IAMs Cannot Be Fixed

Also in re IAMs from David Roberts's Vox article, and rather a separate point:

There are tons of other interesting results buried in this monster paper — for instance, out of various parameters, uncertainty about future productivity growth has by far the largest implications for outcomes, "which suggests that uncertainty in GDP growth dominates the uncertainty in emissions"

Please note - GDP growth is a parameter of IAMs; it's not an output, it's an input. This alone makes them unfit for purpose, as I determined independently, but David wrote up before I got around to it.

Right now, the most frequently used climate IAMs treat economic growth as mostly or entirely exogenous. Here’s a very brief explanation of endogenous vs. exogenous variables in economic models:

Some economic variables are determined by our models, while others are usually assumed to be determined by factors outside of our models. We call the former endogenous variables and the latter exogenous variables.

For econometric applications, the crucial difference between an endogenous and an exogenous variable is that we must assume that exogenous variables are not systematically affected by changes in the other variables of the model, especially by changes in the endogenous variables.

In other words, an exogenous variable is something you plug into the model, not an outcome of the dynamics within the model.

So what does it mean for GDP growth to be treated as exogenous in economic models? It means that growth cannot be systematically affected by endogenous variables like, say, temperature — by definition, in these models, climate impacts cannot affect the rate of GDP growth. Climate impacts might subtract something from total economic output, but they do not alter the rate or trajectory of growth.

This doesn't mean that complacent economic models are better than disturbing economic models nor the other way around. It means that this approach is of no value whatsoever in assessing long term climate risk. At all. Period.

Now people have finally got around to realizing this and are plugging in a fveedback from climate to the growth parameter. If course, they aren't going to publish the results. It's not hard to imagine the rosy models turning collapsitarian with this minor tweak. That doesn't make them right, of course. Presumably an approach with such a fundamental flaw as to tread GDP as a parameter is pretty flawed in other ways.

It's funny how people who criticize climate scientists tend to be such empiricists, obsessing over every bump and wiggle in every chart and using each little excursion as an excuse to dismiss a fairly mature body of physical theory. But when it comes to economics, they are utterly happy with the economists utter indifference to data collection and their silly handwaving predictions.

If our circumstances weren't so dire it would be funny.