The IPAT Equation and Its Variants

Home , Environmental revolution, Factor 10

❙ FORUM The IPAT Equation and Its Variants

Changing Views of Technology and Environmental Impact

Marian R. Chertow School of Forestry and Environmental Studies Yale University New Haven, CT USA

❙ Keywords environmental technology Summary Factor X In the early 1970s Ehrlich and Holdren devised a simple IPAT equation equation in dialogue with Commoner identifying three fac- master equation tors that created environmental impact. Thus, impact (I) technological change technological optimism was expressed as the product of (1) population, (P); (2) affluence, (A); and (3) technology, (T). This article tracks the various forms the IPAT equation has taken over 30 years as a means of examining an underlying shift among many environmentalists toward a more accepting view of the role technology can play in sustainable development. Al- though the IPAT equation was once used to determine which single variable was the most damaging to the environment, an industrial ecology view reverses this usage, recognizing that increases in population and affluence can, in many cases, be balanced by improvements to the environment offered by technological systems. ❙

Address correspondence to: Marian R. Chertow Yale School of Forestry & Environmental Studies 205 Prospect Street New Haven, CT 06511-2189 USA [email protected] www.yale.edu/forestry/popup/faculty/ chertow.html

Volume 4, Number 4

Journal of Industrial Ecology 13 ❙ FORUM

Introduction 1977). On the other hand, the result of this development was a quantum leap in environmental im- In a provocative article, Rockefeller University pact. Prior to World War II, “smoke, sewage, and researcher Jesse Ausubel asks: “Can technology soot were the main environmental concerns” spare the earth?” (Ausubel 1996a). It is a modern (Heaton et al. 1991, 5). In 1972, the revolution- rendering of an epochal question concerning the ary thinker Barry Commoner opined: “Most relationship of humanity and nature, and, espe- United States pollution problems are of relatively cially since Malthus and Darwin, of the effect of recent origin. The postwar period, 1945–46, is a human population on resources. Surely, technology convenient benchmark, for a number of pollut- does not offer, on its own, the answer to environ- ants—man-made radioisotopes, detergents, plas- mental problems. Sustainability is inextricably tics, synthetic pesticides, and herbicides—are due linked with economic and social considerations to the emergence, after the war, of new productive that differ across cultures. This article, however, technologies” (1972a, 345). discusses the imperative of technological change Commoner conclusively assigned blame, as- and the role it can play in human and environmen- serting that his evidence “leads to the general tal improvement, particularly in the United States. conclusion that in most of the technological dis- The vehicle used to begin the discussion of placements which have accompanied the technological change, though phrased math- growth of the United States economy since ematically, is largely a conceptual expression of 1946, the new technology has an appreciably what factors create environmental impact in the greater environmental impact than the technol- first place. This equation represents environ- ogy which it has displaced; and the postwar mental impact, (I), as the product of three vari- technological transformation of productive ac- ables, (1) population, (P); (2) affluence, (A); tivities is the chief reason for the present envi- and (3) technology, (T). The IPAT equation ronmental crisis” (1972a, 349). and related formulas were born, along with the Commoner’s contemporaries, eminent envi- modern environmental movement, circa 1970. ronmentalists Paul Ehrlich and John Holdren, Although first used to quantify contributions to writing in the same set of conference proceed- unsustainability, the formulation has been rein- ings as Commoner, disagree that technology is terpreted to assess the most promising path to the dominant reason for environmental degrada- sustainability. This revisionism can be seen as tion, emphasizing the importance of population part of an underlying shift among many environ- size and growth, noting “if there are too many mentalists in their attitudes toward technology. people, even the most wisely managed technol- This article examines the conversion of the ogy will not keep the environment from being IPAT equation from a contest over which vari- overstressed” (Ehrlich and Holdren 1972a, 376). able was worst for the planet to an expression of Commoner, Ehrlich, and Holdren have been the profound importance of technological devel- extremely influential environmental thinkers opment in Earth’s environmental future. for a generation. Following their work came an unprecedented barrage of regulatory activity in A Historical Perspective the United States, initiated to alter human degradation of the environment. First, the National In many ways, the modern environmental Environmental Policy Act of 1970, followed by movement itself was a reaction to unbridled faith the Clean Air Act, the Clean Water Act, the in technology, especially over the last 50 years. Toxic Substances Control Act, the Resource On the one hand, contemporary researchers con- Conservation and Recovery Act, and more than verge around World War II as the starting point of a dozen other less-well-known statutes, focused a generation of unprecedented technological de- America’s attention on a range of threats to hu- velopment springing from governmental policies man and ecosystem health. Interestingly, the supporting scientific advance (Brooks 1987; Free- regulation of the 1970s relied to a large extent man 1982; Smith 1990; Spiegel-Rosing and Price on technology and engineered solutions to con-

14 Journal of Industrial Ecology FORUM ❙

trol and manage end-of-pipe pollution.1 The so- teractions of population, economic growth, and lution to dirty technology was, through the force technological development. of law, to use more technology to clean it up. The IPAT identity has led, in turn, to the In the pre–Earth Day period, when Americans master equation in industrial ecology (Heaton et were coming to grips with the unintended conse- al. 1991; Graedel and Allenby 1995). These have quences of rapid technological development, been followed by two concepts in sustainability blame tended to be placed on what Commoner research: the Factor 10 Club (1994) and Factor referred to as “ecologically faulty technology.” Four (von Weizsäcker et al. 1997). The first two With twenty years more data to examine, references, IPAT and the master equation, state Ausubel sees Earth Day 1970 to be a possible in- relationships about technology and environmen- flection point in a number of trends, including tal impact, whereas the use of Factor Four, Factor population, and finds the message from history to 10, and even the Factor X debate described later, be “that technology, wisely used, can spare the are attempts to quantify potential impacts. earth” (Ausubel 1996a, 177). In reviewing the literature, an interesting his- This move toward technological optimism, as tory emerges. The original formulation presented discussed later in this article, has gained ground by Ehrlich and Holdren (1971, 1972a,b) was in- in environmental circles. Although there are tended to refute the notion that population was notable extremes in any distribution, and serious a minor contributor to the environmental crisis.2 limits to what scholars and practitioners often Rather, it makes population—which the authors refer to as “technological fixes,” a better under- call “the most unyielding of all environmental standing exists of how technology, combined pressures”—central to the equation by express- with improved design, can greatly aid the quest ing the impact of a society on the ecosystem as: for sustainability. Indeed, the notion that tech- I = P × F (2) nological choice is crucial for environmental improvement lies at the core of industrial ecology. where I = total impact, P = population size, and F Such thinking revises the informal mantra of is impact per capita. As the authors explain, im- U.S. technology policy from simply “cheaper, pact increases as either P or F increases, or if one faster” in the flow of innovations to the market- increases faster than the other declines. Both vari- place to, at a minimum, “cheaper, faster, cleaner” ables have been growing rapidly and are much in- and, to some visionaries, it offers the transforma- tertwined. To show that the equation is nonlinear tive mechanism for achieving sustainability. and the variables interdependent, Ehrlich and Holdren then expanded their equation as follows:3 Origins of the IPAT Equation I = P(I, F) × F (3) The relationship between technological in- This variant shows that F is also dependent novation and environmental impact has been on P, and P depends on I and F as well. For ex- conceptualized mathematically, as noted above, ample, rapid population growth can inhibit the by the IPAT equation. IPAT is an identity simply growth of income and consumption, particularly stating that environmental impact (I) is the in developing countries. On the other hand, product of population (P), affluence (A), and cornucopians such as Julian Simon maintain technology (T). that greater population is the key to prosperity (Simon 1980). Ehrlich and Holdren comment I = PAT (1) extensively on the tangled relationship of these Generally credited to Ehrlich, it embodies factors and note that almost no factor has been simplicity in the face of a multitude of more thoroughly studied (1972a). complex models, and has been chosen by many Technology, at this stage, is not expressed as a scholars (Commoner et al. 1971a; Dietz and separate variable, but is discussed in relationship Rosa 1994, 1997, 1998; Turner 1996; Wernick et to F, per capita impact. First, F is related to per al. 1997) as a starting point for investigating in- capita consumption—of, for example, food, en-

Chertow, The IPAT Equation and Its Variants 15 ❙ FORUM ergy, fibers, and metals. Then, it is related to the Thus, for Commoner, environmental impact technology used to make the consumption pos- is simply the amount of pollutant released rather sible and whether that technology creates more than broader measures of impact; for example, or less impact. The authors note that “improve- the amount of damage such pollution created or ments in technology can sometimes hold the per the amount of resource depletion the pollution capita impact, F, constant or even decrease it, caused.4 His task, then, is to estimate the contri- despite increases in per capita consumption” bution of each of the three terms to total envi- (1972a, 372). Although this statement recognizes ronmental impact. the positive role technology can play, Ehrlich and Much to the consternation of Ehrlich and Holdren generally conclude that technology can Holdren, Commoner’s effort to measure impact delay certain trends but cannot avert them. as amount of pollution released leads to the con- Commoner also plays an important role in clusion that technology is the culprit in almost the formulation of the IPAT equation. every specific case he examines. Commoner goes Commoner’s work in his popular 1971 book The on to compare the relative contributions of the Closing Circle, and much of his scientific analy- three IPAT variables arithmetically: Population, sis during the period of 1970–1972, were con- affluence (Economic good/Population), and tech- cerned with measuring the amount of pollution nology (Pollutant/Economic good), in examples resulting from economic growth in the United such as detergent phosphate, fertilizer nitrogen, States during the postwar period. To do so, he synthetic pesticides, tetraethyl lead, nitrogen and his colleagues became the first to apply the oxides, and beer bottles. He concludes that the IPAT concept with mathematical rigor. In order contribution of population and affluence to to operationalize the three factors that influence present-day pollution levels is much smaller I, environmental impact, Commoner further than that of the technology of production. He defines I as “the amount of a given pollutant in- calls for a new period of technology transforma- troduced annually into the environment.” His tion to undo the trends since 1946 in order “to equation, published in a 1972 conference pro- bring the nation’s productive technology much ceedings (Commoner 1972a), is: more closely into harmony with the inescapable demands of the ecosystem” (1972a, 363). Economic good Pollutant I=× Population × (4) Following the publication of The Closing Circle Population Economic good (Commoner et al. 1971a), full-scale academic war Population is used to express the size of the erupted between Ehrlich and Holdren on the one U.S. population in a given year or the change in hand and Commoner on the other. In a fierce Cri- population over a defined period. Economic good tique and Response published in the Bulletin of the is used to express the amount of a particular good Atomic Scientists in March 1972, Ehrlich and produced or consumed during a given year or the Holdren reject the mathematical basis through change over a defined period and is referred to as which Commoner finds population a modest con- “affluence.” Pollutant refers to the amount of a tributor to environmental impact. In their piece, specific pollutant released and is thus a measure “One-Dimensional Ecology,” Ehrlich and Holdren of “the environmental impact (i.e., amount of charge that Commoner, in his zeal to blame faulty pollutant) generated per unit of production (or technology, overemphasizes pollution, miscon- consumption), which reflects the nature of the ceives certain aspects of demography, understates productive technology” (Commoner 1972a, the growth of “affluence,” and resorts to “biased se- 346). Used in this way, the equation takes on the lection of data . . . and bad ecology.” Their evident characteristics of a mathematical identity. On fear comes from “the possibility that uncritical ac- the right-hand side of the equation, the two ceptance of Commoner’s assertions will lead to Populations cancel out, the two Economic goods public complacency concerning both population cancel out, and what remains is: I = Pollutant. and affluence in the United States” (1972b, 16). Commoner (1972b), in his Response pub- Economic good Pollutant lished in the same issue of the Bulletin, offers a I=× Population × Population Economic good spirited defense of his mathematical and eco-

16 Journal of Industrial Ecology FORUM ❙

logical competence and evokes supporting re- measure I, environmental impact or P, Pollu- views by Sir Eric Ashby and Rene Dubos to the tion. The very first time the reference I = P × A “intemperate onslaught” by Ehrlich and × T appears in writing is as part of the Critique Holdren. Commoner reminds the reader that it and Response in 1972, in which Ehrlich and was Ehrlich who first wrote: “Pollution can be Holdren take Commoner’s equation from a foot- said to be the result of multiplying three factors; note from The Closing Circle: population size, per capita consumption, and an pollution = (population) × (production/capita) ‘environmental impact’ index that measures, in × (pollution emission/production) (6) part, how wisely we apply the technology that goes with consumption” (Commoner 1972b, and, say, “for compactness, let us rewrite this 42). Commoner originally quoted this in an equation”: April 1971 article in Environment written with I = P × A × T (7) colleagues Michael Corr and Paul Stamler. Commoner’s interest in the Environment piece From this basis Ehrlich and Holdren dissect and in a follow-up piece for a symposium held by Commoner’s mathematics. They point out that the U.S. think tank Resources for the Future was Commoner uses his definitions of P, A , and T as to try to operationalize the relationship “first independent variables such that their effects on I proposed by Ehrlich and Holdren, that is, to find are independent of each other. Ehrlich and a way of entering the actual values for the sev- Holdren are careful to describe them as interde- eral factors and thus computing, numerically, pendent as in formula (3) above. Ehrlich and their relative importance” (Commoner 1972b, Holdren also emphasize the multiplicative nature 42). Commoner’s team paraphrased this rela- of population. In a 1974 piece, they identify how tionship, introducing the terms “consumption” population acts as a multiplier of consumption and “production” into the variant as follows as: and environmental damages associated with human activity such that even if no one of the IPAT Population size × per capita consumption terms goes up very much, it is the simultaneous × environmental impact per increases in all the factors that cause extensive unit of production = level of pollution (5) environmental impact. They make the case that At this stage Commoner brought to light a using them as independent variables tends to un- letter Ehrlich and Holdren sent to colleagues in derestimate the impact of population.5 which they reveal that they had urged Com- In another article, Holdren and Ehrlich moner not to engage in debate about which of (1974) offer yet another formulation of what they the factors was most important because it would term the “population/environment equation”: be counterproductive to achieving environmen- environmental disruption = population tal goals. Commoner takes great umbrage at the × consumption/person idea of avoiding public discussion of scientific × damage/unit of production (8) findings in favor of private agreements that, in turn, erode democracy and “the survival of a First, environmental disruption substitutes for I, civilized society” (1972b, 56). Commoner iden- impact, and damage, in the third term, is a pre- tifies what he believes to be behind the debate: sumed outcome of each unit of consumption. that “Ehrlich is so intent upon population con- Note, also, the cross-substitution among the trol as to be unwilling to tolerate open discus- many formulations of consumption and produc- sion of data that might weaken the argument for tion. Sometimes production and consumption it” (1972b, 55). cancel each other out, and sometimes they do Some of the issues raised between the two not. Sometimes the affluence term is used for sides of this debate reflect the formative nature one or the other or for both as in the form “pro- of the ideas. Note that across the several differ- duction/consumption” or “production (con- ent articles and books mentioned, there is not sumption).” In Commoner’s original paraphrase one single way of stating the variables or even of Ehrlich (equation (5)), in which he makes consistency as to whether we are attempting to consumption part of affluence and production

Chertow, The IPAT Equation and Its Variants 17 ❙ FORUM part of technology and the terms do not cancel ample, although the consumption of non- out, he goes on to make the now outdated state- returnable beer bottles went up almost 600 per- ment that “since imports, exports, and storage cent in the period 1950–1967, actual consump- are relatively slight effects, total consumption tion of beer per capita increased a mere five can be taken to be approximately equal to total percent. Thus, the affluence gain to the beer production” (Commoner et al. 1971b, 4). Today, drinker has been slight, whereas the technology imports and exports are not slight effects, so we chosen to package and deliver the beer, which is would not even try to equate U.S. production of no use to the consumer, has changed dramati- with U.S. consumption. Even so, the relation- cally at the expense of the environment. ship of production to consumption is not a focus Despite some differences in orientation be- of these early articles. In fact, only in the late tween Commoner and Ehrlich and Holdren, the 1990s did an analytic discussion of consumption chicken-and-egg nature of this debate—whether begin to take shape in the scientific community population or technology is a bigger contributor (Stern et al. 1997; Berkhout 1998). to environmental damage—is revealing. Does an increased population call for improved technology, or does improved technology increase Apples and Oranges carrying capacity?6 (Boserup 1981; Kates 1997). In comparing Commoner with Ehrlich and Even our latter-day technology optimist, Jesse Holdren, there are often differences in time and Ausubel, is stymied by the technology/popula- spatial scale. Commoner tends to write about tion link (1996a, 1996b). Just as he demon- the present deteriorating environmental condi- strates a revolution in factor productivity in tions in the United States, and sometimes fo- energy, land, materials, and water, especially cuses more locally on a given pollutant. He gives since the time of the Commoner/Ehrlich debate, light attention to resource use with his heavy he goes on to describe how the new technologies focus on pollution. Ehrlich and Holdren have a serve to make “the human niche elastic. If we broader sweep and are less specific in space and solve problems, our population grows and cre- time. They write in several of their articles about ates further, eventually insurmountable prob- the underestimated role of diminishing returns, lems” (1996a, 167). Would technology that is threshold effects, and synergisms, as well as the not, in Commoner’s phrase, “ecologically relation between ecosystem complexity and sta- faulty,” (1972a, 362), but rather ecologically bility. They suggest that direct effects of envi- wise, merely delay the inevitability of environ- ronmental damage such as lead poisoning and mental destruction, or is better technology our air pollution are likely to be less threatening, ul- best horse in the race toward sustainability? timately, than the indirect effects on human In a review of several models of anthropo- welfare from interference with ecosystem struc- genic driving forces of environmental impact, ture and function. They note numerous ex- Deitz and Rosa single out the IPAT equation be- amples of the “public services” of nature cause it “is easily understood, frequently used for (Holdren and Ehrlich 1974), today better illustrative purposes and can discipline our known as ecosystem services (Daily 1997). thinking” (Deitz and Rosa 1997). They draw Commoner and Ehrlich and Holdren even this conclusion despite their finding that the ef- differ on a common meaning of affluence. fects of population and economic growth on en- Ehrlich and Commoner generally consider a per vironmental degradation have not been capita output measure. Commoner does not see extensively researched and are thus uncertain. “true affluence” as the culturally prescribed ac- B. L. Turner (1996) finds the IPAT equation cumulation of television sets and fancy cars, useful as a macro-scale assessment, noting that re- more akin to consumption. Rather, he attempts gional and local scale assessments generally high- to differentiate the technology used to deliver light other drivers of environmental changes such goods from the actual contribution of those as policy, institutions, and complexity of social goods to human welfare as a means of separating factors. Meyer and Turner point out that the IPAT consumption from “true affluence.” For ex- equation is largely the product of biologists and

18 Journal of Industrial Ecology FORUM ❙

ecologists and uses terms undefined in social sci- clear evidence that the IPAT progenitors were ence. They comment that neither A (affluence) exposed to growth accounting in economics, we nor T (technology) is associated with a substan- see that poor definition of a residual term is not tial body of social science theory (Meyer and unique to the environmental field. Turner 1992). Another critique leveled by Deitz and Rosa is Conceptually as well as numerically, P, popu- that although the IPAT equation does allow for lation, and A, defined as a per capita measure of some disaggregation of the forces of environ- wealth, consumption, or production, have gen- mental change, including human impacts, the erally been more accessible to researchers than disaggregation is too simple and does not allow the T term. The product (P × A) represents an for interactions among population, affluence, aggregate measure of total economic activity, and technology (Deitz and Rosa 1998). As it such as total GDP. In this case, T = I/P × A or T stands, a direct relationship is formed such that = unit of environmental impact per unit of eco- a 10 percent increase in P, A , or T creates a 10 nomic activity. Here, T, technology, becomes percent increase in environmental impact. Deitz the residual of an accounting identity; it repre- and Rosa cite several studies that build upon the sents everything that affects the environment IPAT model to enable it to capture more com- that is not population or affluence. As Deitz and plexity and interaction among the variables. Rosa observe, “most social scientists are frus- They actually reformulate the IPAT equation as trated by the truncated visions of the rest of the STIRPAT—meaning “Stochastic Impacts by world offered by the T in the IPAT model” Regression on Population, Affluence and Tech- (1994, 287). In this sense, whether technology, nology”—to be able to disaggregate P, A, and T through the T term, is truly endogenous to the and to be able to use regression methods to esti- master equation could be questioned (Grubler mate and test hypotheses. Their reformulation is 2001). Thus, Deitz and Rosa recommend that an I = aPbAcTde, where the variables a–d can be ei- independent measure of T be used and that re- ther parameters or more complex functions esti- searchers be required to specify T rather than mated using standard statistical procedures and e solve for it. is the error term (Deitz and Rosa 1994, 1997, The notion of T as residual is a reminder of 1998). They have used the STIRPAT model to an important antecedent to the IPAT quest to bridge the social and biological sciences in, for disaggregate the elements of environmental im- example, studies of global climate change. pact. Macroeconomists have been involved The IPAT equation has also been a source for since the 1930s with efforts to disaggregate fac- the development of the literature on energy de- tors of economic growth and productivity in the composition analysis, which disaggregated en- economy (Griliches 1996). Robert Solow’s ergy intensity and extended and refined the Nobel Prize in economics, for example, was asso- mathematics of IPAT (Greening et al. 1998, ciated with his statistical explanation of the 1999; Gurer and Ban 1997). The use of the IPAT causes of U.S. manufacturing growth from equation in research related to climate change, 1909–1949 (Clark 1985). He found that labor specifically energy-related carbon emission stud- and capital, the traditional factor inputs of neo- ies, may be the most enduring legacy of IPAT. classical economics, explained only about 10 Such formulations are typically stated as percent of this growth. The rest, some 90 per- (Holdren 2000): Energy use = Population × cent, was “residual”—a factor representing all GDP/person × energy/GDP; and Carbon emis- other contributions to growth such as education, sions = Population × GDP/person × carbon en- management, and technological innovation. ergy. In this way IPAT has played a prominent Later research identified about 30–40 percent of role, particularly in the Intergovernmental Panel the explanation for income growth to be attrib- on Climate Change assessments (IPCC 1996). utable to technological change, with the poorly Ultimately, the evidence presented suggests understood interdependence of the variables be- that the IPAT equation can be used to support ing the most difficult challenge (Stoneman many different points of view. Ehrlich and 1987; Abramovitz 1993). Although there is no Holdren show how it supports the population

Chertow, The IPAT Equation and Its Variants 19 ❙ FORUM view. Commoner demonstrates how it supports technology-friendly, we might have avoided the the harmful technology or Faustian view. Econo- flowery “greenhouse effect” in favor of “the prob- mist Julian Simon, representative of the lem of global dumpsites” or “the celestial emphy- cornucopian view, believes that increasing popula- sema syndrome” (Marx 1994, note 3). The root tion and wealth is the driving force for new tech- cause of each of these problems, however, does nological development (Simon 1981). That the not come from nature, but rather the human be- IPAT formulation can be interpreted in so many havior and the complex practices that go along ways represents a weakness and a strength: on the with it (Marx 1994). We lean to technologists in one hand, it may simply be too broad and general part because of an “inadequate understanding of to account for the interrelationships among the the part played by ideological, moral, religious, variables. On the other hand, it has not revealed and aesthetic factors in shaping response to envi- a bias and need not be definitive to be extremely ronmental degradation” (1994, 18). useful as a thought model. There really has been no underlying disagreement that each of the terms The Transition to Technological belongs to the equation in some way and so, as a Optimism conceptual analytic approach, IPAT provides readily identifiable common ground. If the approach of the environmental move- The technology factor is also subject to mul- ment of the 1970s was to juxtapose the gains of tiple interpretations. Ehrlich and Holdren cite a economic growth with the devastating reality of fundamental problem associated with reliance on pollution, this approach changed in the 1980s. technology by recognizing that no technology can The Brundtland Commission report in 1987 completely eliminate the environmental impact concluded that if humanity were to have a posi- of consumption. They choose the example of re- tive future, then economy and environment had cycling, which always results in some loss of ma- to be made more compatible. “Sustainable” was terials, if for no other reason than the “sad but paired with “development” to describe this state, unavoidable consequences of the second law of and since that time there has been increasing thermodynamics” (1972a, 370). Commoner is the acceptance that economy and environment can harshest on “ecologically faulty technology” and be mutually compatible (World Commission on its singular environmental impact, but, on the Environment and Development 1987). other hand, is quite receptive to “developing new At the same time, the IPAT equation makes us technologies which incorporate not only the keenly aware of our limited choices. The year fol- knowledge of the physical sciences (as most do lowing the Brundtland report, one environmental moderately well), but ecological wisdom as well” chieftain, under the heading “A Luddite Re- (1972a). Because IPAT factors take us well be- cants,” conceded that “economic growth has its yond technological choice, we quickly cross from imperatives; it will occur . . . seen this way, recon- the realm of technologists, especially scientists ciling the economic and environmental goals so- and engineers, into the realm of social scientists, cieties have set for themselves will occur only if politicians, and the needs, wants, and desires of there is a transformation in technology—a shift, people. The ubiquity of C. P. Snow’s “two cul- unprecedented in scope and pace, to technolo- tures” problem is especially evident in the IPAT gies, high and low, soft and hard, that facilitate realm because organizing the relationships of economic growth while sharply reducing the pres- population, environmental impact, and social sures on the natural environment” (Speth 1988). welfare puts analysts at the juncture of the scien- Here, James Gustave Speth, then president of the tific and humanistic realms. As cultural historian World Resources Institute, converts the 1970s Leo Marx points out, we often name environmen- suspicion, as expressed by Commoner’s condem- tal problems after their biophysical symptoms, nation of “ecologically faulty technology,” into an such as soil erosion or acid rain, and thus address expression of hope for transformed technology them to scientists. Naming practices reflect the (see also Speth 1990, 1991). dominant outlooks of the culture. For example, This line of argument is presented in a publi- Marx points out that if we, as a society, were less cation of the World Resources Institute called

20 Journal of Industrial Ecology FORUM ❙

“Transforming Technology: An Agenda for En- cast the T term in a very positive light. In essence, vironmentally Sustainable Growth in the 21st a new generation of technological optimists finds Century” (Heaton, Repetto, and Sobin 1991). that experiments in changing human behavior to Heaton and colleagues recite a variant of the vary the course of P and A are highly uncertain. IPAT equation resurrected by Speth in a back- Stated another way, Walter Lynn, while Dean of ground paper a year earlier (Speth 1990) and the Cornell University faculty, cited the lack of explain its critical importance to the future of progress in “social engineering,” and the success, the environment in a section of their article even if temporary, of technological fixes. He ob- titled “Why Technological Change Is Key.” served: “Currently, technology provides the only They write: viable means by which our complex interdependent society is able to address these environmen- Human impact on the natural environ- tal problems” (Lynn 1989, 186). ment depends fundamentally on an interaction among population, economic growth, and technology. A simple iden- Enter Industrial Ecology tity encapsulates the relationship: The concepts of the IPAT equation are at the Pollution GNP core of the emerging field of industrial ecology. Pollution =× ×Population (9) GNP Population Industrial ecology has been described as the “marriage of technology and ecology” and exam- Here, pollution (environmental degrada- ines, on the one hand, the environmental im- tion generally) emerges as the product of pacts of the technological society, and, on the population, income levels (the GNP per other hand, the means by which technology can capita term) and the pollution intensity be effectively channeled toward environmental of production (the pollution/GNP term). benefit. According to the first textbook in this In principle, pollution can be con- new field (Graedel and Allenby 1995), indus- trolled by lowering any (or all) of these trial ecology has adopted the following IPAT three factors. In fact, however, heroic ef- variant as its “master equation”: forts will be required to stabilize global Environmental impact=× Population population at double today’s level, and raising income and living standards is a GDP × Environmental impact (10) near-universal quest. Indeed, economic person unit of per capita GDP growth is a basic goal for at least 80 per- This master equation incorporates the same cent of the world’s population. These relationships as the WRI equation, with some powerful forces give economic expansion changes in terminology. Once again we see the forward momentum. In this field of forces, variation between defining pollution, P, as WRI the pollution intensity of production has done, in contrast to defining environmental looks to be the variable easiest to manipu- impact, as in the master equation. WRI states late, which puts the burden of change the equation as an identity—the populations largely on technology. In fact, broadly de- cancel and the GNPs cancel—so that Pollution = fined to include both changes within eco- Pollution. The master equation is not strictly nomic sectors and shifts among them, stated as an identity. Also, Graedel and Allenby technological change is essential just to use gross domestic product (GDP) for the afflu- halt backsliding: Even today’s unaccept- ence term rather than WRI’s gross national prod- able levels of pollution will rise unless the uct (GNP), which reflects a shift by the United percentage of annual growth in global and States in 1991 to the use of GDP in order to con- economic output is matched by an annual form to the practices of most other countries. decline in pollution intensity (Heaton, Although GNP is defined as the total final out- Repetto, and Sobin 1991, 1). put produced by a country using inputs owned by Thus, whereas the IPAT equation can send the residents of that country, GDP counts the us in several directions, recent interpretations output produced with labor and capital located

Chertow, The IPAT Equation and Its Variants 21 ❙ FORUM inside the given country, whoever owns the capi- rich and poor in many countries. GDP has been tal (Samuelson and Nordhaus 1998). criticized as distortionary for measuring only WRI’s technology term is a measure of the quantifiable transactions, leaving out harder-to- pollution intensity of production (the pollution/ measure, but critical, assets such as an educated GNP term). Graedel and Allenby define their populace, healthy citizens, and a clean environ- third term, qualitatively, as the degree of envi- ment (Cobb, Halstead, and Rowe 1995).7 None- ronmental impact per unit of per capita gross do- theless, to the extent that, as a blunt measure, real mestic product, which they call “an expression of GDP per person rises, it implies that wealth and, the degree to which technology is available to subsequently, quality of life are more likely to be permit development without serious environ- improving. Therefore, as in the I = PAT equation, mental consequences and the degree to which both of the first terms presented here are headed that available technology is deployed” (Graedel upward, although estimates of how much vary and Allenby 1995, 7). Although it is interesting widely.8 In the emerging “industrial ecology” view, to observe the back and forth of the use of I, im- using technology to reduce environmental impact pact versus P, pollution in the history of the IPAT can, theoretically, not only compensate for the equation, it is not surprising that such a macro impact of more people, but also the impact of more view makes it difficult to capture true differences affluent people. Increasing wealth without “back- in types and dynamics of specific impacts that are sliding” as described by WRI, or even while de- difficult, if not impossible, to aggregate. creasing overall impact, is a worthy, if challenging Characteristic of each T term is the assump- goal. According to Graedel and Allenby: tion that the pollution it defines can be reduced. Curiously, this usage leaves room for being less The third term, the amount of environ- bad, for example, through pollution reduction or mental impact per unit of output, is pri- eco-efficiency, but does not really express the marily a technological term, though potential human and environmental benefit that societal and economic issues provide can come from technology (McDonough and strong constraints to changing it rapidly Braungart 1998). All in all, WRI’s formulation and dramatically. It is this third term in and that of the master equation are similar along the equation that offers the greatest hope the lines we have seen since the Commoner/ for a transition to sustainable develop- Ehrlich and Holdren debate, but give further ment, and it is modifying this term that definition to A and T. A sense of progress exists is the central tenet of industrial ecology in that Ehrlich and Holdren as well as Com- (Graedel and Allenby 1995, 8). moner, although using a multiterm equation, were really most interested in pursuing a single Until recently, the third term, technology, has cause. Still, the more recent emphasis on pollu- been seen as a continuous source of pollution: tion per unit GNP or GDP is not a satisfactory technology, for example, to mine, manufacture, universal definition of technology, leaving room and drive with all the environmental harms such for continual reconsideration. activities create. Other differences are concealed Let us examine the three terms of the master by the macro nature of the IPAT equation and its equation and IPAT more broadly. In practice, at variants. In the master equation, the T term is de- the global level, there is strong upward pressure fined as the amount of environmental impact each on the first term, population, even as we debate unit of a country’s wealth creates averaged over the range of those increases (Marchetti et al. the population as a whole. In reality, this is an 1996). Similarly, the common desire to improve oversimplification. Countries with clean, energy- quality of life translates into an increasing second efficient production have been able to produce term, affluence, as well. Affluence, as measured by greater wealth with less per-unit environmental gross domestic product (GDP) per person, spreads impact. The poorest countries are least likely to the worth of a country’s economy over the popu- have clean air, water, and sanitary systems. But lation. Clearly, this, too, is a generalization. Per even here anomalies exist, summarized in discus- capita figures miss growing disparities between sions of the environmental Kuznets curve, which

22 Journal of Industrial Ecology FORUM ❙

considers a nuanced relationship between A, after or sulfur dioxide, as shown in figure 1, follow fluence, and I, impact, such that an environmen- the environmental Kuznets curve hypothesis, tal emission might rise as income increases until a worsening at first and then improving as income particular level is reached, at which point emis- increases (Hosier 1996). Many researchers have sion levels begin to fall (Arrow et al. 1995). been interested in the relationship of affluence As indicated below, no universal rule exists: and environmental impact and have determined Impacts such as waste and carbon dioxide emis- that the condition of impact worsening and then sions increase with wealth, whereas other indica- improving with income is most typical of short- tors, such as urban concentrations of particulate term, local indicators such as sulfur, particulates, matter and sulfur dioxide, decline over specified and fecal coliforms, but not to accumulated income levels (see figure 1). Some indicators, stocks of waste or long-term dispersed indicators such as urban concentrations of particulate mat- such as CO2 (Rothman and de Bruyn 1998).

Figure 1 Relationship of affluence (per capita income) to various environmental impacts. Sources: Hosier (1996) from Shafik and Bandyopadhyay, background paper; World Bank data.

Chertow, The IPAT Equation and Its Variants 23 ❙ FORUM

Earlier discussions of T, and IPAT in general 1987). Therefore, using these equations predic- from Ehrlich onward portray a curiously passive tively would enhance the theoretical basis of role for the technology producer. Either technol- IPAT ideas, for example, determining how much ogy is used abstractly, as in Ehrlich and Holdren’s technology versus how much increase in popula- writing, or it is assumed to be static in that the tion and affluence would be reasonable goals for producers just keep on doing what they have countries and for the global commons. Recently, done before. Interventions, then, are policies two agendas have been established that set targets from above rather than revisions of the mind- for technology and environmental impact. sets of the technologists. In contrast, industrial F. Schmidt-Bleek, while with the Wuppertal ecology has deep roots in engineering and the Institute, presented the Carnoules Declaration physical sciences, so it is not surprising that its of the Factor 10 Club in 1994. The Factor 10 practitioners would put stock in the T term, with Club has focused on the need to substantially which they are most familiar professionally. But reduce global material flows. Its advocates be- neither is it easy to assure that any given tech- lieve that the current productivity of resources nology, let alone T, technology collectively, will used must be increased by an average of a factor be beneficial rather than harmful. By offering a of ten during the next 30 to 50 years. “This is systems approach to environmental/technical feasible if we mobilize know-how to generate interactions, industrial ecology research can pro- new products, services, as well as new methods vide an essential link between the episodic use of of manufacturing” (Factor 10 Club 1994, 8). We promising technology and the long-term, less see here reliance on “know-how” and “methods defined goal of sustainable development. of manufacturing” that again emphasizes the T Another critical component industrial ecolo- term of the IPAT equation, in this case to create gists have brought into the discussion of anthropo- a specific sustainability target. genic environmental impacts is the participation Weizsäcker, Lovins, and Lovins (1997) state of private industry. In describing the “industrial” in their book, Factor Four: Doubling Wealth, character of industrial ecology, the first issue of the Halving Resource Use, that the amount of wealth Journal of Industrial Ecology notes that “it views extracted from one unit of natural resources can corporate entities as key players in the protection quadruple. Their goal that we can “live twice as of the environment, particularly where techno- well—yet use half as much” might be expressed logical innovation is an avenue for environmental in IPAT terms as achieving 2(A) with only improvement. As an important repository of tech- .5(T). They define technological progress nei- nological expertise in our society, industrial orga- ther as a reduction in pollution nor as a gain in nizations can provide crucial leverage in attacking labor productivity, but, overall, as a gain in pro- environmental problems by incorporating envi- ductivity of resources. Up until now, tremendous ronmental considerations into product and pro- gains in productivity have come from substitut- cess design” (Lifset 1997, 1). ing resources for human labor. They are con- cerned that such substitution has gone too far and been inconsiderate of overusing resources The Factor X such as energy, materials, water, soil, and air. Neither in the IPAT equation, nor in Their research is devoted to an “efficiency revo- Commoner’s quantitative work, nor in the master lution” that shows the potential for fourfold equation in industrial ecology is there an attempt gains in resource use. to quantify the relationship of technology and Factor Four and the Factor 10 are specific, if environmental impact in a prospective way, al- ambitious, expressions of the potential impact of though this has entered some of the global cli- T, the technology term. They are also future- mate change work cited earlier. Still, a threshold oriented, setting a goal for corporate and policy for sustainability is that it “meets the needs of the direction. The four- to tenfold increase in aggre- present without compromising the ability of fu- gate economic impact, PA, can also be thought of ture generations to meet their own needs” (World as a twofold increase in P, population, over the Commission on Environment and Development next 50 years, and a two- to fivefold increase in A,

24 Journal of Industrial Ecology FORUM ❙

affluence. Picking up on Factor Four and the Fac- of the environmental movement, much has tor 10, Lucas Reijnders of the University of changed, in large part because of the alarm Amsterdam writes of “The Factor X Debate” sounded in the post–Silent Spring era by Ehrlich (Reijnders 1998), in which researchers have gone and Holdren, Commoner, and many other even further than a factor of four or ten to propose thoughtful researchers and policy makers. Still, long-term reductions in resource use as high as 50 much of the change was motivated by pessi- times. This would occur through dematerializa- mism, captured in this statement of Holdren and tion, eco-efficiency, or increased natural resource Ehrlich at the end of their 1974 article: productivity relying mainly on the T of the IPAT Ecological disaster will be difficult equation. Of course, until environmental impact enough to avoid even if population limi- is defined with great specificity, the choice of dif- tation succeeds: if population growth ferent X factors, including the baseline of Factor proceeds unabated, the gains of improved Four and the Factor 10, is arbitrary. Reijnders technology and stabilized per capita con- notes that whereas the debate in the United sumption will be erased, and averting di- States is still largely within the NGO community, saster will be impossible (Holdren and the concept of Factor X and the importance of Ehrlich 1974, 291). quantifying objectives has influenced policy in several European nations, including Austria, Ger- Indeed, these are still controversial issues. En- many, and the Netherlands. vironmentalists of the 1970s who were pessimis- Do Factor X policies put the entire onus for tic continued to sound alarms in the 1990s environmental improvement on the technology (Ehrlich and Ehrlich 1990; Commoner 1992; variable? Some would tag WRI and the indus- Meadows et al. 1992). Deep ecologists will not trial ecologists as implying that nothing can be wake to find themselves warm to technological done about population and increasing wealth. optimism. But a great deal more consciousness This is too pessimistic a reading. The effective- about environmental issues exists internationally, ness of population programs has been demon- especially among global institutions such as the strated in many parts of the world, for example, United Nations, the World Bank, and other fi- in China, Japan, and Thailand (Miller 1994). As nancial players (Schmidheiny and Zorraguin a matter of basic fairness, few would want to 1996). Indeed, the notion that environmental deny the improvement in the standard of living problems can be addressed and even advanced for the world’s peoples implied in the affluence through technical and procedural innovation has term. Neither can the three terms be considered achieved its own name in the European environ- in isolation. Rather, interactive effects exist, as mental sociology literature—“ecological modern- demonstrated by Ehrlich and Holdren. Still, as ization” (Hajer 1996; Mol and Sonnenfeld 2000). Reijnders concludes, “although there is no agree- In the United States, environmental policy, for ment on the relative importance of technology all its warts, has made an enormous contribution in achieving a Factor X for economies as a at the end of the pipe, and is slowly migrating to- whole, one still may note that the Factor X de- ward more integrative policy. Similarly, in corpo- bate is characterized by a remarkable technologi- rate environmental policy, researchers can now cal optimism. This is especially so if one views measure the early stages of a change in emphasis this debate against the background of a widely from regulatory compliance toward overall pro- held supposition that environmentalism has an cess efficiency (Florida 1996), even at the ex- antitechnological bias” (Reijnders 1998, 18). pense of sales in the traditional end-of-pipe environmental industry (U.S. Department of Commerce 1998). The Call of the Optimist Upon reflection, I believe that Commoner Just as all ecological problems are contextual, (1972a) anticipates the work defined by Ausubel, so too are the issues confronted by IPAT, which WRI, and industrial ecologists. He calls for a may shed light on why it has multiple interpre- “new period of technological transformation of tations. Since it was introduced in the early days the [U.S.] economy, which reverses the counter-

Chertow, The IPAT Equation and Its Variants 25 ❙ FORUM ecological trends developed since 1946”—a 2. A precursor to the IPAT formulation by sociolo- transformation that reconnects people and their gist Dudley Duncan in 1964 was the POET ecosystems: model (population, organization, environment, technology). According to Deitz and Rosa Consider the following simple transforma- (1994), the model showed that each of these tion of the present, ecologically faulty, re- components are interconnected but did not lationship among soil, agricultural crops, specify quantifiable relationships. the human population and sewage. Sup- 3. To trace the origins of these equations accurately pose that the sewage, instead of being in- is challenging. The IPAT ideas emerged in 1970 troduced into surface water as it is now, and 1971. Particularly relevant was an exchange whether directly or following treatment, is by Commoner and Ehrlich and Holdren in the Saturday Review during 1970 followed by a meet- instead transported from urban collection ing at the President’s Commission on Population systems by pipeline to agricultural areas, Growth and the American Future held on No- where—after appropriate sterilization pro- vember 17, 1970, the findings of which were not cedures—it is incorporated into the soil. published until 1972. However, Ehrlich and Such a pipeline would literally reincorpo- Holdren and Commoner produced numerous rate the urban population into the soil’s publications in the meantime, which helped ecological cycle, restoring the integrity of steer the IPAT debate in new directions. Ehrlich that cycle . . . Hence the urban population and Holdren used I = P(I,F) × F(P) in the 1972 is then no longer external to the soil cycle findings, but a slightly different version, I = P × . . . But note that this rate of zero environ- F(P) in their earlier article from Science in March of 1971, which is otherwise almost identical to mental impact is not achieved by a return the 1972 conference report. Both equations try to “primitive” conditions, but by an actual to express a similar point, that P and F are inter- technological advance. active and can increase faster than linearly. 4. In fact, one of the reviewers of this article sug- Conclusions gested that this use of the IPAT equation might This article underscores that technology, al- better be called “EPAT,” showing the emphasis on “E” for “Emissions” rather than the totality of though associated with both disease and cure for I for all impacts. environmental harms, is a critical factor in envi- 5. The authors use the example of the impact of lead ronmental improvement. Thus, important rea- emissions from automobiles from 1946 to 1967 and sons can be found to continue to develop find that population has increased 41 percent; con- frameworks such as industrial ecology, that focus sumption, measured as vehicle-miles per person, on cures. The overall shift from pessimism to op- has doubled; and lead emissions per vehicle-mile timism, captured here through changing inter- increased 83 percent. Hence, 1.41 × 2.0 × 1.83 = pretations of the IPAT equation and its variants, 5.16, or, subtracting 1.0, a 416 percent increase in is shown to be partly fatalistic, in that few alter- total impact. This illustrates that although no vari- natives exist to the imperative established by the able more than doubled, the cumulative impact is multiplicative. Had population not grown but been Brundtland Commission; partly pragmatic, in held constant, then total impact would only have that technological variables often seem easier to been 1.0 × 2.0 × 1.83, or 3.66, reflecting a 266 per- manage than human behavior; and partly a con- cent increase, illustrating the multiplier effect (see tinued act of faith, at least in the United States, Holdren and Ehrlich 1974, using corrections to the in the power of scientific advance. variables suggested by Commoner 1972b). 6. Editor’s note: For an application of decomposi- Notes tion analysis to materials flows, see Hoffrén, Luukkanen, and Kaivo-oja, “Decomposition 1. Indeed, the very prescriptive laws of this period Analysis of Finnish Material Flows: 1960–1996,” have been criticized by researchers for becoming Journal of Industrial Ecology, this issue. so specific in their standards as to preclude tech- 7. Further, these authors point out that the GDP nological innovation (NACEPT 1991; Heaton et measures socially and environmentally destruc- al. 1991; U.S. Department of Commerce 1998). tive behavior as an economic gain. Pollution, for

26 Journal of Industrial Ecology FORUM ❙

example, shows up as a double boost to the vironment, edited by R. G. Ridker. Washington DC: economy. The first boost comes when a pollut- U.S. Government Printing Office, pp. 339–363. ing company makes a profit on the product they Commoner, B. 1972b. A bulletin dialogue on “The are selling and the second boost comes when the Closing Circle”: Response. Bulletin of the Atomic company spends large amounts of money on en- Scientists 28(5): 17, 42–56. vironmental cleanup. Commoner, B. 1992. Making peace with the planet. 8. Graedel, however, has some revisionist thinking New York: New Press. about A, the affluence term (Graedel, 2000). Daily, G., ed. 1997. Nature’s services: Societal depen- Simply assigning it a financial measure such as dence on natural ecosystems. Washington, DC: Is- GNP or GDP per capita may overemphasize the land Press. contribution of the market, and, as a result, de- Dietz, T. and E. Rosa. 1994. Rethinking the environ- emphasize the opportunity for changing atti- mental impacts of population, affluence and tudes even as income rises. Graedel has technology. Human Ecology Review 1: 277–300. suggested that the essence of the A term resides Dietz, T. and E. Rosa. 1997. Environmental impacts of in its cultural and behavioral attributes, which population and consumption. In Environmentally he has called “the Madonna factor”—after the Significant Consumption: Research Directions, ed- pop singer’s well-known phrase “a material girl ited by P. Stern et al. Washington, DC: Commit- in a material world.” tee on the Human Dimensions of Global Change, National Research Council. References Dietz, T. and E. Rosa. 1998. Climate change and society: Speculation, construction and scientific in- Abramowitz, M. 1993. The search for the sources of vestigation. International Sociology 13(4): 421–455. growth: areas of ignorance, old and new. Journal Ehrlich, P. and J. Holdren. 1971. Impact of population of Economic History 53(2): 217–243. growth. Science 171: 1212–1217. Arrow, K. et. al. 1995. Economic growth, carrying capac- Ehrlich, P. and J. Holdren. 1972a. Impact of popula- ity and the environment. Science 268: 520–521. tion growth. In Population, Resources, and the En- Ausubel, J. H. 1996a. Can technology spare the vironment, edited by R.G. Riker. Washington DC: earth? American Scientist 84: 166–178. U.S. Government Printing Office. pp. 365–377. Ausubel, J. H. 1996b. The liberation of the environ- Ehrlich, P. and J. Holdren. 1972b. A bulletin dialogue ment. Daedalus 125: 1–17. on the ‘Closing Circle’: Critique: One dimen- Berkhout, F. 1998. Book review of Environmentally sional ecology. Bulletin of the Atomic Scientists Significant Consumption: Research Directions; 28(5): 16–27. Resource Flows: The Material Basis of Industrial Ehrlich, P. and A. Ehrlich. 1990. The population explo- Economies; Towards Sustainable Consumption; sion. New York: Simon and Schuster. Sustainable Consumption and Production. Jour- Factor 10 Club. 1994. Carnoules declaration. nal of Industrial Ecology 2(2): 119–125. Wuppertal, Germany: Wuppertal Institute for Boserup, E. 1981. Population and technological change. Climate, Environment and Energy. Chicago: The University of Chicago Press. Florida, R. 1996. Lean and green: The move to envi- Brooks, H. 1987. What is the national agenda for sci- ronmentally conscious manufacturing. California ence, and how did it come about? American Sci- Management Review 39(1): 80–105. entist 75: 511–517. Freeman, C. 1982. The economics of industrial innova- Clark, N. 1985. The political economy of science and tion. Cambridge, MA: MIT Press. technology. Oxford, Great Britain: Basil Graedel, T. and B. Allenby. 1995. Industrial ecology. Blackwell. Englewood Cliffs, NJ: Prentice Hall. Cobb, C. W., T. Halstead, and J. Rowe. 1995. If the Graedel, T. 2000. The evolution of industrial ecology. GDP is up, why is America down? Atlantic Environmental Science & Technology 34(1): 28–31. Monthly 276: 59–60. Greening, L., W. B. Davis, and L. Schipper. 1998. De- Commoner, B., M. Corr, and P. J. Stamler. 1971a. The composition of aggregate carbon intensity for closing circle: nature, man, and technology. New the manufacturing sector: Comparison of declin- York: Knopf. ing trends from 10 OECD countries for the pe- Commoner, B., M. Corr, and P. J. Stamler. 1971b. The riod 1971–1991. Energy Economics 20: 43–65. causes of pollution. Environment 13(3): 2–19. Griliches, Z. 1996. The discovery of the residual: a Commoner, B. 1972a. The environmental cost of eco- historical note. Journal of Economic Literature nomic growth. In Population, Resources and the En- 34(3): 1324–1330.

Chertow, The IPAT Equation and Its Variants 27 ❙ FORUM

Grubler, A. 2001. Personal communication. Meyer, W. B. and B. L. Turner II. 1992. Human popu- Gurer N. and J. Ban. 1997. Factors affecting energy lation growth and land-use/cover change. An-

related CO2 emissions: Past levels and present nual Review of Ecological Systems 23:39–61. trends. OPEC Review XXI(4): 309–350. Miller, G. T., Jr. 1994. Living in the environment prin- Hajer, M. 1996. Ecological modernisation as cultural ciples, connections, and solutions, eighth edition. politics. In Risk, environment & modernity: To- Belmont, California; Wadsworth Publishing wards a new ecology, edited by Scott Lash et al. Company. London: Sage Publications. Mol, A. and D. Sonnenfeld. 2000. Ecological mod- Heaton, G., R. Repetto, and R. Sobin. 1991. Trans- ernization around the world: an introduction. forming technology: An agenda for environmentally Environmental Politics 9(1): 3–16. sustainable growth in the 21st century. Washing- National Advisory Council for Environmental Policy ton, DC: World Resources Institute. and Technology (NACEPT), Report of the Holdren, J. and P. Ehrlich. 1974. Human population Technology Innovation and Economics Com- and the global environment. American Scientist mittee (TIE). 1991. “Permitting and compliance 62: 282–292. policy: Barriers to U.S. environmental technol- Holdren, J. 2000. Environmental degradation: popu- ogy innovation.” Washington, DC: U.S. Envi- lation, affluence, technology, and sociopolitical ronmental Protection Agency. factors. Environment 42(6): 4–5. Reijnders, L. 1998. The Factor X debate: Setting tar- Hosier, R. 1996. Economic development and the en- gets for eco-efficiency. Journal of Industrial Ecol- vironment: Beyond trade-offs. Lecture at Yale ogy 2(1): 13–22. University. Rothman, D. and S. deBruyn. 1998. Probing into the Intergovernmental Panel on Climate Change. 1996. environmental Kuznets curve hypothesis. Eco- Second assessment. Cambridge, UK: Cambridge logical Economics 25: 143–145. University Press. Samuelson, P. and W. Nordhaus. 1998. Economics, six- Kates, R. 1997. Population, technology, and the hu- teenth edition. Boston: Irwin/McGraw-Hill. man environment: A thread through time. In Schmidheiny, S. and F. Zorraquin. 1996. Financing Technological Trajectories and the Human Environ- change: the financial community, eco-efficiency, ment, edited by J. Ausubel and H. Langford. and sustainable development. Cambridge, Massa- Washington, DC: National Academy Press. pp. chusetts: MIT Press. 33–55. Simon, J. 1980. Resources, population, environment: Lifset, R. 1997. A metaphor, a field and a journal. An oversupply of false bad news.Science 208: Journal of Industrial Ecology 1(1): 1–3. 1431–1437 Lynn, W. 1989. Engineering our way out of endless Simon, J. 1981. Environmental disruption or environ- environmental crises. In Technology and Environ- mental improvement? Social Science Quarterly 62 ment, edited by J. H. Ausubel et. al. and H. E. (1): 30–43. Sladovich. National Academy of Engineering, Smith, B. L. R. 1990. American science policy since Washington, DC: National Academy Press. World War II. Washington, DC: The Brookings Marchetti, C. , P. S. Meyer, and J. H. Ausubel. 1996. Institution. Human population dynamic revisited with the Speth, J. 1988. The greening of technology. The logistic model: How much can be modeled and Washington Post. November 20. predicted? Technological Forecasting and Social Speth, J. 1990. Needed: An environmental revolution in Change 52: 1–30. technology. A background paper prepared for a Marx, L. 1994. The environment and the ‘Two cul- symposium: Toward 2000: Environment, tech- tures’ divide. In Science, Technology and the Envi- nology and the new century. World Resources ronment, edited by J. R. Fleming et al. and H. A. Institute: June 13–15. Gemery. Akron, Ohio: The University of Akron Speth, J. 1991. EPA must help lead an environmental Press, pp. 3–21. revolution in technology. Environmental Law. McDonough, W. and M. Braungart. 1998. The NEXT Vol. 21: 1425–1460. Industrial revolution. The Atlantic Monthly. Oc- Spiegel-Rosing, I. and D. de Solla Price, eds. 1977. tober. pp. 82–92. Science, technology, and society: A cross-disciplin- Meadows, D., D. Meadows, and J. Randers. 1992. Be- ary perspective. London and Beverly Hills: Sage yond the limits: Confronting a global collapse, envi- Publications. sioning a sustainable future. Post Mills, VT: Stern, P., C. T. Deitz, V. Ruttan, R. Socolow, and J. Chelsea-Green. Sweeney, eds. 1997. Environmentally significant

28 Journal of Industrial Ecology FORUM ❙

consumption: Research directions. Washington, U.S. Department of Commerce. 1998. The U.S. envi- DC: Committee on the Human Dimensions of ronmental industry. Office of Technology Policy. Global Change, National Research Council. October. Stoneman, P. 1987. The economic analysis of technology Wernick, I., P. Waggoner, and J. Ausubel. 1997. policy. Oxford: Clarendon Press. Searching for leverage to conserve forests. Jour- Turner, B. L. 1996. Class notes, June. nal of Industrial Ecology 1(3): 125–145. von Weizsäcker, E., A. B. Lovins, and L. Lovins. 1997. World Commission on Environment and Develop- Factor Four: Doubling wealth, halving resource use. ment. 1987. Our common future. Oxford: Oxford London: Earthscan Publications Ltd. University Press.

Chertow, The IPAT Equation and Its Variants 29