In 1972 a team at MIT produced a report 'Limits to Growth' which used systems dynamics modelling to show that the world economy could not continue to grow indefinitely and predicted that 'business as usual' would lead to a collapse in the middle of the twenty first century. Will this happen? Are humans exceeding the boundaries of what the earth will support?

The limits to growth model was ridiculed by economists at the time but curiously it has been revisited repeatedly over the last 50 years and each time the model has been shown to be pretty much on track.

The diagram below from the model shows the predicted downturns under its 'Business as Usual' scenario. First there is a big drop in industrial output and food production.  This leads to a fall in population and quality of life.

The model contained many simplifications and a lot has changed in the last 50 years but there are increasing signs that we are approaching the limits.  Another approach is to map how we are exceeding global limits in different areas as illustrated in the diagram below.  This shows that we have already overstepped the limits in a number of areas.

J. Lokrantz/Azote based on Steffen et al. 2015

We will examine the models in more detail to see how the various factors still apply and their implications for scenarios.