Tuesday, October 02, 2007

Starts with an "F", ends with "CK"

And we are.

The long periods of stability and the generally very gradual changes of earth's thermal profiles argue that in all eras prior to human pollution, nature had found the balance points at which small perturbations were damped by negative feedbacks. This gave rise to mild oscillation in the short term and only in very long term could larger effects such as ice ages play out the more extreme oscillations. And even those were still oscillations, periodic departures from some normal for which there was always a gradual return.

While nature has never given us dead calm stasis, it is not possible for her to tarry around conditions in which positive feedback is at work...instability is always necessarily transient. But if you screw up badly enough, positive feedbacks nature has never shown us come out of the woodwork. It had been realized in the last few years that a thawing arctic could release trapped CO2 from tundra ice and hasten the rising levels of green house gases now that we pushed them above a threshold that could start the thaw. That added carbon in the air sped up models for global warming. It has been suggested that if the warming goes on long enough to affect temps at ocean depths of a few thousand feet, there might be rapid release of CH4 from methane hydrate ice that is known to blanket some slopes at the margins of continental shelfs. That would be a monster positive feedback both because of the enormous amount of methane now sequestered in the deeps and because methane is 20 times better at trapping heat in the atmosphere than is carbon dioxide. The first positive feedback was more obvious and postulated decades ago: ice reflects the warming rays better than dirt and vegetation so a shrinking ice cap meant yet more heat piled on to, you guessed it, melt yet more frost. That is positive feedback and, as predicted, that particular effect is here already and hard at work dismantling our notions of seasonal variation and agriculture's geographic limits. Another positive feedback that is in operation in temperate climates that used to get enough rain: wildfires. Obviously, wholesale combustion of vast forests puts millions of tons of carbon in air, pronto. Only slightly less obvious [unless you are Senator Inhofe] is the connection that a warmer atmosphere dries or outright drives out the rain that fell in a cooler climate: some may get more rain, most will get less ... and that will increase the likelihood and severity of wildfires. Just ask the Australians.

And now comes yet another curve ball: enough melting to break the ice free is all you need to let the winds push the unmelted remnants to warmer latitudes where water temperatures and less glancing solar radiation finish the job well ahead of the schedule a simple model would predict. Another positive feedback.

If you haven't studied engineering or physics and don't have a feeling for what the difference of behavior is between a system in the grip of negative feedbacks vs one in the grip of positive feedbacks then just think of something like a runaway freight train that is approaching a long downhill grade...its present uncontrolled excess of energy is about to yield an even greater excess of uncontrollable energy. Or think of positive feedback as the simplified physical example of the "vicious circle".

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