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.