22 February 2012

Basic climate question

While I'm on the subject of climate, is there anyone with background in system modelling who can tell me what's wrong with the following. It  seems obvious to me, but I'm probably missing something important.

Take a global climate model, without human GHG emissions.

There should be no long-term climate trend, or at least none on the scale of what GHG emissions are supposed to cause.

Perturb the climate model in some way: maybe a major volcano, or a temporary solar variation, or something.

That perturbation, if it is big enough, will detectably affect the climate, including the global average temperature.

The perturbation is a one-off, so it will end. After the perturbation ends, does the global average temperature stay around its new value, or does it move back towards its old value?

My understanding is that the best current models of the climate exhibit positive feedback. That would mean that there are no forces that would cause the climate to move back to its old value. Block out the sun for a bit, the climate will get colder, and when the dust has settled (literally), it will still be colder than it was before.

For the temperature to return to "normal" after a perturbation, there would have to be net negative feedback: because the temperature is higher, something happens that removes the excess heat.

Therefore, without negative feedback, the climate would be a complete "random walk". It could be pushed up or down by solar changes, vulcanism, vegetation changes, even freak weather, and every such perturbation would affect the future climate forever.

That just doesn't seem remotely plausible to me. Surely such random events wouldn't balance so well as to keep the climate roughly stable for so long as paleoclimate data indicates?

I feel I must be missing something. Is there some way that the climate can exhibit positive feedback in response to anthropogenic CO2 emissions, as we are told it does, and yet recover from other random effects?

1 comment:

candide3 said...

there would have to be net negative feedback
As for 'negative feedbacks' that keep the climate within certain bounds, these are principally not what you would call 'feedbacks' but the various physical constraints and limits. To list a few, the amount of incident solar radiation does not change very much and does not depend on what the climate does. The amount of infrared radiation the Earth emits is subject to radiation laws: the hotter the body, the more it radiates. A black body twice as hot radiates 16 times as much energy; that's a strong negative feedback for you. The Earth's albedo cannot change very much either, and some changes to it (e.g. volcanic ash) quickly disappear. The distribution of land masses that affects albedo, ocean currents, winds etc. is one more climate-independent constraint.

That just doesn't seem remotely plausible to me.
However that is just the way it is. Climate is a stochastic dynamical system with multiple equilibria, and potentially any perturbation can affect transitions between the equilibria. The flip side of this coin is the impossibility of knowing exactly what perturbation has caused or will cause the system to transition from one state to another, to pinpoint the straw that breaks the camel's back, so to speak. And a third side is that after a time the 'memory' of the perturbation dissipates, and it becomes impossible to say (looking just at the climate) that there has been a perturbation at all. The climate resembles one of those funny pendulums with magnets: it bobbles randomly within the bounds set by the power being imparted by the magnets' kicks (incident solar radiation) and trickling out through friction (thermal radiation). Nudge it and its motion will visibly change, but as it bobbles on it will soon become impossible to tell that you've ever nudged it. If your nudge has added energy it will be lost through friction, if it has removed energy it will be replenished by the magnets.
I am probably not explaining this very well. Please don't hesitate to ask questions!