveagriculture increaseserosion and sediment load,and leaches nutrients andagricultural chemicals togroundwater, streams, and100 %proportion of landscape0 %naturalecosystemsrivers. In fact, agriculture has become thelargest source of excess nitrogen and phosphorusto waterways and coastal zones (12, 29).Urbanization also substantially degrades waterquality, especially where wastewater treatmentis absent. The resulting degradation ofinland and coastal waters impairs water supplies,causes oxygen depletion and fish kills,increases blooms of cyanobacteria (includingtoxic varieties), and contributes to waterbornedisease (12, 30).Forest ResourcesLand-use activities, primarily for agriculturalexpansion and timber extraction, have caused anet loss of È7to11millionkm 2 of forest in thepast 300 years (17, 32, 33). Highly managedforests, such as timber plantations in NorthAmerica and oil-palm plantations in SoutheastAsia, have also replaced many natural forestsand now cover 1.9 million km 2 worldwide (31).Many land-use practices (e.g., fuel-woodcollection, forest grazing, and road expansion)can degrade forest ecosystem conditions—interms of productivity, biomass, stand structure,and species composition—even withoutchanging forest area. Land use can also degradeforest conditions indirectly by introducingpests and pathogens, changing fire-fuelloads, changing patterns and frequency of ignitionsources, and changing local meteorologicalconditions (34).In many parts of the world, especially inEast Asian countries, reforestation and afforestationare increasing the area of forestedfrontierclearingssubsistenceagricultureandsmall-scalefarmspre-settlement frontier subsistence intensifying intensivestage in land use transitionlands (35). Furthermore, forest managementin many regions is acting to improve forestconditions. For example, inadvertent nitrogenfertilization, peatland drainage, and direct managementefforts increased the standing biomassof European forests by È40% between1950 and 1990, while their area remainedlargely unchanged (36, 37). These forests havebecome a substantial sink of atmosphericcarbon (È0.14 Pg C yr j1 in the 1990s) (37),although other ecosystem services (includingthose provided by peatlands) and biodiversityare likely diminished.protected/recreational landsintensiveagricultureurbanareas?Fig. 1. Land-use transitions. Transitions in land-use activities that may be experiencedwithin a given region over time. As with demographic and economic transitions, societiesappear also to follow a sequence of different land-use regimes: from presettlement naturalvegetation to frontier clearing, then to subsistence agriculture and small-scale farms,and finally to intensive agriculture, urban areas, and protected recreational lands. Differentparts of the world are in different transition stages, depending on their history,social and economic conditions, and ecological context. Furthermore, not all parts ofthe world move linearly through these transitions. Rather, some places remain in onestage for a long period of time, while others move rapidly between stages. [Adaptedfrom (1) and (2)]Regional Climate and Air QualityLand conversion can alter regional climatesthrough its effects on net radiation, the divisionof energy into sensible and latent heat,and the partitioning of precipitation into soilwater, evapotranspiration, and runoff. Modelingstudies demonstrate that land-cover changesin the tropics affect climate largely throughwater-balance changes, but changes in temperateand boreal vegetation influence climateprimarily through changes in the surface radiationbalance (38). Large-scale clearing oftropical forests may create a warmer, drierclimate (39), whereas clearing temperate andboreal forest is generally thought to cool theclimate, primarily through increased albedo(40) (table S2, A and B).Urban ‘‘heat islands’’ are an extreme caseof how land use modifies regional climate.The reduced vegetation cover, impervioussurface area, and morphology of buildings incityscapes combine to lowerevaporative cooling, storeheat, and warm the surfaceair (41). A recent analysisof climate records in theUnited States suggests thata major portion of the temperatureincrease during thelast several decades resultedfrom urbanization and otherland-use changes (9). Landcoverchange has also beenimplicated in changing theregional climate in China;recent analyses suggest thatthe daily diurnal temperaturerange has decreased asa result of urbanization (42).Land-use practices alsochange air quality by alteringemissions and changingthe atmospheric conditionsthat affect reaction rates,transport, and deposition.For example, troposphericozone (O 3)isparticularlysensitive to changes in vegetationcover and biogenicemissions. Land-use practicesoften determine dustsources, biomass burning, vehicle emissionpatterns, and other air pollution sources.Furthermore, the effects of land use on localmeteorological conditions, primarily in urbanheat islands, also affect air quality: Higherurban temperatures generally cause O 3to increase(43).Infectious DiseaseHabitat modification, road and dam construction,irrigation, increased proximity of peopleand livestock, and the concentration orexpansion of urban environments all modifythe transmission of infectious disease and canlead to outbreaks and emergence episodes(44). For example, increasing tropical deforestationcoincides with an upsurge of malariaDownloaded from www.sciencemag.org on May 18, 2009www.sciencemag.org SCIENCE VOL 309 22 JULY 2005 571