IPAT Research Paper

‘Impact’ here indicates a persisting rather than transitory environmental effect. In Commoner’s (1971) definition, it is an external intrusion into an ecosystem which tends to degrade the system’s natural capacity for self-adjustment. For population– environment interactions the impacts of most concern are those that reduce the system’s capacity to provide environmental services. An example of an environmental impact is a country’s carbon dioxide emissions, which degrade the environmental service provided by the atmosphere in regulating heat radiation from the earth’s surface. The decomposition in that case would be population per capita GDP the ‘carbon intensity’ of the economy. At a given level of affluence and carbon intensity, emissions rise in proportion to population.

1. IPAT As A Causal Relationship

The IPAT equation is more widely employed not as an identity but as a causal relationship, intended to point to the environmental damage that results from one or more of population size, a high-consumption lifestyle and environmentally destructive technology, each amplifying the others. Implicitly, it asserts that these factors can together be seen as the main human causes of degradation. That assertion is debatable: in particular, social organizational and behavioral factors would often warrant separate scrutiny as causes of degradation rather than being subsumed within A and T.

This causal use is mostly confined to elementary or rhetorical discourse about the environment. A familiar instance is in contrasting the environmental impacts of population growth in rich and poor regions. If per capita energy use is a proxy for A×T, the environmental impact of an additional person in America or Europe consuming at the average level for those societies would be an order of magnitude greater than that of an average person in Asia or Africa—a disparity cited in many popular writings that seek to assign responsibility for degradation or resource depletion. The relationship is also used simply to organize a discussion, sometimes expressed in a more general functional form: I=f(PAT ).

If P, A, and T were independent of each other, the multiplicative relationship would be equivalent to an additive relationship among growth rates. In the case of carbon dioxide, the growth rate of emissions would equal the sum of the growth rates of the three components. However, P, A and T are not independent of each other. For any defined population and environment, they are variables in a complex economic, demographic and sociocultural system. Each also has major distributional dimensions and is a function of time. Consumption—or any other measure of human welfare—is an output of this system; environmental effects, both intended and unintended, are outputs as well. And even at the global level the system is not autonomous: it is influenced by ‘natural’ changes in the environment and by environmental feedbacks from human activity.

One kind of dependency among P, A and T follows from the common situation of diminishing returns: successive increments to population may do more than proportional environmental damage if they require recourse to ever more powerful technologies to maintain consumption levels. The IPAT equation might then be seen as illustrating a best-case scenario. But other kinds of dependency would tend to lessen the impact. For example, as affluence increases, so typically does a society’s demand for a clean environment and wilderness preservation—that is, there is a high income elasticity of demand for environmental quality. This would support a policy response emphasizing conservation or restoration, and the development and adoption of ‘clean’ technologies. Similarly, population growth rates generally fall, even to negative values, with rising incomes. If such interactions are acknowledged, the PAT decomposition retains heuristic and pedagogical value but does not bear much empirical scrutiny. In particular, it cannot resolve disputes on the relative contributions of factors responsible for environmental degradation.

For this latter task, Preston (1994) has proposed looking at the variances of the growth rates of I, P, A and T over different regions or countries. Writing these as σ²_I, etc., the additive relationship among growth rates implies the following relationship among variances and covariances:

The covariance terms are the interaction effects. If each is relatively small in a given case, there is a simple decomposition of the impact variance into the variance imputed to each factor. Otherwise, the one or more significant interaction terms can be explicitly noted. In Preston’s analysis of carbon emission data for major world regions between 1980 and 1990, population growth makes a minor contribution to the total variance; the major contributors are the growth of A and T, with a substantial offsetting effect from the interaction of A and T.

2. Environmental Spillovers

With a strong management regime or effective community norms and sanctions, population growth in a region need not adversely affect the local environment. Access to a resource can be rationed, behavior strictly governed; or the growth can be diverted elsewhere, usually to urban centers. Ultimately, limits on the environmental services that can be drawn on would be reached and thereafter degradation would probably ensue. Well short of that limit, however, conditions of rapid economic or political change can undermine a management regime or erode norms and sanctions, with adverse environmental results. Excessive deforestation can often be traced to such institutional breakdowns (or to ill-considered efforts at institutional reform) rather than to population growth. In other cases, a resource may have been so abundant that no management or sanctions were needed: this is a setting where the familiar ‘tragedy of the commons’ may unfold as the number of claimants to the resource or their exploitative abilities increase (see Hardin 1968).

Environmental effects of human activities may extend well beyond the region of origin, as with the case of acid rain or watershed destruction. In addition to such literal spillovers, trade and other economic relationships among societies may allow adverse effects to be ‘exported’ to another region: a poorer society may be more willing to incur environmental damage in return for economic gain, or be less able to prevent it. The concept of a community’s ‘ecological footprint’ offers one means of accounting for such displaced effects in terms of the total area required to sustain the community’s population and level of consumption (see Wackernagel and Rees 1996). Where the environmental effects involve degradation of a global commons, as with carbon emissions into the atmosphere, an effective response requires devising an international regulatory regime that takes account of economic level, demographic scale and technological capacity.

At the global level, a calculation of human environmental impact, in the spirit of IPAT, is that of the extent of human domination of various components of the global ecosystem. Examples would include the proportion of the land surface transformed by human activity, the anthropogenic share of atmospheric carbon dioxide, and the proportion of marine fisheries that are fully exploited or depleted (Vitousek et al. 1997). A summary impact measure proposed by Vitousek et al. (1986) is the proportion of total organic material produced through photosynthesis that is used directly by humans or used in human-dominated ecosystems, which they calculated at the time to be around 30 percent of the terrestrial total and 2 percent of the marine total.

3. Nonlinearities In Environmental Systems

An evident difficulty for impact assessment is the nonlinearity of environmental systems. Ecosystems may have multiple equilibria; they may exhibit cyclical or chaotic behavior. Small human populations and primitive technology do not ensure small environmental impact: in prehistoric times minuscule populations of hunter–gatherers may have been responsible for some mammalian extinctions and possibly for large-scale ecological change following the introduction of new fire regimes (see Martin and Klein 1984). Ecosystems may be resilient under the pressure of human activity until a point is reached at which there is sharp discontinuous change (see Holling 1986). Kasperson et al. (1995) identify a series of thresholds in nature–society trajectories as human activity in a region intensifies beyond sustainability: first a threshold of impoverishment, then endangerment and finally criticality—the stage at which human wealth and well-being enter an irreparable decline.

The thresholds may not be evident in advance. Under some greenhouse scenarios, for example, gradual ocean warming leads to an abrupt shift in the course of ocean currents—possibly over a period as short as a few decades—with major effects on weather patterns and the climates of coastal states. As systems models of the environment, such as general circulation models of global climate, expand in size and sophistication, their value in prediction is expected to increase. However, they may always be defeated in this task by the inherent uncertainties created by nonlinearities in nature itself. Application of the precautionary principle, aimed at keeping human environmental impact below the threshold of sustainability, may lower the likelihood of a surprise outcome but cannot eliminate it.

4. Subjectivity Of Impact Assessments

The transformation of natural environments by human activity is typically identified with damage or degradation. In the case of agricultural practices leading to erosion, salination, laterization, etc, that assessment is clearly justified. The spread of crop and forest monocultures is seen as ecologically damaging through the consequent loss of biodiversity and increase in vulnerability. Some human interventions, however, can be seen as positive in ecological terms— and not only in cases of restoration of earlier damage. It is also important to note that scientific judgment in characterizing environmental impact does not necessarily accord with public sentiment. Radically transformed environments—say, English hedged fields or Balinese rice terraces—may be regarded as aesthetically pleasing, provided their ecological stability (at a new level) is maintained. Indeed, Dubos (1980) notes that some of today’s most admired landscapes are products or by-products of human activity, representing ‘areas deforested, swamps drained, hillsides gouged of their stones and sand.’ Prior to the Romantic age, ‘wilderness’ tended to evoke feelings of horror more than of beauty.

Thus, it is necessary to distinguish between an environmental impact of human activity as construed by ecologists (summarized in stylized fashion by the I of the IPAT equation), and the subjective assessment of that effect. In common usage the environment is nature viewed through human eyes, valued by human preferences. Preferences differ among people and over a person’s life; they may be socially constructed; they tend to adapt to or track realities. An environmental impact that is seen by ecologists as an effectively irreversible change in a natural system—for example, extinction of a species or loss of a tropical rainforest, or the much earlier loss of temperate forests—may offer apparently compensating gains at the time and in the locality; the full costs and accompanying regret may only be felt much later or by groups far removed geographically. Moreover, regret wanes as people’s expectations adjust to the new environmental circumstances. An active and timely public response in limiting adverse impacts calls for heightened ecological awareness at the various different levels of society and fuller realization that ecological stability and environmental services cannot be taken for granted.

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