The Limits to Growth (LTG) is a 1972 report that discussed the possibility of exponential economic and population growth with a finite supply of resources, studied by computer simulation. The study used the World3 computer model to simulate the consequence of interactions between the Earth and human systems.
Commissioned by the Club of Rome, the study saw its findings first presented at international gatherings in Moscow and Rio de Janeiro in the summer of 1971. The report's authors are Donella H. Meadows, Dennis L. Meadows, Jørgen Randers, and William W. Behrens III, representing a team of 17 researchers. The model was based on the work of Jay Forrester of MIT, as described in his book World Dynamics.
The report's findings suggest that, in the absence of significant alterations in resource utilization and environmental destruction, it is highly likely that there will be an abrupt and unmanageable decrease in both population and industrial capacity. Although it faced severe criticism and scrutiny upon its release, the report influenced environmental reforms for decades. Subsequent analysis notes that global use of natural resources has been inadequately reformed to alter its expected outcome. Yet price predictions based on resource scarcity failed to materialize in the years since publication.
Since its publication, some 30 million copies of the book in 30 languages have been purchased. It continues to generate debate and has been the subject of several subsequent publications.
Beyond the Limits and The Limits to Growth: The 30-Year Update were published in 1992 and 2004 respectively; in 2012, a 40-year forecast from Jørgen Randers, one of the book's original authors, was published as 2052: A Global Forecast for the Next Forty Years; and in 2022, two of the original Limits to Growth authors, Dennis Meadows and Jørgen Randers, joined 19 other contributors to produce Limits and Beyond.
Purpose In commissioning the MIT team to undertake the project that resulted in LTG, the Club of Rome had three objectives: Gain insights into the limits of our world system and the constraints it puts on human numbers and activity. Identify and study the dominant elements, and their interactions, that influence the long-term behavior of world systems. To warn of the likely outcome of contemporary economic and industrial policies, with a view to influencing changes to a sustainable lifestyle.
Method The World3 model is based on five variables: "population, food production, industrialization, pollution, and consumption of nonrenewable natural resources." At the time of the study, all these variables were increasing and were assumed to continue to grow exponentially, while the ability of technology to increase resources grew only linearly. The authors intended to explore the possibility of a sustainable feedback pattern that would be achieved by altering growth trends among the five variables under three scenarios. They noted that their projections for the values of the variables in each scenario were predictions "only in the most limited sense of the word" and were only indications of the system's behavioral tendencies. Two of the scenarios saw "overshoot and collapse" of the global system by the mid- to latter part of the 21st century, while a third scenario resulted in a "stabilized world."
Exponential reserve index A key idea in The Limits to Growth is the notion that if the rate of resource use is increasing, the number of reserves cannot be calculated by simply taking the current known reserves and dividing them by the current yearly usage, as is typically done to obtain a static index. For example, in 1972, the amount of chromium reserves was 775 million metric tons, of which 1.85 million metric tons were mined annually. The static index is 775/1.85=418 years, but the rate of chromium consumption was growing exponentially at 2.6 percent annually. If instead of assuming a constant rate of usage, the assumption of a constant rate of growth of 2.6 percent annually is made, the resource will instead last
In general, the formula for calculating the amount of time left for a resource with constant consumption growth is:Limits To Growth, pg 60, Derivation: reverts to
where: y = years left; r = the continuous compounding growth rate; s = R/C or static reserve; R = reserve; C = (annual) consumption.
Commodity reserve extrapolation The chapter contains a large table that spans five pages in total, based on actual geological reserves data for a total of 19 non-renewable resources, and analyzes their reserves at the 1972 modeling time of their exhaustion under three scenarios: static (constant growth), exponential, and exponential with reserves multiplied by 5 to account for possible discoveries. A short excerpt from the table is presented below: {| class="wikitable" ! colspan="2" | !! colspan="3" | Years |-valign="top" !align="left"|Resource!!Consumption, projected average annual growth rate!!Static index!!Exponential index!!5× reserves exponential index |- |width="100"|Chromium||width="100" align="right" |2.6%||width="100" align="right" |420||width="100" align="right"|95||width="100" align="right"|154 |- |Gold||align="right"|4.1%||align="right"|11||align="right"|9||align="right"|29 |- |Iron||align="right"|1.8%||align="right"|240||align="right"|93||align="right"|173 |- |Lead||align="right"|2.0%||align="right"|26||align="right"|21||align="right"|64 |- |Petroleum||align="right"|3.9%||align="right"|31||align="right"|20||align="right"|50
The chapter also contains a detailed computer model of chromium availability with current (as of 1972) and double the known reserves, as well as numerous statements on the current increasing price trends for discussed metals:
Interpretations of the exhaustion model Due to the detailed nature and use of actual resources and their real-world price trends, the indexes have been interpreted as a prediction of the number of years until the world would "run out" of them, both by environmentalist groups calling for greater conservation and restrictions on use and by skeptics criticizing the accuracy of the predictions.The Skeptical Environmentalist, p. 121 This interpretation has been widely propagated by media and environmental organizations and authors who, apart from a note about the possibility of the future flows being "more complicated," did not clearly constrain or deny this interpretation.
While environmental organizations used it to support their arguments, a number of economists used it to criticize LTG as a whole shortly after publication in the 1970s (Peter Passel, Marc Roberts, and Leonard Ross), with similar criticism reoccurring from Ronald Baily, George Goodman, and others in the 1990s. In 2011 Ugo Bardi, in "The Limits to Growth Revisited," argued that "nowhere in the book was it stated that the numbers were supposed to be read as predictions"; nonetheless, as they were the only tangible numbers referring to actual resources, they were promptly picked as such by both supporters and opponents.
While Chapter 2 serves as an introduction to the concept of exponential growth modeling, the actual World3 model uses an abstract "non-renewable resources" component based on static coefficients rather than the actual physical commodities described above.
Conclusions After reviewing their computer simulations, the research team came to the following conclusions:
The introduction goes on to say:
Criticism and responses
LTG provoked a wide range of responses, including immediate criticisms almost as soon as it was published.
Peter Passell and two co-authors published a 2 April 1972 article in the New York Times describing LTG as "an empty and misleading work ... best summarized ... as a rediscovery of the oldest maxim of computer science: Garbage In, Garbage Out." Passell considered the study's simulation simplistic and that it assigned little value to the role of technological progress in solving the problems of resource depletion, pollution, and food production. They charged that all LTG simulations ended in collapse, predicting the imminent end of irreplaceable resources. Passell also charged that the entire endeavour was motivated by a hidden agenda: to halt growth in its tracks.
In 1973, a group of researchers at the Science Policy Research Unit at the University of Sussex concluded that simulations in Limits to Growth were very sensitive to a few key assumptions and suggested that the MIT assumptions were unduly pessimistic and the MIT methodology, data, and projections were faulty. However, the LTG team, in a paper entitled "A Response to Sussex," described and analyzed five major areas of disagreement between themselves and the Sussex authors. The team asserted that the Sussex critics applied "micro reasoning to macro problems" and suggested that their own arguments had been either misunderstood or wilfully misrepresented. They pointed out that the critics had failed to suggest any alternative model for the interaction of growth processes and resource availability, and "nor had they described in precise terms the sort of social change and technological advances that they believe would accommodate current growth processes."
During that period, the very idea of any worldwide constraint, as indicated in the study, was met with scepticism and opposition by both businesses and the majority of economists. Critics declared that history proved the projections to be incorrect, such as the predicted resource depletion and associated economic collapse by the end of the 20th century. The methodology, the computer, the conclusions, the rhetoric, and the people behind the project were criticised.Alan Atkisson (2010). Believing Cassandra: How to be an Optimist in a Pessimist's World, Earthscan, p. 13. Yale economist Henry C. Wallich agreed that growth could not continue indefinitely; however, he believed that a natural end to growth was preferable to intervention. Wallich stated that technology could solve all the problems the report was concerned about, but only if growth continued apace. According to Wallich's cautionary statement, prematurely halting progress would result in the perpetual impoverishment of billions.
Julian Simon, a professor at the Universities of Illinois and, later, Maryland, argued that the fundamental underlying concepts of the LTG scenarios were faulty because the very idea of what constitutes a "resource" varies over time. For instance, wood was the primary shipbuilding resource until the 1800s, and there were concerns about prospective wood shortages from the 1500s on. But then boats began to be made of iron, later steel, and the shortage issue disappeared. Simon argued in his book The Ultimate Resource that human ingenuity creates new resources as required from the raw materials of the universe. For instance, copper will never "run out." History demonstrates that as it becomes scarcer, its price will rise, more will be found, more will be recycled, new techniques will use less of it, and at some point a better substitute will be found for it altogether. His book was revised and reissued in 1996 as The Ultimate Resource 2.
To the US Congress in 1973, Allen V. Kneese and Ronald Riker of Resources for the Future (RFF) testified that in their view, "The authors load their case by letting some things grow exponentially and others not. Population, capital, and pollution grow exponentially in all models, but technologies for expanding resources and controlling pollution are permitted to grow, if at all, only in discrete increments." However, their testimony also noted the possibility of "relatively firm long-term limits" associated with carbon dioxide emissions, that humanity might "loose upon itself, or the ecosystem services on which it depends, a disastrously virulent substance," and (implying that population growth in "developing countries" is problematic) that "we don't know what to do about it.", pp 203–205