Population and Environment - K4Health · Population and Environment Alex de Sherbinin,1 David Carr,2 Susan Cassels,3 and Leiwen Jiang4 ... The interactions between human population

ANRV325-EG32-12 ARI 14 September 2007 20:33

Population andEnvironmentAlex de Sherbinin,1 David Carr,2 Susan Cassels,3

and Leiwen Jiang4

1Center for International Earth Science Information Network, Columbia Universityand Population-Environment Research Network, Palisades, New York 10964;email: [email protected] of Geography, University of California, Santa Barbara, California93106-4060; email: [email protected] for Studies in Demography and Ecology, University of Washington, Seattle,Washington 98195; email: [email protected] Institute for International Studies, Brown University, Providence,Rhode Island 02912; email: leiwen [email protected]

Annu. Rev. Environ. Resour. 2007. 32:345–73

First published online as a Review in Advance onJuly 16, 2007

The Annual Review of Environment and Resourcesis online at http://environ.annualreviews.org

This article’s doi:10.1146/annurev.energy.32.041306.100243

Copyright c© 2007 by Annual Reviews.All rights reserved

1543-5938/07/1121-0345$20.00

Key Words

climate change, coastal and marine environments, land-coverchange, land degradation, population dynamics, water resources

AbstractThe interactions between human population dynamics and the en-vironment have often been viewed mechanistically. This review elu-cidates the complexities and contextual specificities of population-environment relationships in a number of domains. It explores theways in which demographers and other social scientists have soughtto understand the relationships among a full range of populationdynamics (e.g., population size, growth, density, age and sex com-position, migration, urbanization, vital rates) and environmentalchanges. The chapter briefly reviews a number of the theories forunderstanding population and the environment and then proceedsto provide a state-of-the-art review of studies that have examinedpopulation dynamics and their relationship to five environmental is-sue areas. The review concludes by relating population-environmentresearch to emerging work on human-environment systems.

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Carrying capacity:the animalpopulation that canbe supported giventhe quantity of food,habitat, and waterpresent in a givenarea

Contents

INTRODUCTION. . . . . . . . . . . . . . . . . 346Global Trends in Population and

Consumption . . . . . . . . . . . . . . . . . 347POPULATION-ENVIRONMENT

THEORIES . . . . . . . . . . . . . . . . . . . . . 348REVIEW BY ENVIRONMENTAL

ISSUE AREA . . . . . . . . . . . . . . . . . . . . 350Land-Cover Change and

Deforestation . . . . . . . . . . . . . . . . . 351Agricultural Land Degradation

or Improvement . . . . . . . . . . . . . . . 353Abstraction and Pollution of Water

Resources . . . . . . . . . . . . . . . . . . . . . 356Coastal and Marine

Environments . . . . . . . . . . . . . . . . . 358Energy, Air Pollution, and Climate

Change . . . . . . . . . . . . . . . . . . . . . . . 360CONCLUSION . . . . . . . . . . . . . . . . . . . . 362

INTRODUCTION

Humans have sought to understand the re-lationship between population dynamics andthe environment since the earliest times (1, 2),but it was Thomas Malthus’ Essay on the Prin-ciple of Population (3) in 1798 that is cred-ited with launching the study of populationand resources as a scientific topic of inquiry.Malthus’ famous hypothesis was that popu-lation numbers tend to grow exponentiallywhile food production grows linearly, neverquite keeping pace with population and thusresulting in natural “checks” (such as famine)to further growth. Although the subject wasperiodically taken up again in the ensuringdecades, with for example George PerkinsMarsh’s classic Man and Nature (1864) (4) andconcern over human-induced soil depletion incolonial Africa (5, 6), it was not until the 1960sthat significant research interest was rekin-dled. In 1963, the U.S. National Academy ofSciences published The Growth of World Popu-lation (7), a report that reflected scientific con-cern about the consequences of global pop-

ulation growth, which was then reaching itspeak annual rate of two percent. In 1968, PaulEhrlich published The Population Bomb (8),which focused public attention on the issueof population growth, food production, andthe environment. By 1972, the Club of Romehad released its World Model (9), which rep-resented the first computer-based population-environment modeling effort, predicting an“overshoot” of global carrying capacity within100 years.

Clearly, efforts to understand the relation-ship between demographic and environmen-tal change are part of a venerable tradition.Yet, by the same token, it is a traditionthat has often sought to reduce environmen-tal change to a mere function of popula-tion size or growth. Indeed, an overlay ofgraphs depicting global trends in population,energy consumption, carbon dioxide (CO2)emissions, nitrogen deposition, or land areadeforested has often been used to demon-strate the impact that population has on theenvironment. Although we start from thepremise that population dynamics do indeedhave an impact on the environment, we alsobelieve that monocausal explanations of en-vironmental change that give a preeminentplace to population size and growth sufferfrom three major deficiencies: They oversim-plify a complex reality, they often raise morequestions than they answer, and they mayin some instances even provide the wronganswers.

As the field of population-environmentstudies has matured, researchers increasinglyhave wanted to understand the nuances ofthe relationship. In the past two decadesdemographers, geographers, anthropologists,economists, and environmental scientists havesought to answer a more complex set ofquestions, which include, among others, howdo specific population changes (in density,composition, or numbers) relate to specificchanges in the environment (such as defor-estation, climate change, or ambient concen-trations of air and water pollutants)? Howdo environmental conditions and changes,

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in turn, affect population dynamics? Howdo intervening variables, such as institutionsor markets, mediate the relationship? Andhow do these relationships vary in time andspace? They have sought to answer thesequestions armed with a host of new tools (geo-graphic information systems, remote sensing,computer-based models, and statistical pack-ages) and with evolving theories on human-environment interactions.

This review explores the ways in whichdemographers and other social scientists havesought to understand the relationships amonga full range of population dynamics (e.g.,population size, growth, density, age and sexcomposition, migration, urbanization, vitalrates) and environmental changes. With theexception of the energy subsection, the focusis largely on micro- and mesoscale studiesin the developing world. This is not becausethese dynamics are unimportant in thedeveloped world—on the contrary, per capitaenvironmental impacts are far greater in thisregion (see the text below on global popu-lation and consumption trends)—but ratherbecause this is where much of the researchhas focused (10). We have surveyed a widearray of literature with an emphasis on peer-reviewed articles from the past decade, butgiven the veritable explosion in population-environment research, we hasten to add thatthis review merely provides a sampling of themost salient findings. The chapter begins witha short review of the theories for understand-ing population and the environment. It thenproceeds to provide a state-of-the-art reviewof studies that have examined populationdynamics and their relationship to the fol-lowing environmental issue areas: land-coverchange and deforestation; agricultural landdegradation and improvement; abstractionand pollution of water resources; coastal andmarine environments; and energy, air pollu-tion, and climate change. In the concludingsection, we relate population-environmentresearch to the emerging understanding ofcomplex human-environment systems.

Land degradation:any human inducedor natural processthat negativelyaffects soil structure,nutrients, organicmatter,moisture-holdingcapacity, acidity andsalinity

Global Trends in Populationand Consumption

At the global level, research has found thatthe two major drivers of humanity’s ecologi-cal footprint are population and consumption(11), so we provide a brief introduction to thestatus and trends in these two indicators.

The future size of world population is pro-jected on the basis of assumed trends in fer-tility and mortality. Current world populationstands at 6.7 billion people (12). The 2006 re-vision of the United Nations World PopulationProspects presents a medium variant projectionby 2050 of 9.2 billion people and still growing,although at a significantly reduced rate. All ofthe projected growth is expected to occur inthe developing world (increasing from 5.4 to7.9 billion), whereas the developed world isexpected to remain unchanged at 1.2 billion.Africa, which has the fastest growing popu-lation of the continents, is projected to morethan double the number of its inhabitants inthe next 43 years—from 965 million to ap-proximately 2 billion. Globally, fertility is as-sumed to decline to 2.02 births per woman(below replacement) by 2050; it is populationmomentum arising from a young age struc-ture that will cause global population to con-tinue to grow beyond 2050 (the 2006 revi-sion does not make prognoses about ultimatestabilization). The medium variant is brack-eted by a low-variant projection of 7.8 billion(and declining) and a high variant of 10.8 bil-lion (and growing rapidly) by 2050. Fertilityin the former is assumed to be half a childlower than the medium variant, and in thelatter, it is assumed to be half a child higher.1

As Cohen (2) points out, minor variations in

1Fertility in most of the developed world is at or belowreplacement levels (2.1 births per woman). Fertility hasdeclined significantly since the middle of the twentiethcentury in many developing countries owing to many fac-tors, such as urbanization, the improved status of womenthrough education and job opportunities, and increasingaccess to contraception. The different projections makedifferent assumptions concerning future progress in reduc-ing fertility.

www.annualreviews.org • Population and Environment 347

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IPAT: This identityholds thatenvironmentalimpacts (I) are theproduct ofpopulation size (P),affluence orconsumption (A),and technology (T).

above- or below-replacement fertility canhave dramatic long-term consequences for theultimate global population size; hence, pro-jections are highly conditional, and their sen-sitivity to the underlying assumptions needsto be properly understood. Finally, the im-pact of the HIV/AIDS epidemic on futuremortality is assumed to attenuate somewhaton the basis of recent declines in prevalencein some countries, increasing antiretroviraldrug therapy, and government commitmentsmade under the Millennium Declaration(13).

Consumption trends are somewhat moredifficult to predict because they depend moreheavily than population projections on globaleconomic conditions, efforts to pursue sus-tainable development, and potential feed-backs from the environmental systems uponwhich the global economy depends for re-sources and sinks. Nevertheless, several in-dicators of consumption have grown at rateswell above population growth in the past cen-tury: Global GDP is 20 times higher than itwas in 1900, having grown at a rate of 2.7%per annum (14); CO2 emissions have grownat an annual rate of 3.5% since 1900, reach-ing an all-time high of 100 million metrictons of carbon in 2001 (15); and the ecolog-ical footprint, a composite measure of con-sumption measured in hectares of biologicallyproductive land, grew from 4.5 to 14.1 bil-lion hectares between 1961 and 2003, andit is now 25% more than Earth’s “biocapac-ity” according to Hails (16). In the case ofCO2 emissions and footprints, the per capitaimpacts of high-income countries are cur-rently 6 to 10 times higher than those inlow-income countries. As far as the future isconcerned, barring major policy changes oreconomic downturns, there is no reason tosuspect that consumption trends will changesignificantly in the near term. Long-termprojections suggest that economic growthrates will decline past 2050 owing to de-clining population growth, saturation of con-sumption, and slower technological change(14).

POPULATION-ENVIRONMENTTHEORIES

As in any contested field—and population-environment studies certainly fit thisdescription—a wide array of theories haveemerged to describe the relationship amongthe variables of interest, and each of thesetheories leads to starkly different conclusionsand policy recommendations. Here we reviewthe most prominent theories in the field ofpopulation and environment.

The introduction briefly touched on thework of Malthus, whose theory still gener-ates strong reactions 200 years after it wasfirst published. Adherents of Malthus havegenerally been termed neo-Malthusians. Inits simplest form, neo-Malthusianism holdsthat human populations, because of their ten-dency to increase exponentially if fertility isunchecked, will ultimately outstrip Earth’sresources, leading to ecological catastrophe.This has been one of the dominant paradigmsin the field of population and the environ-ment, but it is one which many social scien-tists have rejected because of its underlyingbiological/ecological underpinnings, treatinghumans in an undifferentiated way from otherspecies that grow beyond the local “carry-ing capacity.” Neo-Malthusianism has beencriticized for overlooking cultural adaptation,technological developments, trade, and insti-tutional arrangements that have allowed hu-man populations to grow beyond their localsubsistence base.

Neo-Malthusianism underpins the Club ofRome World Model (mentioned above) (9)and implicitly or explicitly underlies manystudies and frameworks. The widely citedIPAT formulation—in which environmentalimpacts (I) are the product of population (P),affluence (A), and technology (T)—is implic-itly framed in neo-Malthusian terms (17), al-though not all research using the identity isMalthusian in approach (18). IPAT itself hasbeen criticized because it does not accountfor interactions among the terms (e.g., in-creasing affluence can lead to more efficient


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technologies); it omits explicit reference toimportant variables such as culture and in-stitutions (e.g., social organization); impactis not linearly related to the right side vari-ables (there can be important thresholds);and it can simply lead to wrong conclusions(19).2

The so-called Boserupian hypothesis,named after agricultural economist EstherBoserup, holds that agricultural productionincreases with population growth owing to theintensification of production (greater laborand capital inputs). Although often depictedas being in opposition to Malthusianism,Malthus himself acknowledged that agri-cultural output increases with increasingpopulation density (just not fast enough),and Boserup acknowledged that there aresituations under which intensification mightnot take place (20). As Turner & Shajaat Ali(21) point out, the main difference betweenthe theories of Malthus and Boserup is thatMalthus saw technology as being exogenousto the population-resource condition andBoserup sees it as endogenous. Cornucopiantheories espoused by some neoclassicaleconomists stand in sharper contrast toneo-Malthunisianism because they posit thathuman ingenuity (through the increasedsupply of more creative people) and marketsubstitution (as certain resources becomescarce) will avert future resource crises (22). Inthis line of thinking, market failures and inap-propriate technologies are more responsiblefor environmental degradation than popula-tion size or growth, and natural resources canbe substituted by man-made ones.

Political ecology also frequently informsthe population-environment literature (23).

2For example, Myers (156), finding that the CO2 emissionsgrew annually by 3.1% from 1950–1980 and that popula-tion grew by 1.9% annually during the same time period,concluded that population growth contributed nearly twothirds of emissions. Yet Preston (157) shows the logical fal-lacy behind this by pointing out that most of the growthin fossil fuel use during this period was in developed coun-tries with limited population growth and that populationgrew much faster in countries with the lowest per capitaemissions.

Thresholds: a pointin a system’scondition in whichabrupt change isobserved and beyondwhich recovery ofearlier conditions isdifficult

Intensification:increasing cropoutput per unit ofland and/or per unitof labor

VCM: vicious circlemodel

Many political ecologists see population andenvironment as linked only insofar as theyhave a common root cause, e.g., poverty, andthat poverty itself stems from economic im-balances between the developed and devel-oping world and within developing countriesthemselves (e.g., 24). In this view, migrants todeforestation hot spots in frontier areas maybe victims of historical inequalities in land ac-cess in their country’s core agricultural areas,or they may be responding to global inequal-ities in which industrialized countries dependon resource extraction from tropical countriesto maintain their high standards of living, orboth. Whatever the impact of the migrant onthe rainforest, it is merely a symptom of moredeeply rooted imbalances. Similarly, politicalecologists see land degradation as stemmingfrom poor farmer’s lack of access to credit,technology, and land rather than populationgrowth per se.

A number of theories—often subscribedto by demographers—state that population isone of a number of variables that affect the en-vironment and that rapid population growthsimply exacerbates other conditions such asbad governance, civil conflict, wars, pollutingtechnologies, or distortionary policies. Theseinclude the intermediate (or mediating)variable theory (23) or the holistic approach(25) in which population’s impact on theenvironment is mediated by social organiza-tion, technology, culture, consumption, andvalues (26, 27). Some also group IPAT in thiscategory because population is only one of thethree variables contributing to environmentalimpacts.

Many theories in the field of populationand environment are built on theoretical con-tributions from a number of fields. A casein point is the vicious circle model (VCM),which attempts to explain sustained high fer-tility in the face of declining environmentalresources (28, 29). In this model, it is hy-pothesized that there are a number of posi-tive feedback loops that contribute to a down-ward spiral of population growth, resourcedepletion, and rising poverty (see the land


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degradation section). At the simplest level, themodel is neo-Malthusian, but it also owes adebt to a number of other theories. First, itbuilds on the intergenerational wealth flowstheory from demography, which holds thathigh fertility in traditional societies is benefi-cial to older generations owing to the net flowof wealth from children to parents over thecourse of their lifetimes (30). It also borrowsfrom a demographic theory that describes fer-tility as an adjustment to risk, which arguesthat in situations where financial and insur-ance markets and government safety nets arepoorly developed, children serve as old-agesecurity (31). Finally, it is partially derivedfrom the ecologist Garrett Hardin’s famous(32) “tragedy of the commons,” which holdsthat as long as incentives exist for each house-hold to privatize open access resources, thenthere will be a tendency at the societal level tooverexploit available resources to the detri-ment of all users.

It is important to note that population-environment theories may simultaneously op-erate at different scales, and thus could allconceivably be correct. At the global level,we cannot fully predict what the aggregateimpacts of population, affluence, and tech-nology under prevailing social organizationwill be on the global environment when theworld’s population reaches 9 or 10 billion peo-ple (2). But many scientists—neo-Malthusianor not—are justifiably concerned with the im-pact that even the current 6.7 billion peopleare having on the planet given consumptionpatterns in the global North and the boom-ing economies of China and India. Mean-while, at the national level the cornucopiantheory may be correct, say, for a country likeDenmark, whereas neo-Malthusianism, polit-ical ecology, and intermediate variable the-ories may each illuminate different facets ofHaiti’s environmental crisis. Finally, Boserup’stheory of intensification has been found tohold true in the historical experience of manydeveloped countries and in many localizedcase studies spanning the developing world(33).

Although theory may seem dry andacademic, theoretical frameworks can be im-portant guides to action. A good theory helpsto develop well-targeted policies. However,bad theory can become the “orthodoxies” thatare very difficult to overcome and that un-derlie government and development agencypolicies and programs (34, 35). Each of theabove theories identifies one or more ultimatecauses for environmental degradation, whichif remedied would “solve” the problem. Inthe case of neo-Malthusianism, populationgrowth is the primary problem, and thesolution is population programs. In the caseof cornucopianism, market failures are theprimary problem, and the solution is to fixthem. For political ecologists, inequalities atdifferent scales are the main problem, andpolicies should address those inequalities.Multivariable theories offer few magic bulletsbut do underscore the need for action onmultiple fronts to bring about sustainability.Unfortunately, many theories in the realmof population and the environment havenot been subjected to the level of rigorousempirical testing that would allow them to becategorized as robust. This is partly becausethe linkages are complex and difficult to disen-tangle. Fortunately for the field as a whole, thepicture is beginning to change, and a numberof studies at the microlevel have used robuststatistical methods and multilevel modelingin order to test theories such as the VCM (36).

We now turn to a review of the five issueareas.

REVIEW BY ENVIRONMENTALISSUE AREA

In this section, we review the literature onpopulation-environment interactions in eachof five issue areas: land-cover change, agricul-tural land degradation, water resource man-agement, coastal management, and energyand climate change. We focus largely on peer-reviewed articles published in the past decadewith an occasional reference to important ear-lier work.


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Land-Cover Change andDeforestationThe conversion of natural lands to croplands,pastures, urban areas, reservoirs, and other an-thropogenic landscapes represents the mostvisible and pervasive form of human impacton the environment (37). Today, roughly 40%of Earth’s land surface is under agriculture,and 85% has some level of anthropogenic in-fluence (38). Although the world’s populationis now 50% urban, urban areas occupy lessthan three percent of Earth’s surface (39). Wecan conclude from this that large-scale land-cover change is largely a rural phenomenon,but many of its drivers can be traced to theconsumption demands of the swelling urbanmiddle classes (40).

As with the demographic and develop-ment transitions, the world remains dividedin various stages of the land-use transition(41) (Figure 1). Although the developed na-tions have achieved replacement (2.1 birthsper woman) or below replacement-level fer-tility, have urbanized, and have economiesdominated by service and technology indus-tries, developing nations continue to experi-ence rapid population growth, remain largelyrural, and have labor forces concentrated in

Land-use andland-cover change(LUCC): changes inhuman use of land(e.g. fromagricultural toresidential) ornatural land covertypes

the primary sector (agriculture and extractiveindustries).

In part because most developed countrieslargely deforested their lands in past cen-turies, today most land conversion from natu-ral states to human uses is occurring in thedeveloping world, particularly in the trop-ics through forest conversion to agriculture.(One exception is the Russian Far East, whichis one of the few developed world regionswith high rates of primary forest conversion—mostly for logging and not for agriculturallands.) Given the scale of these transforma-tions and their intimate but complex linkageswith population dynamics, research on land-use/-cover change (LUCC) and particularlydeforestation constitutes a large portion ofthe population-environment literature. De-mographic variables are linked at differentscales to this phenomenon (42). But there isdisagreement on the impact of population ver-sus other factors, with some studies suggestingthat demographic dynamics contribute morethan any other process to deforestation (43)and others suggesting the superiority of eco-nomic factors (44). Geist & Lambin’s meta-analysis of 152 case studies of tropical defor-estation suggests that, although most cases of

Stage in land-use transition

Presettlement Frontier Subsistence Intensifying Intensive

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Urbanareas

Intensiveagriculture

Figure 1Land-use transitions.Reprinted fromReference 163 withthe permission ofScience.


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deforestation are driven at least partially bypopulation growth, population factors almostalways operate in concert with political, eco-nomic, and ecological processes, and the rel-ative impact of each factor varies dependingon the scale of analysis. In this section, webriefly outline how population dynamics af-fect LUCC through changes in fertility, pop-ulation structure, and migration as well ashow these interactions are largely mediatedby scale. We also reference case studies il-lustrating the sometimes counter-intuitive re-lationship between population variables andLUCC.

In much of the developing world fertilityrates are plummeting, and nowhere have theydeclined so rapidly as in urban areas, where(apart from sub-Saharan Africa) fertility is ator below the replacement level. Conversely,in most developing countries, the regions ofhighest fertility also coincide with the mostremotely settled lands where the agriculturalfrontier continues to advance, areas that areboth biodiverse and ecologically fragile. Thishigh fertility and associated rapid populationgrowth directly contribute to land conversionin these forest frontier areas. Fertility in re-mote areas of the tropics is buoyed by a com-bination of low demand for and supply of con-traception (45). In such regions, children con-stitute an asset to farm families that are of-ten short on labor (30). Furthermore, poorhealth care access contributes to high rates ofchild mortality—promoting so-called “insur-ance births” that guarantee a family a certainnumber of surviving children (31). Childrencompensate for land insecurity through in-come security to parents in their old age (46),and a dearth of education and work oppor-tunities for women also maintains high fer-tility (47). Positive correlations between fer-tility and deforestation have been found instudies in Central (48, 49) and South America(50, 51).

Household age and sex composition andlife cycle stages are also important factors infrontier LUCC (36). Although young chil-dren divert household labor resources from

agriculture, older children contribute labor tothe farm or capture public access resourcessuch as firewood, game, and water. The settle-ment life cycle of farm homesteads also helpsto explain when and where forest clearing willoccur (52, 53). Immediately following settle-ment, deforestation is high as land is clearedfor subsistence crops (51, 54). A later defor-estation pulse may occur as farms move fromsubsistence to market-oriented crops or ex-pand into livestock. These processes are en-abled by children growing old enough to pro-vide labor or capital investments (through, forexample, remittances) to the farm household(53).

Despite the high fertility of remote ruralpopulations, migration remains the primarysource of population growth in forest fron-tiers (44). Indeed, at a key point along theforest transitions causal chain, in-migration isa necessary precedent to frontier deforesta-tion. Migration will remain a major driver offrontier forest conversion, often in a leap-frogmanner, as more established farm householdssend younger family members as migrants tothe new frontier (55).

Although population dynamics are centralto LUCC, in all cases population exerts itsinfluence synergistically with other factors.Demand for agricultural land among smallholders directly impacts forest conversion,whereas, owing to market forces, urban andinternational demand for forest and agricul-tural products further contribute to LUCCthrough logging and large-scale agriculture.Political and institutional factors also playan important role in shaping LUCC. Forexample, government investments in roads,subsidies to the agricultural sector, or landtenure policy can directly influence deforesta-tion rates. Such effects are well researched inthe Brazilian Amazon (56–58). Cultural pref-erences can also affect LUCC, such as thedesire for cattle as a status symbol amongCentral American frontier farmers (59). Thus,intervening variables help explain incon-sistencies in population-LUCC dynamics(60).


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Changing the scale of analysis revealsexamples in which population growth de-clined yet deforestation accelerated, popula-tion growth was accompanied by reforesta-tion, or population growth attended a numberof different human-environment responses(60). Examples of this are evident in the liter-ature for Latin America where many nationshave experienced declining rural populationsbut continued deforestation (48). A dramaticexample is Ecuador whose Amazon region’sforest canopy is facing rapid attrition owingto growing settlements of frontier farmers, al-though overall rural population is decliningbecause of falling fertility and rapid urbaniza-tion (61). This apparent anomaly is explainedby the small populations, which account fora minority of a nation’s rural population, thatmove to forest frontiers and contribute a dis-proportionate amount to the nation’s total de-forestation. In parts of the Brazilian Amazon,forest conversion has been driven increasinglyby exogenous factors, such as the global de-mand for soybeans, and owing to increasinglymechanized farming, the region has also ex-perienced rural population decline (62). Inter-estingly, the same association—rural depopu-lation and continued deforestation in Ecuadorand Brazil—results from a completely differ-ent causal mechanism in the two cases, high-lighting the importance both of scale andplace-based effects. Similar scale-dependentphenomena emerge in Asian forest frontiers.Research in Thailand’s northeast suggests, forexample, the importance of population fac-tors at finer scales and of biophysical factors atcoarser scales for explaining variation in plantbiomass levels (63).

Land-cover dynamics are the most evidentmark of human occupation of Earth. Links topopulation are both obvious (without humanpopulation presence there is no human im-pact on forests apart from acid rain) and ex-ceedingly complex, e.g., at what spatial andtemporal scales does population interact withpolitical, economic, and social processes toproduce LUCC? A challenge for future re-search is to disentangle the contributions of

population and other dynamics across spa-tial and temporal scales. For example, moreresearch is needed at the mesoscale (subna-tional) and to build causal chains across spatialscales. A diversity of research methods needsto be combined to improve our understand-ing of these space-dependent links, includingremote sensing, geographic information sys-tems, ecosystem process and multilevel mod-eling, surveys and interviews, participant ob-servation, and stakeholder analyses.

Agricultural Land Degradationor Improvement

Land-cover change research also considerschanges in the quality of land resources as aresult of human uses, which is the focus ofthis section. Perhaps the most contentious de-bate in the population-environment literatureconcerns the relationship between increasingpopulation density in subsistence agriculturalareas and land degradation or improvement.This is, in part, the result of widely differingestimates regarding the extent of land degra-dation, with global estimates ranging from 20to 51 million km2 (64). This section consid-ers arguments and evidence marshaled by twomajor schools of thought: the vicious circleproponents who believe that increasing popu-lation density in the context of high poverty al-most inevitably leads to land degradation andthe Boserupians who suggest that increasingdensity leads to intensification of agriculturalsystems such that yields per unit area (and percapita) are increased (see the theory section,above).

In the VCM, it is hypothesized that thereare a number of positive feedback loops thatcontribute to a downward spiral of resourcedepletion, growing poverty, and populationgrowth. An elaboration of these linkages canbe found elsewhere (29, 65), but in its sim-plest form, the model describes the follow-ing causal connections: poverty leads to highfertility through mechanisms such as a de-mand for farm labor, insurance births ow-ing to high infant mortality, and women’s


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low status. High fertility contributes to pop-ulation growth, which further increases de-mands for food and resources from an es-sentially static resource base; the decliningper capita resource base reinforces povertythrough soil fertility loss, declining yields, andpoor environmental sanitation; and poverty,in turn, contributes to land degradation byincreasing incentives for short-term exploita-tion (versus long-term stewardship) and be-cause poor farmers lack access to costly fertil-izers and other technologies. The implicationof these reinforcing linkages is that, absent in-tervention, the circle will continue and soilfertility will decline until the land is no longersuitable for crops or pasture.

Economists have been among the ma-jor proponents of the VCM. For example,Panayotou (66) and Dasgupta (28, 67) havesuggested that children are valued by ruralhouseholds, in part, because they transformopen access resources (forests, fisheries, andrangeland) into household wealth, resulting inthe “externalization” of the costs of high fertil-ity. One manifestation is the process of “exten-sification,” whereby farm households in fron-tier areas use additional labor to open up newlands for cultivation (68). Thus, household-level responses to resource scarcity can lead toproblems at the societal level as each house-hold copes with increased risk and uncer-tainty by maximizing its number of survivingchildren.

A number of modeling efforts, such asthe Population-Environment-Development-Agriculture model (69) and work by Pascual& Barbier (70), borrow concepts from theVCM hypothesis. Testing of the VCM is dif-ficult, however, because one is searching fora relatively small “resources effect” on fer-tility when there are at least a score of po-tentially confounding variables, and testingthe direction of causality requires time se-ries data on social and environmental vari-ables, which is quite rare. Economists Filmer& Pritchett (71) found qualified support forthe vicious circle hypothesis using detaileddata from Pakistan on children’s time use, fire-

wood collection activities, and recent fertility.They find that collection activities do absorba substantial part of household resources andthat children’s tasks are often devoted to col-lection activities. However, they could not es-tablish a “fertility effect” on resource or landdegradation. A longitudinal study in the west-ern Chitwan Valley of Nepal (72) found thatthree measures of local resource depletion—the time to collect fodder, the increase in timerequired to collect fodder in the prior threeyears, and household’s dependence on pub-lic lands for fodder—were significantly andpositively correlated with desired family size,even when controlling for household wealthand numerous other factors found to influencedesired fertility. Yet, several other indicatorsof environmental decline had no significantrelationship to either desired fertility or preg-nancy outcomes, and the actual relationship todesired fertility depended in part on whetherthe respondents were men or women. Pascual& Barbier’s (70) modeling of shifting cultiva-tion in the Yucatan found that among poorhouseholds, as population density increased,the response was extensification or a reduc-tion in fallow periods, whereas among better-off households, labor was shifted to off-farmemployment. Thus, although anecdotal evi-dence is abundant and development policy-making has been heavily influenced by VCMassumptions, there is only qualified supportfor the hypothesis in the few existing quanti-tative studies.

The Boserupian or intensification hypoth-esis has been tested in a number of stud-ies spanning Africa, Asia, and Latin America.A frequently cited study by Tiffen et al. (6)examined changes in population density andagricultural productivity in Machakos Dis-trict, Kenya. From 1930 to 1990, the pop-ulation of Machakos District grew sixfold,from 240,000 to 1.4 million people, with a1990 population density of 654/km2. The re-gion is mountainous and semiarid (<500 mmrainfall a year), and in the 1930s, it was suf-fering already from soil erosion (mass wast-ing and gullies). The region was also isolated


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from national markets, and there were colo-nial restrictions on access to certain lands andcrops. In the 1950s and 1960s, a new formof terracing was propagated by local workgroups, agricultural systems shifted from live-stock to intensive farming with emphasis onhigher-value crops, feeder roads were builtto market towns, and market towns devel-oped with agricultural processing facilitiesand other small industries. By 1990, the valueof agricultural production had doubled on aper capita basis. Many factors led to a posi-tive outcome for this region, including infras-tructure development, market growth, privateinvestment, increasing management capacityand skills, self-help groups, food relief dur-ing drought, and secure land tenure. Thisstudy confirms the basic Boserupian hypoth-esis: increased food demand, a denser net-work of social and market interactions, labor-intensive agriculture and economies of scalehelped to avert a Malthusian crisis. Yet evenin this textbook study, other researchers work-ing in the district found important social dif-ferentiation in livelihood improvements, landalienation, and government-imposed limita-tions on mobility—elements that tend to maran otherwise rosy picture (73).

Mortimore (74) found similar “success sto-ries” in three dryland areas of West Africa:Kano State in northern Nigeria, the DiourbelRegion of Senegal, and the Maradi Depart-ment, Niger. Outcomes were assessed in fourdomains: improved ecosystem management,land investments, productivity, and personalincomes. Taking pains to point out that innone of these regions were indicators under allfour domains positive, the author neverthelessfound some common ingredients that resultedin improved or stable soil fertility and yieldsdespite rapid population growth and highdensities. These ingredients include marketsfor agricultural produce, physical infrastruc-ture, producer associations, knowledge man-agement, and incentives for investment andincome diversification. He concludes thatproductivity enhancements respond to eco-nomic incentives and that the capacity of

resource-poor farmers to invest in on-farmimprovements should not be underestimated.

In Asia, there have also been successes,thanks largely to success of the “green revolu-tion,” a package of improved seeds and agri-cultural inputs that resulted in higher yields(75). Turner & Shajaat Ali (21) studied timeseries data (1950–1986) for 265 households insix villages in Bangladesh. They found sup-port for the induced intensification hypoth-esis, with yields largely keeping pace withor exceeding population growth despite highpopulation densities (783 persons per km2).Soil conditions in Bangladesh are, on average,much better than in dryland Africa owing todeposits of alluvium during monsoon seasonflooding and therefore can support far higherdensities. They posit that, as smallholderscome in contact with the market economy,their redundant production is reduced, andtheir aspirations increase. Although croppingintensities on average increased significantly(in one village almost tripling), they also foundincreasing production disparities, with largeland holders accounting for most of the sur-plus production, whereas the growing numberof landless suffered shortfalls and malnutri-tion. They conclude that Bangladesh passedseveral threshold steps at points along its pathtowards intensification in which Malthusianoutcomes of involution and stagnation mighthave occurred but were fortunately averted.

As these case studies make clear, pop-ulation is but one among many factorsthat influence degradation or intensification.Other variables that are of crucial signifi-cance include institutional factors (land tenureregimes, local governance, resource access),market linkages (road networks, crop prices),social conditions (education, inequality oflandholdings), and the biophysical environ-ment itself (original soil quality, slopes, cli-matic conditions). Thus, it would appear thatpopulation growth is neither a necessary norsufficient condition for either declines or im-provements in agricultural productivity to oc-cur. Population growth can either operateas a negative factor, increasing pressure on


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limited arable land, or a positive factor, help-ing to induce intensification through adoptionof improved technologies and higher labor in-puts. Where it does which depends on fac-tors in the economic and institutional realms.This conclusion is supported by two ambi-tious meta-analyses of studies that looked atdryland degradation (or desertification) andagricultural intensification (76, 77). The au-thors reject both single-factor causation andirreducible complexity but propose insteadthat a limited number of underlying drivingforces, including population, and proximatecauses are at work to produce either degrada-tion or intensification.

Although population can perhaps be dis-counted as the only relevant variable, thereis little doubt that rapid population growthin poor rural areas with fragile environmentscan be a complicating factor in the pursuit ofsustainable land use, especially because poli-cies and markets are rarely aligned in sucha way as to produce the most favorable re-sults. Furthermore, trends on the basis of pastprecedents can only be extrapolated with cau-tion, because the exact locations of thresholdsin any given system are still largely unknown(21). One important advance for studies in thisarea will be the development of better mapsof soil quality and land degradation with theaid of remote sensing and local soil samples,as at least part of the debate over popula-tion’s impact can be explained by differing in-terpretations of what constitutes degradationand by a paucity of empirical evidence for therelationship.

Abstraction and Pollution of WaterResources

The water cycle ties together life pro-cesses. It is fundamental to the biochemistryof living organisms; ecosystems are linkedand maintained by water; it drives plantgrowth; it is habitat to aquatic species; andit is a major pathway of sediment, nutri-ent, and pollutant transportation in globalbiogeochemical cycles (78). Population-

environment researchers have not dedicatedthe same level of attention to population dy-namics and water resources as they have toresearch on land-cover change, agriculturalsystems, or climate change. Yet there are clearrelationships between population dynamicsand freshwater abstraction for agricultural,domestic, and industrial uses, as well as emis-sion of pollutants into water bodies.

Human settlement is heavily predicatedupon the availability of water. A map of globalpopulation distributions closely tracks annualrainwater runoff, with lower densities in themost arid regions and as well as the most wa-ter abundant, such as the Amazon and CongoBasins. Whereas the former areas are waterconstrained for agriculture, in the latter ar-eas, year-round rainfall in excess of 2000 mmhas rendered these environments less favor-able for agriculture (owing to soil leaching andoxidation) and more favorable for human andlivestock diseases.

At the global level, irrigation water foragriculture is the biggest single user (about70% of water use), followed by industry(23%) and domestic uses (8%) (79). If “greenwater” is added to the mix (water that feedsrainfed crops), then crop production far andaway outstrips other water uses. As demandfor food increases with growing populationsand changing tastes (including growing de-mand for animal versus vegetable protein withits far greater demands for water), it is ex-pected that water diversions for agriculturewill only increase. Today, humanity is esti-mated to use 26% of terrestrial evapotran-spiration and 54% of accessible runoff (80).Falkenmark & Widstrand (81) establishedbenchmarks for water stress of between 1000and 1700 m3 per person, water scarcity of be-tween 500 and 1000 m3 per person, and ab-solute scarcity of less than 500 m3 per person.Northern and southern Africa and the MiddleEast already suffer absolute scarcity. As popu-lation grows and water resources remain moreor less constant, many countries in the rest ofAfrica are projected to fall below 1000 m3 perperson (82).


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Perhaps because such water resources arehidden underground, groundwater resourcedepletion could potentially remove some agri-cultural areas from the map. Although it iswell known that some Arab countries rely onfossil water for wheat production, less rec-ognized is that 70% of Chinese and 45% ofU.S. irrigation is based on nonrenewable wa-ter resources (C. Vorosmarty, EM Douglas,personal communication). Groundwater lev-els in India have been dropping for morethan a decade owing to the unregulated tap-ping of aquifers (83), rendering some semi-arid regions vulnerable to shortages. A studyin Karnataka State, India, identified a majorshift from surface to groundwater use in thepast decades and found that groundwater useis highly inequitable; large farmers possessing12–16 ha of land make up only 8% of all farm-ers but consume 90% of groundwater (84). Inthe lower delta of the Ganges-BrahmaputraBasin, upstream diversions at the FarakkaBarrage, rather than local demands for irri-gation water, appear to be causing dry sea-son groundwater deficits and intrusion of thesaline front, illustrating how complex basin-wide hydrological connections complicate theattribution of population impacts (85).

Other studies at the local level reveal asimilarly complicated picture. Research in theMwanza region of western Tanzania finds thataccessible runoff varies significantly acrossa relatively small area and that populationdensity closely tracks available water (86).Migrants to towns were generally less likely tohave access to water from standpipes and morelikely to rely on unimproved wells. Rural-urban migration is not correlated to relativewater scarcity in places of origin but ratherto proximity to roads and to towns. The re-searchers conclude that high fertility—a tradi-tional adaptation to peak labor demands dur-ing the short cropping season—increases theproblems of water access and supply mainte-nance in agricultural and domestic spheres.But they also note that gloomy prognosesabout future water shortages often fail to ac-knowledge that large portions of developing

country populations never have had the kindof access to water, or levels of consumption,deemed necessary by international bodies.

In the Pangani Basin of northeasternTanzania, a complex set of factors is leadingto water conflicts (87). Population is one fac-tor: Owing to high fertility and migration,rural population is doubling every 20 years,and the population of towns is doubling every10 years. But other factors include water ex-traction and land alienation for export flowerproduction and protected areas, growth andmobility of livestock herds, declining summerrunoff from glaciers on Mount Kilimanjaroowing to global warming, and hydroelectricitygeneration. The greatest conflict is betweenfarmers and pastoralists, as farmers progres-sively moved into areas previously consideredtoo marginal for agriculture and pastoralistswere squeezed by restrictions on grazing areasowing to newly established protected areas. Inrecent years, the pressure on land has led tostresses on water and other resources, leadingto heavy out-migration from the basin.

Researchers in the densely populated SaoPaulo State in Brazil examined water re-sources in the Piracicaba and Capivari RiverBasins within the Campinas AdministrativeRegion (AR) (88). Campinas is Brazil’s four-teenth largest city, as well as its third largestindustrial center, and an important agricul-tural region as well. The Metropolitan Re-gion of Campinas (the 19 core municipali-ties of the AR) saw high, though declining,average annual population growth rates dur-ing the 1970–2000 period: 6.5% (1970–1980),3.5% (1980–1991), and 2.5% (1991–2000).The authors find that problems in the formof urban growth and the patterns of popula-tion distribution during these three decadeshave accentuated water quality problems be-cause the rapidity and low density of growthmeant that water supply and sanitation infras-tructure could not keep up. By mid-1995 only5% of waste was treated before reenteringstreams, and large reaches of the Piracicabaand Capivari River Basin tributaries weredeemed of poor quality. Water supply


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Natural increase:endogenous growth(local births minusdeaths), excludingmigration

infrastructure (mostly surface reservoirs asgroundwater is scarce) did not keep pace withpopulation growth, and the situation was re-ported as critical as of the mid-1990s. In re-sponse, state water basin agencies are apply-ing some institutional solutions such as feesfor water withdrawals and restrictions on res-idential development, as well as some techni-cal ones, particularly the treatment of wastewaters.

In summary, as in other areas, the relation-ship between population dynamics and waterresources is complex. At the aggregate level,other things being equal, population growthmost assuredly does reduce per capita wateravailability. It is in this light that the GlobalInternational Waters Assessment listed pop-ulation growth first in a series of root causesof the “global water crisis” (89). Yet there ismore to population change than growth alone,and rarely are other factors equal, so the spe-cific impacts of population dynamics on wateroften come down to a complex array of place-specific factors that relate to economic andclimatic changes, agricultural and industrialtechnologies, sewage treatment, and institu-tional mechanisms, to name but a few. One ofthe challenges to research in this area is thecommon property nature of water resources,and another challenge is caused by rapid reg-ulatory changes as water resources becomescarcer, which alters the institutional context.The field could use more basin or watershedstudies to understand how variables such aspopulation and climate change may affect fu-ture water availability and required institu-tional responses (90). Basin-level population-development-environment modeling wouldalso help understand and resolve competitionbetween urban and rural water uses as theworld becomes more urbanized (91).

Coastal and Marine Environments

From the earliest times, the preponderanceof global economic activity has been concen-trated in the coastal zone (92), with settle-ments often growing on the continental mar-

gins to take advantage of overseas trade andeasy access to the resources of the rural hin-terlands. As a result, the coastal zone has at-tracted large and growing populations, withmuch of their growth attributable to migra-tion rather than natural increase (93). Today,10% of the world’s population lives at lessthan 10 m above sea level (even though thisarea only accounts for 2.2% of the world’sland area), and coastal zones have higher pop-ulation densities than any other ecologicallydefined zone in the world (39, 94). Coastaland marine environments are very importantfor human health and well-being, and theyare also quite vulnerable to anthropogenicimpacts. Yet, until recently most population-environment research has focused on terres-trial ecosystems, possibly because the human“footprint” on coastal and marine ecosystemsis harder to discern.

Not surprisingly, over half of the world’scoastlines are at significant risk fromdevelopment-related activities (95), and thepotential (and realized) environmental dam-age is substantial. Population growth is of-ten named as the driver of coastal and marineenvironmental problems, whereas proximatecauses can be traced to specific practices (96).A recent study highlights how the Kuna popu-lation (an indigenous population in CaribbeanPanama) has practiced coral mining and land-filling for decades in response to populationgrowth (97). Since 1970, live coral cover de-clined 79%, and at the same time, the Kunapopulation increased by 62%. The Kunagradually enlarged their island landmass toadjust for their growing population by build-ing coral walls out into the water and thenfilling in the enclosed areas with corals, sea-grass, and sand. In addition to direct loss ofcoral reef, consequences include coastal ero-sion and a local increase in sea level. This ex-ample provides a clear and direct link betweenpopulation growth and coastal degradation.

Population growth can lead to manyother coastal and marine environmentaldisturbances. For instance, tropical man-groves are being converted to fish and shrimp


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aquaculture farms, which undermines coastalprotection and decreases natural habitatthat many fish species use for reproduction.Expanding coastal cities undermine naturalprotection from storms and hurricanes aswell as increase pollution and runoff. Addi-tionally, untreated sewage and agriculturalrunoff continue to be a worldwide problem.Although listed as a driver, like other issues,the impact of population size and growthdepends on many other factors such as thesensitivity of coastal systems to stress, localinstitutions, and global markets. For example,demand for shrimp is the ultimate driver ofmangrove loss, and sewage treatment systemsand no-till agriculture could significantlyreduce nutrient loading in coastal areas.

The relationship between human activitiesand environmental impacts is hard to assessand regulate in coastal and marine environ-ments because the environmental resourcesare almost always governed by common prop-erty resource (CPR) management systems,whereas terrestrial environments are gener-ally managed by the government or privatesector. CPR management systems may be es-pecially vulnerable to disruption caused by in-migration or urbanization. However, the so-cial and economic context largely determineswhether in-migration and population pres-sure disrupt the CPR system and thus causeenvironmental degradation (98–100). Thus, asignificant recurring theme in this researchis that the social and economic context inwhich the population is changing as well aswhen, how, and with whom people interactis more important in determining the impacton the environment than simply demographicchange (101, 102).

Studies in developing countries on migra-tion and the marine environment have focusedon a mediating variables approach, such ashow technology, local knowledge, social in-stitutions of kinship or marriage, and marketsmediate the role of population in resourceextraction and consequent environmentaldegradation or enhancement. For example,some work has hypothesized that migrants

Common propertyresource (CPR): aresource that is solarge or widespreadthat it is difficult toexclude people fromusing it

Social capital: theresources (networks,relationships of trust,access to widerinstitutions ofsociety) upon whicha household orindividual may draw

misuse resource extraction technologies,which leads to environmental degradation(103). In a coastal Brazilian population, tech-nological change imposed by outsiders wholacked knowledge of the ecological and socialcontext of the community contributed to de-creased ecological resilience (104), and rapidin-migration and technological changes in seacucumber fishing techniques in the Galapagosled to a collapse in the sea cucumber industry(105). In both cases, the results seem to be afunction of the migrants’ limited local knowl-edge as well as expansionist attitudes andshort-term time horizons for profiting fromthe extraction of coastal and marine resources.

Thus, population-environment re-searchers have begun to incorporate othersocial theories such as social capital andmigrant incorporation to understand whenpopulation pressures do not necessarilydegrade the environment (106). Most studieshave found that, in systems with strong landtenure or social capital, migrants do not dis-rupt the environment and are able to developlocal knowledge that mitigates environmen-tal impacts (107–109). A case study in theSolomon Islands contests the notion that seatenure regimes are weakened by in-migrationand population growth. Rather, potentiallynegative impacts of population pressure onthe environment are diminished significantlywith greater reciprocal ties among close kinor neighbors (110, 111). Similarly, intermar-riage between a migrant and a nonmigrantin Sulawesi, Indonesia, has been shown tomitigate the otherwise negative associationbetween migrant households and coral reefdegradation (106).

Migration has been the most studied com-ponent of population dynamics in coastal andmarine environments. Yet, urbanization andtourism are other primary human drivers af-fecting coastal ecosystem quality (112, 113).Fourteen of the worlds largest 17 cities arelocated along a coast; this affects freshwaterflows to coastal estuaries, sewage emissions,and ecological processes at the land-sea inter-face (114). Also, without careful planning in


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anticipation of a growing tourist market, cul-tural and ecological resources may be over-exploited, resulting in unsustainable develop-ment, as is the case in Turkey (115).

Human impacts on coastal and marineenvironments are not a simple function ofpopulation size or density. As the afore-mentioned studies suggest, technology,knowledge systems, social cohesion, commonproperty systems, migrant incorporation,and the economic and ecological context inwhich these interactions take place all playan important role in population and envi-ronment research, especially in developingcountries. Nonetheless, coastal and marineenvironments continue to be among the mostthreatened ecosystems in the world, owing inpart to the sheer scale of detrimental humanactivities associated with urbanization alongthe coasts, continued population growth, anda growing number of tourists in search ofcoastal amenities.

An unresolved issue in this area ofresearch—as in the case of LUCC research—is how to spatially and temporally link popula-tions and human activity to a specific environ-mental outcome. This is especially difficult inmarine and coastal ecosystems because envi-ronmental boundaries are fluid. Also impor-tant is the impact of local and global consump-tion on marine and coastal environments. Forinstance, per capita consumption of seafood ishigh in many traditionally seafaring countrieseven though population sizes are low (116),and specialized tastes for rare species can havedramatic impacts on fish stocks (117). Furtherresearch is needed to assess how population-environment linkages in marine and coastalareas are influenced by the global food tradeconnecting consumers and producers fromopposite sides of the world.

Energy, Air Pollution, and ClimateChange

Even when they are connected to the electricgrid, some two billion poor people in the de-veloping world still largely rely on biomass to

meet their energy needs. That leaves approx-imately 4.7 billion people with more energy-intensive lifestyles who consume, with littlehelp from the world’s poorest, the energyequivalent of 77 trillion barrels of oil a year(118).3 More than 80% of global energy con-sumption is derived from fossil fuels (119), andit is this dependence on fossil energy that isresponsible for the release of the greenhousegases and airborne pollutants that are alter-ing atmospheric composition and processeson a global scale. As concern mounts over thehealth impacts of urban air quality (particu-larly in developing countries) and the poten-tial adverse effects of climate change acrossmultiple systems and sectors, population-environment researchers have paid particularattention to understanding the demographicdrivers of energy consumption. Although itis clear that there are vast differences in con-sumption levels (per capita energy consump-tion in the United States is 48 times whatit is in Bangladesh and 4.7 times the worldaverage), it would be wrong to suggest thatpopulation variables are irrelevant. Hence, wereview a number of empirical studies that ex-amine population-energy linkages in a sys-tematic and quantitative manner.4

In studies of energy consumption re-searchers have found that it is more appro-priate to use the household rather than in-dividuals as the unit of analysis because alarge portion of energy consumption relatedto space conditioning (heating and air con-ditioning), transportation, and appliance useis shared by household members. This shar-ing results in significant economies of scale,with large households generally showinglower per capita energy use than small ones

3Of this total, the equivalent of 66 trillion barrels is actuallyfrom fossil fuels (petroleum, natural gas, and coal). Theremaining energy supply (13.7% of the world total) comesfrom renewable sources and nuclear energy.4Although there is extensive research on the reciprocal im-pact of air pollution and projected climate change on de-mographic variables such as morbidity, mortality, and mi-gration (158, 159), this is beyond the scope of this review.


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(29, 120). Energy studies have identified arange of household characteristics as key de-terminants of travel patterns (121–123) andof other types of residential energy demand,such as for heating, cooking, and operatingdomestic appliances (124–127). In a pioneer-ing study, MacKellar et al. (128) used the IPATidentity to decompose the annual increase inenergy consumption of the more developedregions during the period 1970–1990. Theyfound that, because growth in the number ofhouseholds outpaces population growth ow-ing to trends in fertility, divorce, and aging,growth in household numbers accounted for41% of the total increase in energy consump-tion, whereas population growth accountedfor only 18%. However, this study did not takeinto account the lower energy requirementsof smaller households, so it likely exaggeratedthe contribution of the growth in householdnumbers to energy use.

O’Neill & Chen (129) drew on householdsurvey data to quantify the influence of house-hold size, age, and composition on residentialand transportation energy use in the UnitedStates and found that changes in householdsize have caused 14% of the increase inper capita energy use over the past severaldecades. Lenzen et al. (130) assessed the im-portance of various demographic characteris-tics on household energy demand in Australia,Brazil, Demark, India, and Japan, and theyfound similar patterns across countries: Theaverage age of residents is positively asso-ciated with per capita energy consumption,whereas household size and urban locationare negatively associated. To explore the im-portance of adopting adequate demographicvariables in understanding transport-relatedenergy consumption, Prskawetz et al. (131)combined a cross-sectional analysis of car usein Austria with detailed population/householdprojections and tested the sensitivity of pro-jections of future car use across a wide rangeof households by size, age, and sex of house-holder and the number of adults and children.They found that car use will likely increase by20% in the period 1996–2046 if the number

Energy intensity:the amount of energyrequired to produceone unit of output;more efficient use ofenergy results inlower intensities

of households is the unit of analysis. However,it will only increase by 3% if one applies acomposition that differentiates householdsby size, age, and sex of the householders.Therefore, household characteristics canimpact aggregate demand for car use via dif-ferences in demand across households as wellas likely changes in household composition.

In studying demographic impacts (viaenergy consumption) on air pollution, scien-tists have identified a number of importantfactors that jointly determine pollutantemissions, including the familiar elementsof the IPAT identity—population, affluence,and technology as reflected in energy andemissions intensities (132). Selden et al.(133) analyzed the reduction of U.S. majorair pollution emissions from 1970 to 1990and found that changes in economic scale,economic composition, energy mix, energyintensity, and emissions intensity all playedimportant roles. In quantifying the impacts ofpopulation on air pollution, researchers havereached different conclusions depending onwhich pollutants are under study, in whichlocations, at what scale, and for which timeperiods. For instance, a study of Californiacounties shows that population size signif-icantly contributes to the increase of thereactive organic gases NOx and CO and haslittle impact on PM10 and SOx, which are de-rived more from production activities (134).Population size shows no significant relationto ground-level ozone because ozone is verydifficult to measure at specific sites owingto its nature as a diffuse secondary pollutant(135). In research using national-level data,researchers found an almost linear positivecorrelation between population size and CO2

emissions (128, 132, 136, 137) and an invertedU-shaped curve for SO2 (136). However,a more recent study of Canadian provincesover the period 1970–2000 suggests that pop-ulation size has an inverted U-shaped curvewith CO2 emissions as well, which is at oddswith previous literature investigating thesevariables for other regions and time periods(138). The different patterns of impacts may


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reflect the nature of complicated interactionsbetween different pollutants and regionalgeographic/climatic conditions (139, 140),income, and technological levels (139, 141).

The same inconsistencies in the relation-ship between population size and emissionsof various pollutants are in evidence whenexamining other population-related variables.Cramer (134) in his study of California coun-ties and Cole & Neumayer (136) in theircross-national studies found that other vari-ables such as the percent of population that aremigrants, age composition, household size,and level of urbanization have the same ba-sic relationship as overall population size onemission levels of each of the pollutants theystudied. However, caution should be used ininterpreting these results because the studiesonly cover short time periods (10 to 20 years)in which there were only small changes in thedemographic variables.

Because of the complexity of populationinteractions as well as political issues, popu-lation issues were not considered in formu-lation of the Kyoto Protocol (142) and havealso been largely excluded from the Intergov-ernmental Panel on Climate Change (IPCC)assessment reports (143), although populationprojections are an integral part of the SpecialReport on Emissions Scenarios (SRES) (144).The original emissions scenarios were con-structed in 1996 using population projectionswith a base year of 1990. Although the projec-tions used in the SRES were largely consistentwith actual population sizes for the 1990–2005period, the projections to 2050 and beyondwere higher than more recent projections (seethe text, above, on global trends in populationand consumption) (11, 145, 146). Therefore,even though the 1996 scenarios continue toserve as a primary basis for assessing futureclimate change and possible response strate-gies, the Fourth Assessment Report of theIPCC is based on slightly lower populationprojections than the Third Assessment Re-port under the A2 scenario, which describes aneconomically divided world with slow techno-logical progress and high population growth.

Consideration of demographic factors be-yond population size, such as changes in agestructure, urbanization, and living arrange-ments, which as discussed above are impor-tant in modeling future energy use, is notaccounted in the SRES population assump-tions. Making progress in this area requiresa better understanding of the scope for fu-ture demographic change as well as meth-ods for including demographic heterogeneitywithin energy-economic growth models usedfor emissions scenario development.

Simultaneous and consistent projectionsof population, urbanization, and householdsare challenging demographic tasks (147).Recently, Dalton et al. (148) introduced het-erogeneous households into a general equi-librium population-environment-technologymodel of the U.S. economy. Because differenttypes of households have unique demands forgoods, capital stock, and labor supply, andthese characteristics have direct and indirectimplications for energy demand, they wereincorporated into cohorts by age groups(or “dynasties”). These dynamics and otherrelationships implied by household projec-tions create nonlinear interacting effects thatinfluence each dynasty’s future saving andconsumption decisions. Their research showsthat including age heterogeneity among U.S.households reduces emissions by almost 40%in the low-population scenarios by year 2050,and effects of aging on emissions can be aslarge as, or larger than, effects of technicalchange in some cases. Those effects arebelieved to be much larger for the developingworld, where more significant demographicchanges such as population growth, aging,household nuclearization, and urbanizationare occurring.

CONCLUSION

One of the reasons natural scientists havefound population to be so appealing as a hu-man dimension of environmental change isthat data are readily available (in contrast toother human variables such as values, culture,


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and institutions), projections are reasonablyreliable (149), and population can be treatedin models in a manner that is analogous toall the other quantitative variables. This haspromoted something of a reductionist view ofpopulation-environment interactions. Fortu-nately, a growing number of natural scientistsare beginning to appreciate that humans inter-act with the environment in more ways thantheir raw numbers often imply. Populationsare composed of people who collectively formsocieties, and people and societies cannot eas-ily be reduced to food and material demandsthat result in some aggregate impact on theenvironment.5 This makes human societies atonce messy for modeling and fascinating tostudy. The new understanding builds on theconcept of coupled human-environment sys-tems, which are more than the sum of theirparts (150, 151).

In the human-environment system, theimpacts are not unidirectional but reciprocal.For example, the environmental changeimpacts on morbidity and mortality are agrowing area of interest, and some havesought to close the circle by looking athow environmentally induced mortality mayaffect population projections (2). There isalso growing research on the health impactsof landscape or climatic changes on humans,in the one instance through the creation ofmosquito breeding habitats that contributeto malaria (152), and in the other throughheat stress or famine (153). Research on thehuman-environment system also takes ad-vantage of new data sources (remote sensing,biophysical data, as well as georeferencedhousehold surveys), new technologies (high-powered computers, geographic informationsystems, spatial statistics), and new models(agent-based, multilevel, and spatially explicitmodeling). Much of the research reviewed in

5Clarke (160, p. 9) writes, “There is a danger in talkingabout populations as if they are just numbers rather thangroups of peoples, who have never been so demographi-cally, socially, economically or even politically diverse. Vari-ations in the roles of women around the world. . .admirablyexemplify this diversity.”

this chapter has sought to deconstruct pop-ulation into its component parts and to un-derstand how human social institutions in alltheir complexity (e.g., markets, policies, com-munities) mediate the impact of populationvariables on the use of resources, waste gener-ation, and environmental impacts. Thus, theycould be said to fit into this growing under-standing of the human-environment system.

Much population-environment research,whether at the local or global scales, is moti-vated by a broader concern for sustainability.Underlying some of the research, and con-tributing to some of the controversy, has beena concern for distributional justice in twoforms: that the 5.4 billion citizens of devel-oping countries might be able to raise theirliving standards and hence their consumptionlevels from their previously low levels and thatthe costs of biodiversity conservation and cli-mate change adaptation not be unfairly borneby the poorest. Whether research proves thatpopulation dynamics have a dominant or neg-ligible effect on environmental outcomes ineach of the domains we surveyed, it is still leftto human societies to address these inequitiesin consumption and costs as well as to seeklong-term solutions. Here, research on cul-ture, consumption, values, institutions, and al-ternative industrial and food systems will addto what is known about the demographic di-mension as societies seek to transition to sus-tainable systems (10, 154).

Although we have sought to objectivelyreview the literature rather than take a nor-mative stance concerning the environmentalimpacts associated with population dynamics,at the global scale there is no question thathumanity faces significant challenges in thecoming decades owing to the scale and pace ofchanges in human numbers, population distri-bution, and consumption patterns. To quoteCohen’s definitive study on the global carry-ing capacity, “The Earth’s human populationhas entered and rapidly moves deeper into apoorly charted zone where limits on humanpopulation size and well-being have been an-ticipated and may be encountered” (2, p. 11).


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In recent decades, scientists have increasinglywarned of the potential to reach the upper lim-its of the planet’s productive, absorptive, andrecuperative capacities (155). A challenge formicro- and mesoscale researchers is to under-

stand how changes at the local and nationalscale relate to global-scale changes and how,in turn, their research can inform policies andprograms at these lower scales that will atten-uate environmental impacts at all levels.

SUMMARY POINTS

1. There is more to population dynamics than population size and growth. Recent re-search has illuminated the ways in which a number of population variables–age and sexcomposition, household demographics, and the elements of the population balancingequation (fertility, mortality and migration)–are related to environmental change.

2. Most demographers and many other social scientists subscribe to a mediating variabletheory, which states that population dynamics affect the environment through othervariables such as culture, consumption levels, institutions, and technology.

3. Across the environmental issues covered in our review, population dynamics usuallyact in concert with other significant factors such as local institutions, policies, markets,and cultural change. Teasing out the relative contribution of each factor can often bedifficult.

4. The scale of analysis can significantly affect findings concerning the role of populationdynamics in environmental change.

5. Evidence for the impacts of population on land and resource degradation has beenmixed in part because time series data at appropriate scales and measurements ofappropriate variables are rare and because the population “signal” is often difficult toisolate from other signals.

6. Both freshwater resources and coastal and marine ecosystems are often managed ascommon property resources (CPRs); hence levels of resource degradation or depletiondepend more on the existence of effective management systems than on populationvariables per se.

7. In research on population and energy use, the household has been found to be a moreuseful unit of analysis than the individual, and population-environment researchershave made major strides in understanding how household size, composition, andincome are related (via energy use) to environmental impacts.

8. Emerging understanding of complex human-environment systems is informing workin the area of population and the environment, and vice versa.

FUTURE ISSUES

1. Greater exploration of the linkages between micro- (farm or household level) andmacroscale (global) processes manifested at meso- (subnational) scales in population-environment research across the different issue areas is needed.

2. Careful microscale longitudinal studies measuring population variables, householdconsumption, biophysical variables, institutional arrangements, and technologies em-ployed over time should be conducted.


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3. Given the environmental footprints of urban areas on rural hinterlands, one un-resolved issue relates to the impact of population spatial distribution. For example,what would environmental impacts be if the same population were spread more evenlyacross the landscape rather than concentrated in urban areas?

4. Population-environment researchers could contribute to better understanding cur-rent consumption levels and the effects of future aspirations of the growing middleclasses of Asia and Latin America as they relate to the sustainability transition.

5. Advances in demographic modeling are needed to develop a new popula-tion/household model with moderate data requirements, manageable complexity,explicit representation of demographic events, and output that includes sufficientinformation for population-environment studies.

6. A new generation of IPAT modeling is needed that explicitly accounts for the in-teractions among the IPAT terms, including the reciprocal impacts of environmentalchanges on population dynamics, and that is made part of integrated assessment mod-eling.

7. Future research could explore the increase in human mobility and collapse of geo-graphical space as it affects population-environment relationships.

DISCLOSURE STATEMENT

The authors are not aware of any biases that might be perceived as affecting the objectivity ofthis review.

ACKNOWLEDGEMENTS

The authors wish to acknowledge the financial support of the International Union for theScientific Study of Population and the NASA-funded Socioeconomic Data and ApplicationsCenter (SEDAC) (NASA contract NAS5-03117 with Goddard Space Flight Center), whichunderwrite the work of the Population-Environment Research Network. The views expressedhere are those of the authors and not necessarily those of the sponsoring and supportingorganizations.

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RELATED RESOURCES

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Eakin H, Luers AL. 2006. Assessing the vulnerability of social-environmental systems. Annu.Rev. Environ. Resour. 31:365–94

Gleick PH. 2003. Water use. Annu. Rev. Environ. Resour. 28:275–314McGranahan G, Satterthwaite D. 2003. Urban centers: an assessment of sustainability. Annu.

Rev. Environ. Resour. 28:243–74Lambin EF, Geist HG, Lepers E. 2003. Dynamics of land-use and land-cover change in tropical

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AR325-FM ARI 21 September 2007 16:39

Annual Review ofEnvironmentand Resources

Volume 32, 2007Contents

I. Earth’s Life Support Systems

Feedbacks of Terrestrial Ecosystems to Climate ChangeChristopher B. Field, David B. Lobell, Halton A. Peters, and Nona R. Chiariello � � � � � �1

Carbon and Climate System Coupling on Timescales from thePrecambrian to the AnthropoceneScott C. Doney and David S. Schimel � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 31

The Nature and Value of Ecosystem Services: An OverviewHighlighting Hydrologic ServicesKate A. Brauman, Gretchen C. Daily, T. Ka’eo Duarte, and Harold A. Mooney � � � � � 67

Soils: A Contemporary PerspectiveCheryl Palm, Pedro Sanchez, Sonya Ahamed, and Alex Awiti � � � � � � � � � � � � � � � � � � � � � � � � � 99

II. Human Use of Environment and Resources

Bioenergy and Sustainable Development?Ambuj D. Sagar and Sivan Kartha � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �131

Models of Decision Making and Residential Energy UseCharlie Wilson and Hadi Dowlatabadi � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �169

Renewable Energy Futures: Targets, Scenarios, and PathwaysEric Martinot, Carmen Dienst, Liu Weiliang, and Chai Qimin � � � � � � � � � � � � � � � � � � � � � �205

Shared Waters: Conflict and CooperationAaron T. Wolf � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �241

The Role of Livestock Production in Carbon and Nitrogen CyclesHenning Steinfeld and Tom Wassenaar � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �271

Global Environmental Standards for IndustryDavid P. Angel, Trina Hamilton, and Matthew T. Huber � � � � � � � � � � � � � � � � � � � � � � � � � � � � �295

Industry, Environmental Policy, and Environmental OutcomesDaniel Press � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �317

vii

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AR325-FM ARI 21 September 2007 16:39

Population and EnvironmentAlex de Sherbinin, David Carr, Susan Cassels, and Leiwen Jiang � � � � � � � � � � � � � � � � � � � �345

III. Management, Guidance, and Governance of Resources and Environment

Carbon Trading: A Review of the Kyoto MechanismsCameron Hepburn � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �375

Adaptation to Environmental Change: Contributionsof a Resilience FrameworkDonald R. Nelson, W. Neil Adger, and Katrina Brown � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �395

IV. Integrative Themes

Women, Water, and DevelopmentIsha Ray � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �421

Indexes

Cumulative Index of Contributing Authors, Volumes 23–32 � � � � � � � � � � � � � � � � � � � � � � � �451

Cumulative Index of Chapter Titles, Volumes 23–32 � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �455

Errata

An online log of corrections to Annual Review of Environment and Resources articlesmay be found at http://environ.annualreviews.org

viii Contents

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