Musings/Träumereien/Devaneios

(original review, 2006)

"Climate is a Chaotic System
Chaotic Systems cannot be predicted
Climate, therefore, cannot be predicted.
The IPCC has stated this explicitly."

I've been hearing this almost since forever. But is it right?

Predicting a Chaotic System is BY DEFINITION impossible. Climate is a Chaotic System. Ensembles are used to try to mitigate the nature of a Chaotic System by extending how far out one might be able to present probabilities for different scenarios. This is done with weather forecasts all the time. They use ensembles which is why they present the possibility of rain next Thursday as a probability. But as you know, these are often wrong. So if they are right, there is an element of luck involved as the conditions that will (or will not), cause it to rain in five days are changing by the minute. The calculation of climate variables (that is long term averages) is much easier than weather forecasting, since weather is ruled by vagaries of stochastic fluctuations, while climate is not. We can demonstrate this sort of climate response clearly in the Lorenz model, or any more complex climate model. Perturbing the initial conditions gives a completely different trajectory (weather), but this averages out over time, and the statistics of different long-term runs are indistinguishable. However a steady perturbation to the system can generate a significant change to the long-term statistics. This disproves the common but misguided claim that chaotic weather prevents meaningful climate prediction. In fact, all climate models do predict that the change in globally-averaged steady state temperature, at least, is almost exactly proportional to the change in the net radioactive forcing, indicating a near-linear response of the climate, at least on the broadest scales.

If you're into stuff like this, you can read the full review.