Posted by: reasic | 26th Sep, 2007

Feedback Mechanisms

This post is originally written Daniel Rhodes-Mumby at his blog, “This, Our Earth“. I was very impressed with Daniel’s understanding of this subject, and his ability to communicate that understanding, especially considering that he is only 15 years old. So, I figured I’d share this post on the subject of feedbacks.

Greenhouse gases offer a direct forcing of our climate, but perhaps more important are processes called feedback mechanisms, which can serve to multiply a climate shift manyfold or, alternatively, blunt it. A simple example of a feedback mechanism is the melting of ice. If we take a bit of ice covering a bit of ground and then start directing heat energy at it then it will melt eventually, of course.

Now, ice reflects most of the energy directed at it back towards the sources. However, the ground doesn’t; it tends to absorb more heat than it reflects. Therefore, once a bit of ice melts, the ground beneath it warms more. This will warm any nearby ice as well and thus eventually reveal some more ground to warm, which will help melt more ice, which will warm more ground, and so on and so forth. This begins a cycle whereby the effect of a little thing at the start - a small piece of ice melting - can be amplified to have potential effects far beyond its actual scale.

This particular example is an example of a positive feedback cycle. There are two types of feedback mechanisms: negative and positive. A positive feedback cycle reinforces a trend; as the amount of ice goes down in the example, the warming trend quickens as less ice is available to reflect energy. A negative feedback cycle blunts a trend; an example is the increase in vegetation caused by larger quantities of carbon dioxide in the atmosphere. The vegetation takes advantage of the bountiful carbon dioxide and withdraws some from the atmosphere, helping keep carbon dioxide levels stable.

Feedback mechanisms vary in strength and time-scale. The melting of ice tends to happen slowly at first and then at an ever-increasing rate; the vegetation growth takes much longer and is variable in speed from then onwards. Sometimes a feedback mechanism of such magnitude and speed can occur that it can’t be blunted until it’s run its full course; an example would be the clathrate gun hypothesis, in which essentially solid methane at the bottom of the seabed is released by warming temperatures in such massive quantities that warming increases manyfold.

Feedback mechanisms are almost inevitably blunted at some point, though; when the ice runs out, when vegetation becomes overwhelmed with carbon dioxide or when the methane runs out. However, by the time they stop, the effects are usually all manifested, although some mechanisms can have effects which take years to come into play.

Feedback mechanisms can spawn new possibilities which wouldn’t even be considered otherwise; they can kick-start other feedbacks as well. And the new possibilities will be the topic of the next post.

Originally posted at This, Our Earth.