It has come up in the comments a couple of times now, so I would like to state for the record that the following is a lousy analogy of a negative feedback. As far as I know, Richard Lindzen came up with this in his speech at the recent Heartland climate sceptic conference.
The analogy is this:
In your car, the gas and brake pedals act as negative feedbacks to reduce speed when you are going too fast and increase it when you are going too slow.
(You can find Lindzen's presentation, with that quote in it, at WUWT)
Lindzen goes on to imply that the climate models act like a car with the brake and gas pedals reversed and how crazy is that so of course the models are wrong. My head spins with the number of wacko concepts jammed into that short phrase, so I won't get into it.
But the whole analogy is seriously flawed. The brake and gas pedals in the car are not feedbacks, they are forcings. They are the primary controls of your car's speed. In the climate system, adding CO2 into the atmosphere is like putting pressure on the gas pedal. Volcanos spewing suphates into the statosphere are applying the brakes. These also, are primary forcings and not feedbacks (though on longer timescales the carbon cycle becomes a feedback mechanism of its own. Things can be both primary forcings and feedbacks, the difference is only in what causes it.)
Back to the car, an actual example of a feedback would be something like air resistance. You apply pressure to the gas pedal and your speed increases. As the speed increases so does the wind resistance acting to reduce your speed, a negative feedback. A feedback is a reaction to a change in the system that itself causes additional changes. It can be reinforcing, like water vapour in the atmosphere reinforces CO2 warming, or it can be mitigating, like greater levels of infrared radiation escaping from the top of the atmophere as temperatures rise.
I can't think of any positive feedbacks in an automobile. That doesn't say anything about anything except automobiles. GM did not design the climate system (why do I find that thought reassuring?)
But really, it is not that tricky a concept, Richard Lindzen thinks he is talking to an audience of willing fools.
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Positive feedback? Automatic transmission going up a hill?
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Thinks? He was :)
Ted: no, that's still negative feedback. The system adjusts itself for optimum torque. It measures the error signal between actual and desired, and uses the sign and magnitude of that feedback signal to adjust settings to return torque to desired value. Which can be either way - negative feedback does not mean only reducing the effect of a forcing, just as positive feedback does not only mean increasing it. Rather it means adjusting the magnitude up or down as necessary to bring it back in line.
General factlet: negative feedback is one of the mechanisms involved in self regulating systems. The study of such systems is called "Cybernetics", a term coined by the father of the subject, Norbert Wiener, back in 1948 (so it is almost as old as I am);
+ve feedback in a car?
There's an elegant little oscillator circuit that turns your indicators on and off. Uses +ve fb to build up charge in one half circuit until it flips and then builds up the other half.
Metabistable eivai.
ugh, that analogy is so bad it makes my head hurt. He couldn't come up with anything better than that?
Given the attempted analogy and the depth of comprehension of forcings and feedback it demonstrated, it would seem the fools were not confined to the audience.
Does it count as negative feedback that when I try to learn more about climate science, I end up encountering the various denialists, who then make me stupider, and therefore I am less able to learn climate science?
Interesting paper on empirical estimation of feedback from Lindzen and Choi
Abstract
Climate feedbacks are estimated from fluctuations in the outgoing radiation budget from the latest version of Earth Radiation Budget Experiment (ERBE) nonscanner data. It
appears, for the entire tropics, the observed outgoing radiation fluxes increase with the increase in sea surface temperatures (SSTs). The observed behavior of radiation fluxes implies negative feedback processes associated with relatively low climate sensitivity. This is the opposite of the behavior of 11 atmospheric models forced by the same SSTs. Therefore, the models display much higher climate sensitivity than is inferred from ERBE, though it is difficult to pin down such high sensitivities with any precision. Results also show, the feedback in ERBE is mostly from shortwave radiation while the feedback in the models is mostly from longwave radiation. Although such a test does not distinguish the mechanisms, this is important since the inconsistency of climate feedbacks constitutes a very fundamental problem in climate prediction.
power steering?
Forgotten driver:
Old gramophones demonstrate a nice example of negative feedback with rotating weights that extend as the speed of the turntable increases. Eventually the weights touch a braking system and decrease the rpm whereupon the weights retract allowing the speed to increase again. The spring acts as the gas pedal with a foot permanently dumped on it.
And in such manner (though with a touch of sophistication) does the ratiocinating part of the human brain act when it senses the car going too fast or too slow. I suggest therefore that Lindzen's example does indeed nicely demonstrate feedback.
alleagra:
Thanks for the comment, but check me if I'm wrong:
Climate does not have a "motorist"--unless you believe in a benevolent God, about which I am skeptical--to make the conscious decision to hit the cooling "brakes". A "negative feedback" proper should be automatic and impersonal, right?
The accelerating and then braking car has the volitional driver. What does climate have other than the laws of physics?
Skip
Coby I have a question and I hope this post is a suitable place to ask:
When you say something is a positive feedback, you mean that it is some mechanism which leads to a greater change in (say) temperature for a given change in (say) CO2, compared to a world in which this mechanism was absent.
Such a mechanism could work by responding to increased CO2, or to increased temperature. Do you draw any distinction between these two, do they have names? And are the feedbacks people talk about of the former or the latter kind?
Power steering is an example of the second kind of feedback: your input (torque on steering wheel) is translated into greater output (force on wheels). I can't think of any car-based examples of the second kind of feedback (i.e. a positive response to the output variable), but I think Linzden was trying (and failing) to invent one.
Typo: That should have been "Power steering is an example of the first kind of feedback" of course.