Tuesday, December 19, 2006

Climate Stability?

William Connolley has this post asserting that climate is stable. His post, and some of the comments, got me wondering about what exactly was meant thereby. The climate has varied over a considerable range over the life of the Earth, but not, evidently, enough to kill off all life or even the advanced species. It is essentially certain that geology was a major player in some past climate matters.

If we stick to the last few million years, there don't seem to be any major geologic convulsions that have left big footprints, but there are those pesky ice ages, alternating with warmer periods like the present. The ice ages seem to be correlated with the Milankovitch Cycle, the slow periodic changes in the Earth's orbital parameters that have some affect on average insolation. The correlation is hardly simple though, and not, apparently, fully understood.

A system is stable if small perturbations cause small changes in the system behavior. The short term behavior of the Earth's weather is undoubtedly unstable in that sense. There are clearly instabilities, or at least irregular oscillations on the scale of years and tens of years. Conversely, it's pretty obvious that the very long term climate is quite stable - otherwise life would not be here. What about on the scale of a hundred years or a thousand?

In general, one expects dynamic stability if negative feedbacks (effects that decrease the departure from equilibrium) outweigh positive feedbacks. The climate system contains both, operating on various time scales. My question (for William or anybody) is what do the models have to say on these accounts?

Are the models stable, or is it necessary to put in nonphysical feedbacks "by hand?" Does the stability behavior of the models mimic that of the climate system, and if not, do we know why not?