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Rocketdoc Notes – Week of April 25, 2021

Discussing “Limits to Growth”

As part of my report on Global Warming I have mentioned the book “Limits to Growth” (LtG) several times (Ref 1). Most of you are unaware of the book and the movement it triggered and have asked for more information. This blog is my effort to provide that information.

In this blog I will use a lot of the information generated for a master thesis titled “A Thesis in the Field of Sustainability”by Gaya Branderhorst at Harvard University in 2020 (Ref 2). This thesis reviews of all three of the LtG books (1972, 1992, and 2004) and compares the computer-generated predictions over time with empirical data. For background information let me add that I read the 1972 LtG book when in first came out and was very impressed. Professor Forrester and the Club of Rome had a great deal of credibility and computer modeling of the entire earth was a brand-new capability and many of us were in awe of the results. Now, almost fifty years later we are very familiar with the phrase “Garbage-in, Garbage-Out” (GiGo) and know that computer modeling of very interactive, very sensitive complex processes is fraught with small errors that build-up in long-term models and make for inaccurate projections.

But we didn’t understand all that back then and I remember the commotion when the book was published. Three scientists from MIT created a computer model that analyzed the exponential economic and population growth assuming a finite supply of resources. The model was 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. This is little changed from Malthus’s writings back in 1798 but had new credibility because it was computer generated. 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. The essence of the LtG message is that a continued pursuit of growth on a finite planet will inevitably lead to an overshoot and collapse pattern. The term overshoot and collapse comes from ecology and describes a three-phase pattern.

In the first phase a population is growing until it reaches the number that can be sustained by its environment, i.e., its “carrying capacity”. The population can grow beyond its carrying capacity, but it can only remain there temporarily. Because there aren’t enough resources to sustain that population size, population growth slows down and subsequently ends altogether as the mortality rate rises. This is the second phase, at which the population is said to be in

overshoot. In the third phase, the mortality rate has surpassed the birth rate and the

population starts to decline, at a higher speed than it was growing in the first phase. This

third phase is called a collapse because of the steepness that typically marks the decline.

The classic collapse is best in figure 1 below which shows the results from the World3 computer model from reference 1. The changes over time in the five variables are for “Business as Usual” (BAU) human responses, where historic averages for resource consumption, food production, birth rates, industrial output and pollution generation were maintained.

Figure 1 Limits to Growth World Projection for Business as Usual (BAU)

The model said that given business as usual, i.e., no changes to historical growth trends, the limits to growth on earth would become evident by 2072, leading to "sudden and uncontrollable decline in both population and industrial capacity". This includes the following:

· Global industrial output per capita reaches a peak around 2008, followed by a rapid decline

· Global food per capita reaches a peak around 2020, followed by a rapid decline

· Global services per capita reaches a peak around 2020, followed by a rapid decline

· Global population reaches a peak in 2030, followed by a rapid decline

The book’s solution was to alter growth trends existing in 1972 so that sustainable ecological and economic stability could be achieved. This involved birth control to limit population growth and limiting spending on increasing standard of living to conserve natural resources.

The results also showed that the sooner the world's people started putting on limits, the better the chance of achieving stability.

Why did LtG cease to be a major topic for discussion in the twenty-first century? Basically, technology advanced fast enough during the last half of the twentieth century to overwhelm the limiting assumptions in the book. First, we had Norman Borlaugh, the "Father of the Green Revolution", who pioneered the development of high-yielding varieties of cereal grains, expansion of irrigation infrastructure, modernization of management techniques, distribution of hybridized seeds, synthetic fertilizers, and pesticides to farmers. These improvements essentially doubled the yield per acre in the third world. The prediction of “Peak oil” in 2004 was wrong. We found ways to pump more the oil natural resource from old oil wells and the invention of “Fracking” about 2008 greatly expanded oil resources that could be economically drilled and pumped. We also found substitutes for many of materials in limited supply in the Limits to Growth models, thereby indefinitely delaying the crash in resources.

The authors of LtG understood the limitations on their model imposed by fixed assumptions and introduced alternative scenarios. There were eleven other scenarios in the first book, including “comprehensive technology” (CT) and “stabilized world” (SW). CT assumes a range of technological solutions, including reductions in pollution generation through improved technology, increases in agricultural land yields as mentioned above, and resource efficiency improvements that are significantly above historic averages (Reference 1, p. 147). The SW scenario assumes that in addition to the technological solutions, global societal priorities changed from a certain year onwards. In SW a change in values and policies translated into, amongst other things, lower desired family size, perfect birth control availability, and a deliberate choice to limit industrial output (limiting standard of living) and prioritize health and education services. SW was the only scenario in which declines were avoided.

Updated alternative scenarios from reference 3 are summarized in figure 2 below.

Figure 2 – Scenario Description and World3 Model Results Summary

Figure 3 below shows the World3 outputs for the four scenarios described above. The scenarios are Business as Usual (BAU) with the same assumptions as the original model, BAU2 with twice the natural resources as the original model, CT with 2004 technology development rates, and SW where humanity educates themselves and manages to achieve a stable population and with limits on standard of living.

Figure 3 – BAU (upper left), BAU2 (upper right), CT (lower left), and SW (lower right) scenarios

The results from the World3 model are as follows:

· In BAU the world hits a limit on food and natural resources about 2020 causing the population and industrial output to drop leading to a general collapse.

· In BAU2 with double the natural resources pollution keeps climbing to where Global Warming destroys food production about 2030 and starts the death spiral.

· In CT the World3 model predicts that the costs associated with the technologies needed to maintain food production leads to a fall in industrial output and food production that limits world population. There is no death spiral.

· In SW we manage to limit population growth and the rate of world industrialization to where everyone gets fed and has an adequate, but not overly satisfying, standard of living.

I have issues with some of this modeling. For instance, advancing technologies almost always reduce the cost of products so the premise in CT that the costs of advancing technology will limit food production and industrial production just doesn’t make sense.

The obvious goal for humanity is to achieve a stable population with enough food per person and enough industrial production to give every person a good standard of living. Also, it would be nice if pollution was controlled so it wasn’t a health hazard. As you can see from theWorld3 modeling the key is to achieve a stable population as soon as possible. Unfortunately, this is also one of the hardest objectives to achieve. The projected range of world population between now and the year 2100 is shown in figure 4 below. This data is based on UN projections.

Figure 4 – UN World Population Projections out to year 2100

Why is there such a wide dispersion in projections? The wide dispersion is because of the current high birth rates in some parts of the world. See Figure 5 below to see these dispersions.

Figure 5 – Population in Various Regions of the World

Obviously, South and East Asia, plus Sub-Saharan Africa, are key to levelling off the world’s population. If we cannot reach some sort of Zero Population Growth (ZPG) in the world and especially in those regions we could find ourselves in some form of the BAU2 scenario, and heaven help us. What will probably happen is that we will achieve the SW scenario in the developed world, and the parts of the world noted above will experience a BAU scenario with shortages of food and standard of living. This is essentially close to where we are now, and I can’t see that changing without wholesale changes in politics and government.

With respect to the big picture, the World3 model (I have a copy and have run it successfully) had some simplifying assumptions that were too pessimistic and gave results that could not be believed once we got to the year 2000. This hurt their credibility and the movement never regained its early fervor. The political factions involved have switched to Global Warming as their central theme and now support the BAU2 scenario. We will see where that leads us.

The bottom line on LtG has been that the overall idea is correct and resources on Earth are finite. We can either limit the population and the resulting resources depletion, or we can start using the resources in the solar system and move some of the population off planet. Either works for me, but I think we should pursue both avenues.

Thanks for your attention.

Dana Andrews


1. Meadows, D. H., Meadows, D. L., Randers, J., & Behrens, W. W. (1972). The limits to

growth: A report for the Club of Rome's project on the predicament of mankind. New York: Universe Books.

2. Branderhorst, Gaya. 2020. Update to Limits to Growth: Comparing

the World3 Model With Empirical Data. Master's thesis, Harvard Extension School.

3. Meadows, D. H., Meadows, D. L., & Randers, J. (2004). The limits to growth: The 30-

year update. White River Junction VT: Chelsea Green Publishing Co.

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