Le Chatelier's Principle

Blame Cynthia ;) for the fact that I once again went trolling in the dark waters of Luboiania. Lumo has a post about how Le Chatelier's Principle (LCP) shows that positive feedbacks and global warming can't happen.

Now I don't happen to believe that LCP is truly a principle in the sense that special relativity and natural selection are principles of nature. LCP, by contrast, is more like a rule of thumb, and what it says is that stable equilibria tend to be, er, stable. Unstable equilibria, and non-equilibria, don't behave that way.

A bottle of nitroglycerine exists in a slightly stable equilibrium. Supply it with a small amount of kinetic energy and it will quite likely re-equilibrate at a very slightly higher temperature. Or it might just decide to blow itself (and you, dear experimenter) to hell.

The climate of the Earth is not in equilibrium, it's in a quasi-steady state of balance between energy input and output. Moreover, we have good reason to believe that both positive and negative feedbacks exist in the climate system. The positive feedbacks mean that relatively small input changes can and regularly do drive fairly large excursions in climate. The negative feedbacks mean that these excursions tend to be limited.

The big Kahuna of the negative feedbacks is CO2 accumulation->temperature increase->weathering rate increase->CO2 withdrawal feedback loop. Weathering of some kinds of rocks dissolves CO2 from the atmosphere and incorporates it in carbonate rocks, where it tends to sit, until geological upheaval, cometary impact, or cement manufacture turn it loose again in the atmosphere. This type of CO2 impoundment isn't permanent, but it is for a long time - tens of millions to billions of years. This feedback mechanism is supposedly responsible for termination of Snowball Earth episodes, where the Planet froze nearly completely, including much or most of the ocean. In it's icebound state, weather came to a halt so CO2 from volcanic outgassing accumulated until its "greenhouse effect" heated things up enough to melt all, whereupon the whole place stayed hellishly hot until rainfall and weather managed to get the CO2 level down again.

Depending on that particular feedback to save our bacon, should we manage to crank the greenhouse way up, is a very long term strategy - millions of years. Nor can we expect shorter term negative feedbacks to gallup quickly to the rescue. Smaller scale historical climate excursions have big components in the tens of thousands to millions of years scale.

(Only kidding Cynthia, but your post did get me to take a look at Lumo) ;)

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