The New Scientist recently had an article on Limits To Growth, and this week's issue features a letter from me in response to this article. They've edited my letter somewhat, the original is reproduced below:
"The article states that economists' objections were that future innovation was not included in the model. This is to misrepresent economists' concerns to a certain extent: what is missing from LTG are prices and incentives.
LTG is essentially a 'fixed factor' model so that output is associated with certain inputs. Assuming a path for output and some endowment of input factors, we can always make the model overshoot and collapse, no matter how abundant we choose these input factors to be.
Economic models on the other hand require that these inputs be purchased by the sectors creating output. Under standard assumptions, scarcity drives up prices - a continuously rising price may spur innovation, but if it doesn't then it will instead restrict demand. Rising prices incentivise innovation, substitution or a smooth contraction in activity. Because of this, it is quite hard to construct an economic model that displays overshoot and collapse i.e. in most economic models we are automatically in the 'stabilising scenario'.
I think LTG is likely to prove closer to the truth than e.g. the endogenous growth models with exhaustible resources of Dasgupta & Heal, Stiglitz, and Solow. However, this is not because LTG is right and economists are wrong. It is because prices and and incentives have not responded to the finite nature of resources in a manner consistent with a model with rational and perfectly foresighted agents. These are wrong assumptions in much the same way that a model without prices and incentives contains the wrong assumptions.
Non-economists will get nowhere in convincing the economists, by producing models that lack economic mechanisms and incentives. Instead economists and non-economists alike have to work together to tease out the correct economic mechanisms and incentives."