When is Positive Feedback Really Negative Feedback?

April 6th, 2009 by Roy W. Spencer, Ph. D.

I get an amazing number of e-mails from engineers who point out that the climate system can not be dominated by positive feedback, because that would mean the climate is unstable, in which case it would have careened out of control long ago.

So, I have to keep explaining to them that climate researchers have ‘redefined’ positive feedback. We borrowed the concept from electric circuit theory, which was elucidated back in the 1940s. And, yes, all of you engineers are right…in your terms, the climate system IS dominated by negative feedback. The Earth DOES lose extra energy to outer space when it warms, which then stabilizes the climate system against perturbations.

But in the climate research world, the dividing line between ‘positive’ and ‘negative’ feedback is not whether extra energy is gained or lost with warming, but whether the increase is greater (or not) than the ‘temperature-only’ increase in infrared energy loss with warming.

That temperature-only response is what you feel when you turn up the stove, or make a fire bigger — hotter objects give off more radiant energy. Positive feedback in the climate system is defined as any indirect change in response to the warming (for instance, fewer low clouds) which then reduces the energy loss below the temperature-only response. This causes more warming to occur before energy equilibrium is once more restored.

In the absence of feedbacks, this temperature-only response is estimated to be about 3.3 Watts per sq. meter per degree C at the effective radiating temperature of the Earth, which is about 255 Kelvin (-18 deg. C).

Of course, what this also means is that if positive feedbacks exceeded that 3.3 Watts, then we really DO have an unstable climate system. So, in some sense, the climate system is always 3.3 Watts in positive feedback away from oblivion.

If positive feedbacks decrease the response from 3.3 to 1 Watt, then warming from a doubling of atmospheric CO2 would be almost 4 deg. C. That’s a lot of warming.

But if negative feedbacks dominate, and they increase that 3.3 Watts to, say, 3.7 Watts then there would only be 1 deg. C of warming with doubling of CO2.

Or, if (like our latest research suggests), the loss of radiant energy is as large as 6 Watts per degree of warming, then manmade global warming becomes only 0.6 deg. C.

Obviously, the feedbacks operating in the real climate system is the most important question in global warming research. But, unfortunately, there is relatively little research going on in this area. Everyone is too busy playing with computerized climate models.

But you engineers are indeed correct: When we climate researchers talk about positive feedback, what we are really talking about (in electrical engineering terms) is weak negative feedback.

Comments are closed.