At the beginning of the 1970s, a group of young scientists set out to explore our future. Their findings shook a generation and may be even more relevant than ever today.
The question the group set out to answer was: what would happen if the world’s population and industry continued to grow rapidly? Could growth continue indefinitely or would we start to hit limits at some point? In those days, few believed that there were any limits to growth – some economists still don’t. Even those who accepted that on a finite planet there must be some limits usually assumed that growth would merely level off as we approached them.
These notions, however, were based on little more than speculation and ideology. The young scientists tried to take a more rigorous approach: using a computer model to explore possible futures. What was shocking was that their simulations, far from showing growth continuing forever, or even levelling out, suggested that it was most likely that boom would be followed by bust: a sharp decline in industrial output, food production and population. In other words, the collapse of global civilisation.
These explosive conclusions were published in 1972 in a slim paperback called The Limits to Growth. It became a bestseller – and provoked a furious backlash that has obscured what it actually said. For instance, it is widely believed that Limits predicted collapse by 2000, yet in fact it made no such claim. So what did it say? And 40 years on, how do its projections compare with reality so far?
The first thing you might ask is, why look back at a model devised in the days when computers were bigger than your fridge but less powerful than your phone? Surely we now have far more advanced models? In fact, in many ways we have yet to improve on World3, the relatively simple model on which Limits was based. “When you think of the change in both scientific and computational capabilities since 1972, it is astounding there has been so little effort to improve upon their work,” says Yaneer Bar-Yam, head of the New England Complex Systems Institute in Cambridge, Massachusetts.
It hasn’t happened in part because of the storm of controversy the book provoked. “Researchers lost their appetite for global modelling,” says Robert Hoffman of company Whatlf Technologies in Ottawa, Canada, which models resources for companies and governments. “Now, with peak oil, climate change and the failure of conventional economics, there is a renewed interest.”
The other problem is that as models get bigger, it becomes harder to see why they produce certain outcomes and whether they are too sensitive to particular inputs, especially with complex systems. Thomas Homer-Dixon of the University of Waterloo in Ontario, Canada, who studies global systems and has used WorId3, thinks it may have been the best possible compromise between over-simplification and unmanageable complexity. But Hoffman and Bar-Yam’s groups are now trying to do better.
World3 was developed at the Massachusetts Institute of Technology. The team took what was known about the global population, industry and resources from 1900 to 1972 and used it to develop a set of equations describing how these parameters affected each other. Based on various adjustable assumptions, such as the amount of non-renewable resources, the model projected what would happen over the next century.
The team compares their work to exploring what happens to a ball thrown upwards. World3 was meant to reveal the general behaviour that results – in the case of a ball, going up and then falling down – not to make precise predictions, such as exactly how high the ball would go, or where and when it would fall. “None of these computer outputs is a prediction,” the book warned repeatedly.
Assuming that business continued as usual, World3 projected that population and industry would grow exponentially at first. Eventually, however, growth would begin to slow and would soon stop altogether as resources grew scarce, pollution soared and food became limited. “The Limits to Growth said that the human ecological footprint cannot continue to grow indefinitely, because planet Earth is physically limited,” says Jørgen Randers of the Norwegian School of Management in Oslo, one of the book’s original authors.
What’s more, instead of stabilising at the peak levels, or oscillating around them, in almost all model runs population and industry go into a sharp decline once they peak. “If present growth trends in world population, industrialisation, pollution, food production and resource depletion continue unchanged, the limits to growth on this planet will be reached sometime within the next 100 years. The most probable result will be a sudden and rather uncontrollable decline in both population and industrial capacity,” the book warned.
This was unexpected and shocking. Why should the world’s economy collapse rather than stabilise? In World3, it happened because of the complex feedbacks between different global subsystems such as industry, health and agriculture. More industrial output meant more money to spend on agriculture and healthcare, but also more pollution, which could damage health and food production.
And most importantly, says Randers, in the real world there are delays before limits are understood, institutions act or remedies take effect. These delayed responses were programmed into World3. The model crashed because its hypothetical people did not respond to the mounting problems before underlying support systems, such as farmland and ecosystems, had been damaged.
Instead, they carried on consuming and polluting past the point the model world could sustain. The result was what economists call a bubble and Limits called overshoot. The impact of these response delays was “the fundamental scientific message” of the study, says Randers. Critics, and even fans of the study, he says, didn’t get this point.
The other message missed was not that humanity was doomed, but that catastrophe could be averted. In model runs where growth of population and industry were constrained, growth did level out rather than collapse – the stabilised scenario (see graph, right inset).
Yet few saw it this way. Instead, the book came under fire from all sides. Scientists didn’t like Limits because the authors, anxious to publicise their findings, put it out before it was peer reviewed. The political right rejected its warning about the dangers of growth. The left rejected it for betraying the aspirations of workers. The Catholic church rejected its plea for birth control.
The most strident criticisms came from economists, who claimed Limits underestimated the power of the technological fixes humans would surely invent. As resources ran low, for instance, we would discover more or develop alternatives.
Yet the Limits team had tested this. In some runs, they gave World3 unlimited, non-polluting nuclear energy – which allowed extensive substitution and recycling of limited materials – and a doubling in the reserves of nonrenewables that could be economically exploited. All the same, the population crashed when industrial pollution soared. Then fourfold pollution reductions were added as well: this time, the crash came when there was no more farmland.
Adding in higher farm yields and better birth control helped in this case. But then soil erosion and pollution struck, driven by the continuing rise of industry. Whatever the researchers did to eke out resources or stave off pollution, exponential growth was simply prolonged, until it eventually swamped the remedies. Only when the growth of population and industry were constrained, and all the technological fixes applied, did it stabilise in relative prosperity.
The crucial point is that overshoot and collapse usually happened sooner or later in World3 even if very optimistic assumptions were made about, say, oil reserves. “The general behaviour of overshoot and collapse persists, even when large changes to numerous parameters are made,” says Graham Turner of the CSIRO Ecosystem Sciences lab in Crace, Australia.
This did not convince those who thought technology could fix every problem. And with so much criticism, the idea took hold that Limits had been disproved. That mantra has been repeated so often that it became the received wisdom, says Ugo Bardi of the University of Florence in Italy, author of a recent book about Limits. “The common perception is that the work was discredited scientifically. I heard it again at a meeting last April,” says Homer-Dixon. “It wasn’t.”
It wasn’t just confusion. “Misunderstanding was enhanced by a media campaign very similar to the one that has been recently directed against climate science,” says Bardi.
One of the most common myths is that Limits predicted collapse by 2000. Yet as a brief glance at the “standard run” shows, it didn’t (see graph, right). The book does mention a 1970 estimate by the US Bureau of Mines that the world had 31 years of oil left. The bureau calculated this by dividing known reserves by the current rate of consumption. Rates of consumption, however, were increasing exponentially, so Limits pointed out that in fact oil had only 20 years left if nothing changed. But this calculation was made to illustrate the effects of exponential growth, not to predict that there were only 20 years of oil left.
When Matthew Simmons, a leading oil-industry banker, finally read Limits in the 1990s, he was surprised to find none of the false predictions he had heard about. On the contrary, he concluded, population and energy growth largely matched the basic simulation. He felt Limits got so much attention, then lost it, partly because the oil shock of 1973 focused minds on resource shortages that were then largely resolved.
There have been other recent re-appraisals of the book. In 2008, for instance, Turner did a detailed statistical analysis of how real growth compares to the scenarios in Limits. He concluded that reality so far closely matches the standard run of World3.
Does that mean we face industrial collapse and widespread death? Not necessarily. A glance at Turner’s curves shows we haven’t yet reached the stage of the standard run, later this century, when such events are predicted.
In the model, overshoot and collapse are preceded by exponential growth. Exponential growth starts out looking just like linear growth, says Bar-Yam: only later does the exponential curve start heading skywards. After only 40 years, we can’t yet say whether growth is linear or exponential.
We already know the future will be different from the standard run in one respect, says Bar-Yam. Although the actual world population up to 2000 has been similar, in the scenario the rate of population growth increases with time – one of the exponential drivers of collapse. Although Limits took account of the fact that birth rates fall as prosperity rises, in reality they have fallen much faster than was expected when the book was written. “It is reasonable to be concerned about resource limitations in fifty years,” Bar-Yam says, “but the population is not even close to growing [the way Limits projected in 1972].”
The book itself may be partly responsible. Bar-Yam thinks some of the efforts in the 1970s to cut population growth were at least partly due to Limits. “If it helped do that, it bought us more time, and it’s a very important work in the history of humanity,” he says.
Yet World3 still suggests we’ll hit the buffers eventually. The original Limits team put out an updated study using World3 in 2005, which included faster-falling birth rates. Except in the stabilising scenario, World3 still collapsed.
Otherwise, the team didn’t analyse the correspondence between the real world and their 1972 scenarios in detail – noting only that they generally match. “Does this correspondence with history prove our model was true? No, of course not,” they wrote. “But it does indicate that assumptions and conclusions still warrant consideration today.”
This remains the case. Forty years on from its publication, it is still not clear whether Limits was right, but it hasn’t been proved wrong either. And while the model was too pessimistic about birth and death rates, it was too optimistic about the future impact of pollution. We now know that overshoot – the delayed response to problems that makes the effects so much worse – will eventually be especially catastrophic for climate change, because the full effects of greenhouse gases will not be apparent for centuries.
There will be no more sequels based on World3, though. The model can no longer serve its purpose, which was to show us how to avoid collapse. Starting from the current conditions, no plausible assumptions produce any result but overshoot. “There is no sense in only describing a series of collapse scenarios,” says Dennis Meadows, another of the original authors of Limits.
Randers, meanwhile, is editing a book called The Next Forty Years, about what we can do when limits start to bite. “I don’t like the resulting future, but it should be described, particularly because it would have been so easy to make a much better future,” he says. The only hope is that we can invent our way out of trouble. Our ingenuity has allowed us to overcome many limits, says Homer-Dixon, and we can’t predict what revolutionary technological innovations humanity might come up with. Yet he is pessimistic: “The question is, can we deliver ingenuity at an increasing rate indefinitely.” Because that is what we’ll need to do if growth continues.
Instead of declaring we are doomed, or proclaiming that technology will save us, we should explore the future more rigorously, says Bar-Yam. We need better models. “If you think the scientific basis of those conclusions can be challenged, then the answer is more science,” he says. “We need a much better understanding of global dynamics.”
We need to apply that knowledge, too. The most important message of Limits was that the longer we ignore the problems caused by growth, the harder they are to overcome. As we pump out more CO2, it is clear this is a lesson we have yet to learn.
Debora MacKenzie is a consultant for New Scientist based in Brussels, Belgium
January 11, 2012 “New Scientist”