Friday, April 13, 2007

The Great C02 - Tempurature Lag Debate

As someone who is a skeptic of both denialists and AGW, I've been following one part of the debate with interest. Al Gore's film, An Inconvenient Truth, shows a 600,000 year graph of CO2 and of T, "showing" that rising C02 causes rising T. Later it came out, in places like the film "The Great Global Warming Swindle", that in fact what the ice core data shows is that CO2 rise lags T rise by about 800 years.

This has given rise to huge debate (here at RealClimate, or an excellent debate here) over what this means.

The AGW view is that something causes T to rise. There are lots of known somethings: change in solar output or changes in Earth's orbit. The T rise causes oceans to release some of their dissolved C02 (since warmer water can hold less). C02 is a greenhouse gas, so T increases even more. This is "positive feedback". After a while (around 5000 years on average), another something causes T to fall. C02 is re-absorbed into the oceans and we end up back where we started.

My problem is understanding why T ever falls back (eventually) after CO2 rises. If there is positive feedback (or converging feedback), that still suggests a one-way trip. The system would drive itself into a high-CO2/high-T state and stay there. And yes there is solar or orbital forcing going on. But it seems that a small forcing can kick off the T rise, but a huge forcing must be needed to drive the system back to a low CO2 state. I haven't seen this explained anywhere.

Many AGW skeptics get caught up on the lag as proof that significant AGW isn't happening. Their argument is that if C02 lags then it can't be the cause of global warming without repealing the laws of cause and effect. It's a good argument because it throws Gore's false claim back in his face. But the argument only works if there is no feedback; which is impossible given basic physics of water and gases.

They also wonder why positive feedback isn't runaway. If rising T causes rising C02 which causes more rising T, then we should end up with a climate like Venus. However this needn't happen if the feedback decreases. That is if the amount of feedback decreases as T rises then the system won't go runaway.


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