Talking points for the World3 model

World3 cannot predict the future.

"You are here" graphic from

Positive feedback cycles produce exponential growth (pp. 18-23)

The "Demographic Transition" (pp. 23-32)

Limits to Industrial growth: Sources and sinks (pp. 44-47)

Food production and renewable resources (pp. 48-66)

Deconstructing the nightmare scenario

Arable land is created by developing previously unused land, and destroyed by erosion and urban sprawl. But the total amount of arable land does not collapse:

Food production depends on the amount of arable land and the yield per hectare. Let's graph these variables:

Why do agricultural yields collapse?

Land yields are determined by several factors, but the most important one is agricultural inputs such as fertilizer. Let's graph this variable:

Agricultural inputs in turn are determined by how much is invested in agriculture, which in turn is affected by industrial output:

Let's examine the factors that influence industrial output:

Now put these in a graph to detect the cause of the problem:

Malthus in, Malthus out

At last we find the root cause of the problem! Observe these two graphs:

World3 presumes that as natural resources are used up, a larger and larger fraction of total output is used to extract the remaining resources.

The two circles "p_fr_cap_al_obt_res_1" and "p_fr_cap_al_obt_res_2" both represent the fraction of capital allocated to obtaining resources under different circumstances. The key variable s_fr_cap_al_obt_res is switched to one or the other.

Here's the result. As resources are exhausted, more and more of the world's industrial capital gets diverted to extracting resources. Less and less is left for agriculture, leading to food shortages and the nightmare scenario.

Is this scenario a real threat?

Quite likely it is not, because the World3 model does not include resource switching. See the Web reading by Jerry Taylor of the (libertarian) Cato Institute.

So are we out of the woods?

Maybe. Unfortunately, there is also pollution to worry about. Here are some readings: