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Population andEnvironment

Alex de Sherbinin,1 David Carr,2 Susan Cassels,and Leiwen Jiang4

1Center for International Earth Science Information Network, Columbia Universand Population-Environment Research Network, Palisades, New York 10964;email: [email protected]

2Department of Geography, University of California, Santa Barbara, California93106-4060; email: [email protected]

3Center for Studies in Demography and Ecology, University of Washington, SeattWashington 98195; email: [email protected]

4Watson Institute for International Studies, Brown University, Providence,Rhode Island 02912; email: leiwen [email protected]

Annu. Rev. Environ. Resour. 2007. 32:34573

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 articles 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 Wordsclimate change, coastal and marine environments, land-cover

change, land degradation, population dynamics, water resources

Abstract

The interactions between human population dynamics and the evironment have often been viewed mechanistically. This review e

cidates the complexities and contextual specificities of populatioenvironment relationships in a number of domains. It explores t

ways in which demographers and other social scientists have souto understand the relationships among a full range of populati

dynamics (e.g., population size, growth, density, age and sex coposition, migration, urbanization, vital rates) and environmenchanges. The chapter briefly reviews a number of the theories

understanding population and the environment and then proceeto provide a state-of-the-art review of studies that have examinpopulation dynamics and their relationship to five environmental

sue areas. The review concludes by relating population-environmresearch 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 . . . . . . . . . . . . . . . . . 347

POPULATION-ENVIRONMENT

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

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

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

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

R e s o u r c e s . . . . . . . . . . . . . . . . . . . . . 3 5 6

Coastal and MarineEnvi ro nm ents . . . . . . . . . . . . . . . . . 358

Energy, Air Pollution, and ClimateChange . . . . . . . . . . . . . . . . . . . . . . . 360

CONCLUSION . . . . . . . . . . . . . . . . . . . . 362

INTRODUCTION

Humans have sought to understand the re-

lationship between population dynamics andtheenvironment 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 population

and resources as a scientific topic of inquiry.Malthus famous hypothesis was that popu-lation numbers tend to grow exponentially

while food production grows linearly, neverquite keeping pace with population and thus

resulting in natural checks (such as famine)to further growth. Although the subject wasperiodically taken up again in the ensuring

decades, with for example George PerkinsMarshs classic Man and Nature (1864) (4) and

concern overhuman-inducedsoildepletion 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

peak annual rate of two percent. In 1968, PEhrlich published The Population Bomb (which focused public attention on the iss

of population growth, food production, athe environment. By 1972, the Club of Rom

had released its World Model (9), which reresented the firstcomputer-basedpopulatioenvironment modeling effort, predicting

overshoot of global carrying capacity with100 years.

Clearly, efforts to understand the relatioship between demographic and environmetal change are part of a venerable traditio

Yet, by the same token, it is a traditithat has often sought to reduce environme

tal change to a mere function of popu

tion size or growth. Indeed, an overlay graphs depicting global trends in populatio

energy consumption, carbon dioxide (COemissions, nitrogen deposition, or land ar

deforested has often been used to demostrate the impact that population has on tenvironment. Although we start from t

premise that population dynamics do indehave an impact on the environment, we a

believe that monocausal explanations of evironmental change that give a preemine

place to population size and growth suffrom three major deficiencies: They oversiplify a complex reality, they often raise mo

questions than they answer, and they min some instances even provide the wroanswers.

As the field of population-environmstudies has matured, researchers increasin

have wanted to understand the nuances the relationship. In the past two decaddemographers, geographers, anthropologis

economists,and environmental scientists hasought to answer a more complex set

questions, which include, among others, hodo specific population changes (in densicomposition, or numbers) relate to speci

changes in the environment (such as defoestation, climate change, or ambient conce

trations of air and water pollutants)? Hodo environmental conditions and chang

346 de Sherbinin et al.


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in turn, affect population dynamics? How

do intervening variables, such as institutionsor markets, mediate the relationship? Andhow do these relationships vary in time and

space? 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 which

demographers 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, vital

rates) and environmental changes. With the

exception of the energy subsection, the focusis largely on micro- and mesoscale studies

in the developing world. This is not becausethese dynamics are unimportant in the

developed worldon 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 research

has 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 that

this review merely provides a sampling of themost salient findings. Thechapter beginswitha short review of the theories for understand-

ing population and the environment. It thenproceeds to provide a state-of-the-art review

of studies that have examined populationdynamics and their relationship to the fol-lowing environmental issue areas: land-cover

change and deforestation; agricultural landdegradation and improvement; abstraction

and pollution of water resources; coastal andmarine environments; and energy, air pollu-tion, and climate change. In the concluding

section, we relate population-environmentresearch to the emerging understanding of

complex human-environment systems.

Land degradationany human induceor natural processthat negatively

affects soil structunutrients, organicmatter,moisture-holdingcapacity, acidity ansalinity

Global Trends in Populationand Consumption

At the global level, research has found thatthe two major drivers of humanitys ecologi-

cal footprint are population and consumption(11), so we provide a brief introduction to the

status 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 PopulationProspectspresents a medium variant projectionby 2050 of 9.2 billion people andstillgrowing,

although at a significantly reduced rate. All ofthe projected growth is expected to occur in

the developing world (increasing from 5.4 to

7.9 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 more

than double the number of its inhabitants inthe next 43 yearsfrom 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 population

momentum 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. Fertility

in the former is assumed to be half a childlower than the medium variant, and in the

latter, 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 progressin reduc-ing fertility.

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IPAT: This identityholds thatenvironmentalimpacts (I) are the

product ofpopulation size (P),affluence orconsumption (A),and technology (T).

above- or below-replacement fertility can

havedramatic long-termconsequences fortheultimate 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 prevalence

in some countries, increasing antiretroviraldrug therapy, and government commitments

made under the Millennium Declaration(13).

Consumption trends are somewhat more

difficult to predict because they depend moreheavily than population projections on global

economic conditions, efforts to pursue sus-

tainable development, and potential feed-backs from the environmental systems upon

which 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 it

was in 1900, having grown at a rate of 2.7%per annum (14); CO2 emissions have grown

at an annual rate of 3.5% since 1900, reach-ing an all-time high of 100 million metric

tons of carbon in 2001 (15); and the ecolog-ical footprint, a composite measure of con-sumption measured in hectares of biologically

productive land, grew from 4.5 to 14.1 bil-lion hectares between 1961 and 2003, andit is now 25% more than Earths biocapac-

ity according to Hails (16). In the case ofCO2 emissions and footprints, the per capita

impacts of high-income countries are cur-rently 6 to 10 times higher than those inlow-income countries. As far as the future is

concerned, barring major policy changes oreconomic downturns, there is no reason to

suspect that consumption trends will changesignificantly in the near term. Long-termprojections suggest that economic growth

rates 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 fieldand populatioenvironment studies certainly fit t

descriptiona wide array of theories haemerged to describe the relationship amo

the variables of interest, and each of thetheories leads to starkly different conclusioand policy recommendations. Here we revi

the most prominent theories in the fieldpopulation and environment.

The introduction briefly touched on twork of Malthus, whose theory still genates strong reactions 200 years after it w

first published. Adherents of Malthus hagenerally been termed neo-Malthusians.

its simplest form, neo-Malthusianism ho

that human populations, because of their tedency to increase exponentially if fertility

unchecked, will ultimately outstrip Eartresources, leading to ecological catastroph

This has been one of the dominant paradigin the field of population and the enviroment, but it is one which many social scie

tists have rejected because of its underlyibiological/ecological underpinnings, treati

humans in an undifferentiated way from othspecies that grow beyond the local carr

ing capacity. Neo-Malthusianism has becriticized for overlooking cultural adaptatiotechnological developments, trade, and ins

tutional arrangements that have allowed hman populations to grow beyond their losubsistence base.

Neo-Malthusianism underpins the ClubRome World Model (mentioned above)

and implicitly or explicitly underlies mastudies and frameworks. The widely citIPAT formulationin which environmen

impacts (I) are the product of population (Paffluence (A), and technology (T)is impl

itly framed in neo-Malthusian terms (17), though not all research using the identityMalthusian in approach (18). IPAT itself h

been criticized because it does not accoufor interactions among the terms (e.g., i

creasing affluence can lead to more efficie



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technologies); it omits explicit reference to

important 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).2The so-called Boserupian hypothesis,

named after agricultural economist Esther

Boserup, holds that agricultural productionincreases withpopulation growth owing to the

intensification of production (greater laborand capital inputs). Although often depictedas being in opposition to Malthusianism,

Malthus himself acknowledged that agri-cultural output increases with increasing

population density (just not fast enough),

and Boserup acknowledged that there aresituations under which intensification might

not take place (20). As Turner & Shajaat Ali(21) point out, the main difference between

the theories of Malthus and Boserup is thatMalthus saw technology as being exogenousto the population-resource condition and

Boserup sees it as endogenous. Cornucopiantheories espoused by some neoclassical

economists stand in sharper contrast toneo-Malthunisianism because they posit that

human ingenuity (through the increasedsupply of more creative people) and marketsubstitution (as certain resources become

scarce) will avert future resource crises (22). Inthis line of thinking, market failures and inap-propriate technologies are more responsible

for environmental degradation than popula-tion size or growth, and natural resources can

be substituted by man-made ones.Political ecology also frequently informs

the population-environment literature (23).

2For example,Myers (156),finding that theCO2 emissionsgrew annually by 3.1% from 19501980 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 poin a systemscondition in whichabrupt change is

observed and beyowhich recovery ofearlier conditions difficult

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

VCM: vicious cirmodel

Many political ecologists see population and

environment 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 countries

themselves (e.g., 24). In this view, migrants todeforestation hot spots in frontier areas maybe victims of historical inequalities in land ac-

cess in their countrys 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, or

both. Whatever the impact of the migrant onthe rainforest, it is merely a symptom of more

deeply rooted imbalances. Similarly, political

ecologists see land degradation as stemmingfrom poor farmers lack of access to credit,

technology, and land rather than populationgrowth per se.

A number of theoriesoften subscribedto by demographersstate that population isone of a number of variables that affect the en-

vironment and that rapid population growthsimply exacerbates other conditions such as

bad governance, civil conflict, wars, pollutingtechnologies, or distortionary policies. These

include the intermediate (or mediating)variable theory (23) or the holistic approach(25) in which populations impact on the

environment is mediated by social organiza-tion, technology, culture, consumption, andvalues (26, 27). Some also group IPAT in this

category because population is only one of thethree variables contributing to environmental

impacts.Many theories in the field of population

and 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, the

model is neo-Malthusian, but it also owes adebt to a number of other theories. First, itbuilds on the intergenerational wealth flows

theory 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 borrows

from a demographic theory that describes fer-tility as an adjustment to risk, which argues

that in situations where financial and insur-ance markets and government safety nets arepoorly developed, children serve as old-age

security (31). Finally, it is partially derivedfrom the ecologist Garrett Hardins famous

(32) tragedy of the commons, which holds

that as long as incentives exist for each house-hold to privatize open access resources, then

there 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 theworlds population reaches 9 or 10 billion peo-

ple (2). But many scientistsneo-Malthusianor notare justifiably concerned with the im-pact that even the current 6.7 billion people

are 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 like

Denmark, whereas neo-Malthusianism, polit-ical ecology, and intermediate variable the-

ories may each illuminate different facets ofHaitis environmental crisis. Finally, Boserupstheory of intensification has been found to

hold true in the historical experience of manydeveloped countries and in many localized

case studies spanning the developing world(33).

Although theory may seem dry aacademic, theoretical frameworks can be im

portant guides to action. A good theory heto develop well-targeted policies. Howev

bad theory can becomethe orthodoxies thare very difficult to overcome and that u

derlie government and development agenpolicies and programs (34, 35). Each of tabove theories identifies one or more ultim

causes for environmental degradation, whiif remedied would solve the problem. the case of neo-Malthusianism, populati

growth is the primary problem, and tsolution is population programs. In the ca

of cornucopianism, market failures are tprimary problem, and the solution is to them. For political ecologists, inequalities

different scales are the main problem, apolicies should address those inequaliti

Multivariable theories offer few magic bullbut do underscore the need for action multiple fronts to bring about sustainabili

Unfortunately, many theories in the reaof population and the environment ha

not been subjected to the level of rigoroempirical testing that would allow them to categorized as robust. This is partly becau

thelinkagesare complex anddifficult to dise

tangle. Fortunatelyfor thefield as a whole,tpicture is beginning to change, and a numbof studies at the microlevel have used robustatistical methods and multilevel modeli

in order to test theories such as the VCM (3We now turn to a review of the five iss

areas.

REVIEW BY ENVIRONMENTALISSUE AREA

In this section, we review the literature population-environment interactions in ea

of five issue areas: land-cover change, agrictural land degradation, water resource maagement, coastal management, and ener

and climate change. We focus largely on pereviewed articles published in the past deca

with an occasional reference to important elier work.



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Land-Cover Change andDeforestationThe conversion of natural lands to croplands,pastures,urbanareas,reservoirs,andotheran-

thropogenic landscapes represents the mostvisible and pervasive form of human impact

on the environment (37). Today, roughly 40%of Earths land surface is under agriculture,and 85% has some level of anthropogenic in-

fluence (38). Although the worlds populationis now 50% urban, urban areas occupy less

than three percent of Earths 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 urban

middle classes (40).

As with the demographic and develop-ment transitions, the world remains divided

in 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 economies

dominated 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 chang(LUCC): changehuman use of land

(e.g. fromagricultural toresidential) ornatural land covertypes

the primary sector (agriculture and extractive

industries).In part because most developed countries

largely deforested their lands in past cen-

turies, today most land conversion from natu-ral states to human uses is occurring in the

developing world, particularly in the trop-ics through forest conversion to agriculture.(One exception is the Russian Far East, which

is 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 linkages

with population dynamics, research on land-use/-cover change (LUCC) and particularly

deforestation constitutes a large portion of

the population-environment literature. De-mographic variables are linked at different

scales to this phenomenon (42). But there isdisagreement on the impactof populationver-

sus other factors, withsome studies suggestingthat demographic dynamics contribute morethan any other process to deforestation (43)

and others suggesting the superiority of eco-nomic factors (44). Geist & Lambins 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

Proportion

oflandscape

Naturalecosystems

Frontierclearings

100 %

0 %

Subsistenceagriculture

andsmall-scale

farms

Protected/recreational lands

Urbanareas

Intensiveagriculture

Figure 1

Land-use transitioReprinted fromReference 163 witthe permission ofScience.



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deforestation are driven at least partially by

population 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, we

briefly outline how population dynamics af-fect LUCC through changes in fertility, pop-ulation structure, and migration as well as

how 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 they

declined 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 most

remotely settled lands where the agriculturalfrontier continues to advance, areas that areboth biodiverse and ecologically fragile. This

high fertility and associated rapid populationgrowth directly contribute to land conversion

in these forest frontier areas. Fertility in re-mote areas of the tropics is buoyed by a com-

bination of low demand forand 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 mortalitypromoting so-called insur-

ance births that guarantee a family a certainnumber of surviving children (31). Children

compensate 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 in

frontier LUCC (36). Although young chil-dren divert household labor resources from

agriculture, older children contribute laborthe farm or capture public access resourc

such as firewood, game, and water. The settment life cycle of farm homesteads also he

to explain when and where forest clearing woccur (52, 53). Immediately following sett

ment, deforestation is high as land is clearfor subsistence crops (51, 54). A later defoestation pulse may occur as farms move fro

subsistence to market-oriented crops or epand into livestock. These processes are eabled by children growing old enough to pr

vide labor or capital investments (through,example, remittances) to the farm househo

(53).Despite the high fertility of remote ru

populations, migration remains the prima

source of population growth in forest frotiers (44). Indeed, at a key point along t

forest transitions causal chain, in-migrationa necessary precedent to frontier deforestion. Migration will remain a major driver

frontier forest conversion, often in a leap-frmanner, as more established farm househo

send younger family members as migrantsthe new frontier (55).

Although population dynamics are cent

to LUCC, in all cases population exerts

influence synergistically with other factoDemand for agricultural land among smholders directly impacts forest conversiwhereas, owing to market forces, urban a

international demand for forest and agrictural products further contribute to LUC

through logging and large-scale agricultuPolitical and institutional factors also pan important role in shaping LUCC. F

example, government investments in roasubsidies to the agricultural sector, or la

tenure policy can directly influence deforestion rates. Such effects are well researchedthe Brazilian Amazon (5658). Cultural pr

erences can also affect LUCC, such as tdesire for cattle as a status symbol amo

Central American frontierfarmers (59). Thintervening variables help explain incosistencies in population-LUCC dynam

(60).



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Changing the scale of analysis reveals

examples 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 populations

but continued deforestation (48). A dramaticexample is Ecuador whose Amazon regions

forest canopy is facing rapid attrition owingto growing settlements of frontier farmers, al-though overall rural population is declining

because of falling fertility and rapid urbaniza-tion (61). This apparent anomaly is explained

by the small populations, which account for

a minority of a nations rural population, thatmove to forest frontiers and contribute a dis-

proportionate amount to the nations 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 increasingly

mechanized farming, the region has also ex-perienced rural population decline (62). Inter-

estingly, the same associationrural depopu-lation and continued deforestationin Ecuador

and Brazilresults from a completely differ-ent causal mechanism in the two cases, high-lighting the importance both of scale and

place-based effects. Similar scale-dependentphenomena emerge in Asian forest frontiers.Research in Thailands northeast suggests, for

example, the importance of population fac-tors at finer scales and of biophysical factors at

coarser scales for explaining variation in plantbiomass levels (63).

Land-cover dynamics are the most evident

mark of human occupation of Earth. Links topopulation are both obvious (without human

population presence there is no human im-pact on forests apart from acid rain) and ex-ceedingly complex, e.g., at what spatial and

temporal scales does population interact withpolitical, economic, and social processes to

produce 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, more

research is needed at the mesoscale (subna-tional) and to build causal chains across spatial

scales. 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 considers

changes in the quality of land resources as a

result of human uses, which is the focus ofthis section. Perhaps themost contentious de-

bate in the population-environment literatureconcerns the relationship between increasing

population density in subsistence agriculturalareas and land degradation or improvement.This is, in part, the result of widely differing

estimates regarding the extent of land degra-dation, with global estimates ranging from 20

to 51 million km2 (64). This section consid-ers arguments and evidence marshaled by two

major schools of thought: the vicious circleproponents who believe that increasing popu-lationdensityinthecontextofhighpovertyal-

most inevitably leads to land degradation andthe Boserupians who suggest that increasingdensity leads to intensification of agricultural

systems such that yields per unit area (and percapita) are increased (see the theory section,

above).In the VCM, it is hypothesized that there

are a number of positive feedback loops that

contribute to a downward spiral of resourcedepletion, growing poverty, and population

growth. 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 womens



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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 declining

per capita resource base reinforces povertythrough soil fertility loss, declining yields, and

poor 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 soil

fertility 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) have

suggested that children are valued by ruralhouseholds, in part, because they transform

open access resources (forests, fisheries, andrangeland) into household wealth, resulting intheexternalizationof the costs of highfertil-

ity. Onemanifestation is theprocess 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 as

the 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 for

a 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 for

the vicious circle hypothesis using detaileddatafrom Pakistanon childrens timeuse,fire-

wood collection activities, and recent fertilThey find that collection activities do abso

a substantial part of household resources athat childrens tasks are often devoted to c

lection activities. However, they could not tablish a fertility effect on resource or la

degradation. A longitudinal study in the weern Chitwan Valley of Nepal (72) found ththree measures of local resource depletion

the time to collect fodder, the increase in timrequired to collect fodder in the prior thryears, and households dependence on pu

lic lands for fodderwere significantly apositively correlated with desired family si

even when controlling for household weaandnumerousotherfactorsfoundtoinfluendesired fertility. Yet, several other indicato

of environmental decline had no significarelationship to either desired fertility or pre

nancy outcomes, and the actual relationshipdesired fertility depended in part on wheththe respondents were men or women. Pasc

& Barbiers (70) modeling of shifting cultivtion in the Yucatan found that among po

households, as population density increasthe response was extensification or a redution in fallow periods, whereas among bett

off households, labor was shifted to off-fa

employment. Thus, although anecdotal edence is abundant and development policmaking has been heavily influenced by VCassumptions, there is only qualified supp

for the hypothesis in the few existing quantative studies.

The Boserupian or intensification hypoesis has been tested in a number of stuies spanning Africa, Asia, and Latin Ameri

A frequently cited study by Tiffen et al. examined changes in population density a

agricultural productivity in Machakos Dtrict, Kenya. From 1930 to 1990, the poulation of Machakos District grew sixfo

from 240,000 to 1.4 million people, with1990 population density of 654/km2. The

gion is mountainous and semiarid (


11/31

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 work

groups, agricultural systems shifted from live-stock to intensive farming with emphasis on

higher-value crops, feeder roads were builtto market towns, and market towns devel-oped with agricultural processing facilities

and other small industries. By 1990, the valueof agricultural production had doubled on a

per capita basis. Many factors led to a posi-tive outcome for this region, including infras-tructure development, market growth, private

investment, increasing management capacityand skills, self-help groups, food relief dur-

ing drought, and secure land tenure. This

study confirms the basic Boserupian hypoth-esis: increased food demand, a denser net-

work of social and market interactions, labor-intensive agriculture and economies of scale

helped to avert a Malthusian crisis. Yet evenin thistextbookstudy, other researchers work-ing in the district found important social dif-

ferentiation in livelihood improvements, landalienation, and government-imposed limita-

tions on mobilityelements that tend to maran otherwise rosy picture (73).

Mortimore(74) found similarsuccess sto-ries in three dryland areas of West Africa:Kano State in northern Nigeria, the Diourbel

Region 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 in

none of these regions were indicators under allfourdomains positive, the author neverthelessfound somecommoningredients thatresulted

in improved or stable soil fertility and yieldsdespite rapid population growth and high

densities. These ingredients include marketsfor agricultural produce, physical infrastruc-ture, producer associations, knowledge man-

agement, and incentives for investment andincome diversification. He concludes that

productivity 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 (19501986) 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 high

population densities (783 persons per km2).Soil conditions in Bangladesh are, on average,

much better than in dryland Africa owing todeposits of alluvium during monsoon season

flooding and therefore can support far higher

densities. They posit that, as smallholderscome in contact with the market economy,

their redundant production is reduced, andtheir aspirations increase. Although cropping

intensities on average increased significantly(inone village almosttripling), theyalso foundincreasing production disparities, with large

land holders accounting for most of the sur-plusproduction,whereas the growing number

of landless suffered shortfalls and malnutri-tion. They conclude that Bangladesh passed

several threshold steps at points along its pathtowards intensification in which Malthusianoutcomes of involution and stagnation might

have occurred but were fortunately averted.As these case studies make clear, pop-

ulation is but one among many factors

that influence degradation or intensification.Other variables that are of crucial signifi-

cance includeinstitutional 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 nor

sufficient condition for either declines or im-provements in agricultural productivity to oc-

cur. Population growth can either operateas a negative factor, increasing pressure on



12/31

limited arable land, or a positive factor, help-

ingto induce intensification through adoptionof improved technologies and higherlabor 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 instead

that 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, there

is little doubt that rapid population growthin poor rural areas with fragile environments

can 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 past

precedents can only be extrapolated with cau-tion, because the exact locations of thresholds

in any given system are still largely unknown(21). Oneimportant advance forstudies in this

area 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-tions impact can be explained by differing in-terpretations of what constitutes degradation

and by a paucity of empirical evidence for therelationship.

Abstraction and Pollution of Water

ResourcesThe water cycle ties together life pro-

cesses. It is fundamental to the biochemistryof living organisms; ecosystems are linkedand maintained by water; it drives plant

growth; 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 dedicat

the same level of attention to population dnamics and water resources as they haveresearch on land-cover change, agricultu

systems, or climate change. Yet there are clrelationships between population dynam

and freshwater abstraction for agriculturdomestic, and industrial uses, as well as emsion of pollutants into water bodies.

Human settlement is heavily predicatupon the availability of water. A map of glo

population distributions closely tracks annurainwater runoff, with lower densities in tmost arid regions and as well as the most w

ter abundant, such as the Amazon and ConBasins. Whereas the former areas are wa

constrained for agriculture, in the latter

eas, year-round rainfall in excess of 2000 mhas rendered these environments less favo

able for agriculture(owing to soil leachingaoxidation) and more favorable for human a

livestock diseases.At the global level, irrigation water

agriculture is the biggest single user (abo

70% of water use), followed by indus(23%) and domestic uses (8%) (79). If gre

water is added to the mix (water that feerainfed crops), then crop production far a

away outstrips other water uses. As demafor food increases with growing populatioand changing tastes (including growing d

mand for animal versusvegetable protein wits far greater demands for water), it is epected that water diversions for agricultu

will only increase. Today, humanity is esmated to use 26% of terrestrial evapotra

spiration and 54% of accessible runoff (8Falkenmark & Widstrand (81) establishbenchmarks for water stress of between 10

and 1700 m3 per person, water scarcity of btween 500 and 1000 m3 per person, and a

solute scarcity of less than 500 m3 per persoNorthern and southern Africa and the MidEast already suffer absolute scarcity. As pop

lationgrows and waterresources remainmoor less constant, many countries in the rest

Africa are projected to fall below 1000 m3 pperson (82).



13/31

Perhaps because such water resources are

hidden underground, groundwater resourcedepletion could potentially remove someagri-cultural areas from the map. Although it is

well 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 more

than a decade owing to the unregulated tap-ping of aquifers (83), rendering some semi-arid regions vulnerable to shortages. A study

in Karnataka State, India, identified a majorshift from surface to groundwater use in the

past decades and found that groundwater use

is highly inequitable; large farmers possessing1216 haof landmake uponly 8% ofall farm-

ers but consume 90% of groundwater (84). Inthe lower delta of the Ganges-Brahmaputra

Basin, 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 that

accessible 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 andmore

likely to rely on unimproved wells. Rural-urban migration is not correlated to relativewater scarcity in places of origin but rather

to proximity to roads and to towns. The re-searchersconclude thathigh fertilitya tradi-

tional adaptation to peak labor demands dur-ing the short cropping seasonincreases theproblems of water access and supply mainte-

nance in agricultural and domestic spheres.But they also note that gloomy prognoses

about 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 northeastern

Tanzania, 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 every

10 years. But other factors include water ex-traction and land alienation for export flowerproduction and protected areas, growth and

mobility of livestock herds, declining summerrunoff from glaciers on Mount Kilimanjaro

owing to global warming,andhydroelectricitygeneration. The greatest conflict is betweenfarmers and pastoralists, as farmers progres-

sively moved into areas previously consideredtoo marginal for agriculture and pastoralists

were squeezed by restrictions on grazing areasowing to newly established protected areas. Inrecent years, the pressure on land has led to

stresses 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 River

Basins within the Campinas Administrative

Region (AR) (88). Campinas is Brazils 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 19702000 period: 6.5% (19701980),3.5% (19801991), and 2.5% (19912000).

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 growth

meant that water supply and sanitation infras-tructure could not keep up. By mid-1995 only

5% of waste was treated before reenteringstreams, and large reaches of the Piracicabaand Capivari River Basin tributaries were

deemed of poor quality. Water supply



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

migration

infrastructure (mostly surface reservoirs as

groundwater 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 fees

for water withdrawals and restrictions on res-idential development, as well as some techni-cal ones, particularly the treatment of waste

waters.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 growth

most assuredly does reduce per capita wateravailability. It is in this light that the Global

International Waters Assessment listed pop-

ulation growth first in a series of root causesof the global water crisis (89). Yet there is

moretopopulationchangethangrowthalone,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 and

climatic changes, agricultural and industrialtechnologies, sewage treatment, and institu-

tional mechanisms, to name but a few. One ofthe challenges to research in this area is the

common property nature of water resources,and another challenge is caused by rapid reg-ulatory changes as water resources become

scarcer, which alters the institutional context.The field could use more basin or watershedstudies to understand how variables such as

population 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 competition

between 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 a

easy access to the resources of the rural hiterlands. As a result, the coastal zone has tracted large and growing populations, w

much of their growth attributable to migtion rather than natural increase (93). Tod

10% of the worlds population lives at lethan 10 m above sea level (even though tharea only accounts for 2.2% of the worl

land area), and coastal zones have higher poulation densities than any other ecologica

defined zone in the world (39, 94). Coasand marine environments are very importafor human health and well-being, and th

are also quite vulnerable to anthropogenimpacts. Yet, until recently most populatio

environment research has focused on terr

trial ecosystems, possibly because the humfootprint on coastal and marine ecosyste

is harder to discern.Not surprisingly, over half of the worl

coastlines are at significant risk frodevelopment-related activities (95), and tpotential (and realized) environmental dam

age is substantial. Population growth is oten named as the driver of coastal and mari

environmental problems, whereas proximcauses can be traced to specific practices (9

A recent study highlights how the Kuna poplation (an indigenous populationin CaribbePanama) has practiced coral mining and lan

filling for decades in response to populatigrowth (97). Since 1970, live coral cover dclined 79%, and at the same time, the Ku

population increased by 62%. The Kugradually enlarged their island landmass

adjust for their growing population by buiing coral walls out into the water and thfilling in the enclosed areas with corals, se

grass, and sand. In addition to direct losscoral reef, consequences include coastal er

sion and a local increase in sea level. This eample provides a clear and direct link betwepopulation growth and coastal degradation

Population growth can lead to maother coastal and marine environmen

disturbances. For instance, tropical magroves are being converted to fish and shrim



15/31

aquaculture farms, which undermines coastal

protection and decreases natural habitatthat many fish species use for reproduction.Expanding coastal cities undermine natural

protection 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 the

sensitivity of coastal systems to stress, localinstitutions, and global markets. For example,demand for shrimp is the ultimate driver of

mangrove loss, and sewage treatment systemsand no-till agriculture could significantly

reduce nutrient loading in coastal areas.

The relationship between human activitiesand environmental impacts is hard to assess

and regulate in coastal and marine environ-ments because the environmental resources

are 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 cause

environmental degradation (98100). Thus, asignificant recurring theme in this researchis that the social and economic context in

which the population is changing as well aswhen, how, and with whom people interact

is more important in determining the impacton the environment than simply demographicchange (101, 102).

Studies in developing countries on migra-tionandthemarineenvironmenthavefocused

on a mediating variables approach, such ashow technology, local knowledge, social in-stitutions of kinship or marriage, and markets

mediate the role of population in resourceextraction and consequent environmental

degradation or enhancement. For example,some work has hypothesized that migrants

Common properresource (CPR):resource that is solarge or widesprea

that it is difficult texclude people frousing it

Social capital: thresources (networrelationships of truaccess to widerinstitutions ofsociety) upon whica household orindividual may dra

misuse resource extraction technologies,

which leads to environmental degradation(103). In a coastal Brazilian population, tech-nological change imposed by outsiders who

lacked knowledge of the ecological and socialcontext of the community contributed to de-

creased ecological resilience (104), and rapidin-migration and technological changes in seacucumberfishing techniques in the Galapagos

led to a collapse in the sea cucumber industry(105). In both cases, the results seem to be a

function of the migrants limited local knowl-edge as well as expansionist attitudes andshort-term time horizons for profiting from

the extraction of coastal and marine resources.Thus, population-environment re-

searchers have begun to incorporate other

social theories such as social capital andmigrant incorporation to understand when

population pressures do not necessarilydegrade the environment (106). Most studies

have found that, in systems with strong landtenure or social capital, migrants do not dis-rupt the environment and are able to develop

local knowledge that mitigates environmen-tal impacts (107109). A case study in the

Solomon Islands contests the notion that seatenure regimes are weakened by in-migration

and population growth. Rather, potentiallynegative impacts of population pressure onthe environment are diminished significantly

with greater reciprocal ties among close kinor neighbors (110, 111). Similarly, intermar-riage between a migrant and a nonmigrant

in Sulawesi, Indonesia, has been shown tomitigate the otherwise negative association

between migrant households and coral reefdegradation (106).

Migration has been the most studied com-

ponent of population dynamics in coastal andmarine environments. Yet, urbanization and

tourism are other primary human drivers af-fecting coastal ecosystem quality (112, 113).Fourteen of the worlds largest 17 cities are

located 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



16/31

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 of

population size or density. As the afore-mentioned studies suggest, technology,knowledge systems, social cohesion, common

property systems, migrant incorporation,and the economic and ecological context in

which these interactions take place all playan important role in population and envi-ronment research, especially in developing

countries. Nonetheless, coastal and marineenvironments continue to be among the most

threatened ecosystems in the world, owing in

part to the sheer scale of detrimental humanactivities associated with urbanization along

the coasts, continued population growth, anda growing number of tourists in search of

coastal amenities.An unresolved issue in this area of

researchas 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 theimpact of local andglobal consump-tion on marine and coastal environments. For

instance, 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). Further

research is needed to assess how population-environment linkages in marine and coastalareas are influenced by the global food trade

connecting consumers and producers fromopposite sides of the world.

Energy, Air Pollution, and ClimateChange

Even when they are connected to the electric

grid, some two billion poor people in the de-veloping world still largely rely on biomass to

meet their energy needs. That leaves appro

imately 4.7 billion people with more energintensive lifestyles who consume, with lithelp from the worlds poorest, the ener

equivalent of 77 trillion barrels of oil a ye(118).3 More than 80% of global energy co

sumptionis derived fromfossil fuels (119), ait is this dependence on fossil energy thatresponsible for the release of the greenhou

gases and airborne pollutants that are alting atmospheric composition and proces

on a global scale. As concern mounts over thealth impacts of urban air quality (particlarly in developing countries) and the pote

tial adverse effects of climate change acromultiple systems and sectors, populatio

environment researchers have paid particu

attention to understanding the demographdrivers of energy consumption. Although

is clear that there are vast differences in cosumption levels (per capita energy consum

tion in the United States is 48 times whit is in Bangladesh and 4.7 times the woaverage), it would be wrong to suggest th

population variables are irrelevant. Hence,review a number of empirical studies that e

amine population-energy linkages in a sytematic and quantitative manner.4

In studies of energy consumption searchers have found that it is more apprpriate to use the household rather than i

dividuals as the unit of analysis becauselarge portion of energy consumption relatto space conditioning (heating and air co

ditioning), transportation, and appliance uis shared by household members. This sh

ing results in significant economies of scawith large households generally showilower per capita energy use than small on

3Ofthis total,the equivalent of 66 trillionbarrelsis actufrom fossil fuels (petroleum, natural gas, and coal). Tremaining energy supply (13.7% of the world total) comfrom renewable sources and nuclear energy.

4Although there is extensive research on the reciprocalpact of air pollution and projected climate change on mographic variables such as morbidity, mortality, and gration (158, 159), this is beyond the scope of this revi



17/31

(29, 120). Energy studies have identified a

range of household characteristics as key de-terminants of travel patterns (121123) andof other types of residential energy demand,

such as for heating, cooking, and operatingdomestic appliances (124127). In a pioneer-

ingstudy, MacKellar et al. (128) used theIPATidentity to decompose the annual increase inenergy consumption of the more developed

regions during the period 19701990. Theyfound that, because growth in the number of

households outpaces population growth ow-ing to trends in fertility, divorce, and aging,growth in household numbers accounted for

41% of the total increase in energy consump-tion, whereas population growth accounted

foronly18%.However,thisstudydidnottake

into account the lower energy requirementsof smaller households, so it likely exaggerated

the contribution of the growth in householdnumbers to energy use.

ONeill & Chen (129) drew on householdsurvey data to quantify theinfluence of house-hold size, age, and composition on residential

and transportation energy use in the UnitedStates and found that changes in household

size have caused 14% of the increase inper capita energy use over the past several

decades. Lenzen et al. (130) assessed the im-portance of various demographic characteris-tics on householdenergy demandin 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 location

are negatively associated. To explore the im-portance of adopting adequate demographicvariables in understanding transport-related

energy consumption, Prskawetz et al. (131)combined a cross-sectional analysis of car use

in Austriawithdetailedpopulation/householdprojections and tested the sensitivity of pro-jections of future car use across a wide range

of 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 19962046 if the number

Energy intensitythe amount of enerequired to produone unit of output

more efficient useenergy 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 (via

energy consumption) on air pollution, scien-tists have identified a number of important

factors that jointly determine pollutantemissions, including the familiar elementsof the IPAT identitypopulation, affluence,

and technology as reflected in energy andemissions intensities (132). Selden et al.

(133) analyzed the reduction of U.S. major

air pollution emissions from 1970 to 1990and found that changes in economic scale,

economic composition, energy mix, energyintensity, and emissions intensity all played

important roles. In quantifying the impacts ofpopulation on air pollution, researchers havereached different conclusions depending on

which pollutants are under study, in whichlocations, at what scale, and for which time

periods. 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 very

difficult 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 19702000 suggests that pop-ulation size has an inverted U-shaped curve

with CO2 emissions as well, which is at oddswith previous literature investigating these

variables for other regions and time periods(138). The different patterns of impacts may



18/31

reflect the nature of complicated interactions

between 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 emissions

of 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-

ablessuchas thepercentof 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 they

studied. However, caution should be used in

interpreting 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), althoughpopulation

projections 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 theSRES were largely consistent

with actualpopulation sizes for the 19902005period, the projections to 2050 and beyond

were 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 future

climate change and possible response strate-gies, the Fourth Assessment Report of theIPCC is based on slightly lower population

projections than the Third Assessment Re-port under theA2 scenario,which describesan

economicallydivided world withslowtechno-logical progress and high population growth.

Consideration of demographic factors byond population size, such as changes in a

structure, urbanization, and living arrangments, which as discussed above are impo

tant in modeling future energy use, is naccounted in the SRES population assum

tions. Making progress in this area requia better understanding of the scope for fture demographic change as well as met

ods for including demographic heterogenewithin energy-economic growth models usfor emissions scenario development.

Simultaneous and consistent projectioof population, urbanization, and househo

are challenging demographic tasks (14Recently, Dalton et al. (148) introduced herogeneous households into a general eq

librium population-environment-technolomodel of the U.S. economy. Because differe

types of households have unique demands fgoods, capital stock, and labor supply, athese characteristics have direct and indir

implications for energy demand, they weincorporated into cohorts by age grou

(or dynasties). These dynamics and othrelationships implied by household projetions create nonlinear interacting effects th

influence each dynastys future saving a

consumption decisions. Their research shothat including age heterogeneity among Uhouseholds reduces emissions by almost 40in the low-population scenarios by year 20

and effects of aging on emissions can belarge as, or larger than, effects of technic

change in some cases. Those effects abelieved to be much larger for the developiworld, where more significant demograp

changes such as population growth, aginhousehold nuclearization, and urbanizati

are occurring.

CONCLUSION

One of the reasons natural scientists ha

found population to be so appealing as a hman dimension of environmental change

that data are readily available (in contrastother human variables such as values, cultu



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and institutions), projections are reasonably

reliable (149), and population can be treatedin models in a manner that is analogous toall the other quantitative variables. This has

promoted something of a reductionist view ofpopulation-environment interactions. Fortu-

nately, a growing number of natural scientistsare beginning to appreciatethathumans inter-act with the environment in more ways than

their raw numbers often imply. Populationsare composed of people who collectively form

societies, and people and societies cannot eas-ily be reduced to food and material demandsthat result in some aggregate impact on the

environment.5 This makes human societies atonce messy for modeling and fascinating to

study. The new understanding builds on the

concept of coupled human-environment sys-tems, which are more than the sum of their

parts (150, 151).In the human-environment system, the

impacts are not unidirectional but reciprocal.For example, the environmental changeimpacts on morbidity and mortality are a

growing area of interest, and some havesought to close the circle by looking at

how environmentally induced mortality mayaffect population projections (2). There is

also growing research on the health impactsof landscape or climatic changes on humans,in the one instance through the creation of

mosquito breeding habitats that contributeto malaria (152), and in the other throughheat stress or famine (153). Research on the

human-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 information

systems, spatial statistics), and new models(agent-based, multilevel, and spatially explicit

modeling). 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 politicallydiverse. Vari-ationsin theroles of women around theworld. . .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 been

a concern for distributional justice in twoforms: that the 5.4 billion citizens of devel-

oping countries might be able to raise their

living standards and hence their consumptionlevels from their previously low levels andthat

the costs of biodiversity conservation and cli-mate change adaptation not be unfairly borne

by the poorest. Whether research proves thatpopulation dynamics have a dominant or neg-ligible effect on environmental outcomes in

each of the domains we surveyed, it is still leftto human societies to address these inequities

in 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 objectively

review the literature rather than take a nor-mative stance concerning the environmental

impacts associated with population dynamics,at the global scale there is no question thathumanity faces significant challenges in the

coming decades owing to the scale and pace ofchanges in human numbers,population distri-

bution, and consumption patterns. To quoteCohens definitive study on the global carry-ing capacity, The Earths human population

has entered and rapidly moves deeper into apoorly charted zone where limits on human

population size and well-being have been an-ticipated and may be encountered (2, p. 11).



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In recent decades, scientists have increasingly

warned of the potentialto reach the upperlim-its of the planets productive, absorptive, andrecuperative capacities (155). A challenge for

micro- and mesoscale researchers is to under-

stand how changes at the local and nation

scale relate to global-scale changes and hoin turn, their research can inform policies aprograms at these lower scales that will atte

uate environmental impacts at all levels.

SUMMARY POINTS

1. There is more to population dynamics than population size and growth. Recent re-searchhas illuminated theways in whicha number of population variablesage and sex

composition, 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 scaleof analysis can significantly affect findings concerning therole 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 to

isolate from other signals.

6. Both freshwater resources and coastal and marine ecosystems are often managed as

commonpropertyresources (CPRs); hence levels of resourcedegradation or depletiondepend more on the existence of effective management systems than on population

variables 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 researchers

have 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, household

consumption, 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 middle

classes 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 environmental

changes 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 Applications

Center (SEDAC) (NASA contract NAS5-03117 with Goddard Space Flight Center), whichunderwrite the work of the Population-Environment Research Network. The views expressed

here are those of the authors and not necessarily those of the sponsoring and supportingorganizations.

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