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 similar to an active urban center.

Feedback mechanisms are said to be either positive or negative, and the balance and interplay between these two different species of feedback is often the backbone of self-regulating dynamical systems: the global climate is no exception. Positive feedback mechanisms are those in which the action of the mechanism serves to increase the parameter representing the input of the mechanism itself. If the efficacy of the mechanism for producing some compound A depends (in part) on the availability of another compound B and the mechanism which produces compound B also produces compound A, then the operation of these two mechanisms can form a positive feedback loop—as more B is produced, more A is produced, which in turn causes B to be produced at a greater rate, and so on. Consider, for example, two teenage lovers (call them Romeo and Juliet) who are particularly receptive to each other’s affections. As Romeo shows more amorous interest in Juliet, she becomes more smitten with him as well. In response, Romeo—excited by the attention of such a beautiful young woman—becomes still more affectionate. Once the two teenagers are brought into the right sort of contact—once they’re aware of each other’s romantic feelings—their affection for each other will rapidly grow. Positive feedback mechanisms are perhaps best described as “runaway” mechanisms; unless they’re checked (either by other mechanisms that are part of the system itself or by a change in input from the system’s environment), they will tend to increase the value of some parameter of the system without limit. In the case of Romeo and Juliet, it’s easy to see that once the cycle is started, the romantic feelings that each of them has toward the other will, if left unchecked, grow without bound. This can, for obvious reasons, lead to serious instability in the overall system—most interesting systems cannot withstand the unbounded increase of any of their 179