(Part of the How to Talk to a Global Warming Skeptic guide)

Objection: Climate is an inherently chaotic system, and as such its behavior can not be predicted.

Answer: Firstly, let’s make sure we define climate: an average of weather patterns over some meaningful time period. We may thus discount the chaotic annual fluctuations of global mean temperature. That’s weather, and one or two anomalous years does not represent a climate shift.

Quite a few people believe that climate is a chaotic system, and maybe on some large-scale level it is. But it is not chaotic on anything approaching the time scales of which humans need to be mindful.

The notion that climate is chaotic tends to be taken as a given, with little supporting argument. Certainly the march of the seasons is nice and regular, determined directly by the orbital inclination of the earth. If a large volcanic eruption occurs, global temperature drops for a few years quite predictably. Diurnal cycles show the direct influence of insolation changes on the system.

Clearly, if you turn down the sun, the temperature drops. Clearly, if you throw a bunch of SO2 into the stratosphere, the temperature drops. Clearly, if you turn the surface completely white, the temperature drops. And clearly, if you double the amount of an important GHG in the atmosphere, the temperature rises.

What about longer timeframes, say, glacial/interglacial cycles? These are by no means perfectly regular, but they are far from random. They are also a broadly deterministic effect following from a known cause: orbital variations.

Granted, the data is quite chaotic on the multi-century time scale, though it clearly follows a 120Kyr cycle. But who’s to say that if we had enough data and understanding, these spikes and dips could not be thoroughly explained by solar influences, volcanic eruptions, greenhouse gas changes, ice sheet dynamics, etc.?

The ocean-atmosphere climate system is certainly a complex system, and capable of some surprising behaviours, but there is no evidence that it is chaotic in the formal sense.

I see no problem with speaking in a meaningful way about future expectations. Model outputs do produce specific year-to-year fluctuations — fluctuations that are not hindcasted well (that’s the weather, after all) — but nobody’s interested in knowing the exact temperature of any particular year. It is the decadal and century trends we want to anticipate.

It is the climate’s broadly deterministic responses to forcings that are of interest, and all evidence points to such determinism.

(The original article has a great deal of interesting discussion under it, for those with the stomach for talk of strange attractors, dynamical systems, and stochastic processes.)