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In writing an editorial for this issue on mineral resources, I was immediately reminded of The Limits to Growth (Meadows et al. 1972), a book that I read avidly from cover to cover as a young post-doc. For anybody interested in humanity's effect on the environment and its near-term consequences, it is still a fascinating read. The authors summarised a computer model of the likely effects of sustained economic growth on the Earth and the human population. Based on historical data from 1900 to 1970, they observed exponential growth in total human population, resource consumption, food consumption, industrialisation and environmental pollution. They fitted exponential growth curves to the data and assumed in their “standard model” that there was a 250-year supply of all resources at 1970 rates of consumption. They then developed what they considered to be appropriate feedback loops between the different parameters, such as between resource consumption and rate of industrialisation, food consumption and available arable land, gross national product (GNP) per capita, and birth rate. These feedback loops were added to the model, which was constructed to project how the global system would develop out to the year 2100. The results were startling.

In the standard model, the global system collapses because of resource depletion, which leads to increased diversion of industrial capital into resource extraction, which, in turn, reduces industrial growth. The industrial system collapses for lack of investment, taking with it the service and agricultural systems. The global population continues to rise for a while and then declines as death rates increase due to lack of food and health services. When does this start to happen? According to the 1972 study … about 2015; global population starts to decline from around 2030.

Of course, the authors recognised the large uncertainties and didn't put great emphasis on the timescale. But they did note that, even making large allowances for increased resource availability and improved industrial efficiency, collapse still occurred before 2100. If, for example, they double the amount of available resources, collapse would be delayed by a few years, but then occurs due to increased pollution (including climate change), which damages food production and the health of the population.

The Limits to Growth has been subject to intense scrutiny and criticism since its publication. By allowing consumption to increase exponentially but with fixed or slowly increasing resource availability, the authors ensured that the system would collapse at some point. Furthermore, it has been argued that the authors made insufficient allowance for technical innovation. Some of the detail (such as the GNP per capita of China) has been invalidated by political and technological changes. Nevertheless, as shown in a recent study (Turner 2014), many of the projections made by Meadows et al. (1972) were remarkably accurate.

As can be seen in Figure 1, population, industrial output, food and services per capita, which are relatively straightforward to measure, all follow the predicted curves closely. Resources and pollution are less easy to measure, requiring the use of energy reserves and atmospheric CO2 as proxies (Turner 2014), and it is these which determine the collapse of the system in the standard model. Arguably, however, and despite the global financial crisis in 2007−2008 and its aftermath, there is no evidence yet of system collapse. This tends to confirm the view that the standard model is too pessimistic. It is, however, interesting to consider some of the more “optimistic” scenarios treated in The Limits to Growth.

In one scenario, Meadows et al. (1972) assumed unlimited resources and that a low-CO2–producing energy source replaced fossil fuels. But, this still led to collapse of the system because of pollution. Meadows et al. (1972) then added strict pollution controls with the assumption that pollution per unit of industrial output declines to 25% of its level in 1970. In this new scenario, population and industrial output per person rise well beyond their peak values in the standard model but the system again collapses well before 2100 because capital is diverted from industrial output into food production, and the population declines as food per capita becomes inadequate.

So how do we avoid collapse of the global system? Meadows et al. (1972) explored the parameters required for global sustainability and equilibrium. This required them to set birth rate equal to death rate with globally available birth control, and to limit the growth in industrial output by shifting societal values away from material goods towards education and services. Pollution and resource consumption also needed to be reduced to one-quarter of the 1970 values per unit of industrial output, and capital be diverted to food production, even where it is not financially profitable. Clearly, as seen in Figure 1, there has been little progress towards such a system. One can argue that our current political structures and values are inconsistent with changes in these directions. Nevertheless, The Limits to Growth has had enormous political influence, not least here in Europe, and was the stimulus for much of the environmental movement. Don't we all wish that our work could be so widely read and discussed!