screeech! go the speakers and people in the auditorium slap their hands to the sides of their heads as the audio system spews out earsplitting noise. In this example of runaway positive feedback, the microphones were either placed too close to the speakers or the speaker volume was turned up too high. In either case, sound from the speaker enters the microphone, and the microphone causes the speakers to emit more sound, which causes more sound to enter the microphone, and so forth until someone turns the sound system off.

A particularly troubling example of positive feedback involves climate change caused by the use of chlorofluorocarbons (CFCs). CFCs are used as refrigerants, as aerosol propellants, and as solvents in the manufacture of high technology items such as computer circuits. Although safe for people, CFCs have proven to be very toxic for the atmosphere. As an unavoidable byproduct of their use, CFCs escape in large quantities into the atmosphere. Ozone, or triatomic oxygen, protects organisms at the surface of the earth by absorbing harmful ultraviolet radiation. Some of this radiation has a frequency which will damage DNA molecules. Such damage can lead to fatal mutations, skin cancer, and other problems.

In addition to destroying the protective layer of atmospheric ozone, CFCs act as greenhouse gases. A greenhouse gas is any volatile chemical compound (carbon dioxide is another example) that traps radiant solar energy after it enters the earth's atmosphere. When sunlight, which passes easily through a greenhouse gas, strikes a dark surface, it is absorbed and reemitted as infrared radiation. With greenhouse gases in the atmosphere, this infrared radiation cannot pass as easily out through the earth's atmosphere as it could if the greenhouse gases were not present. This heat is therefore trapped near the earth's surface by an atmospheric blanket of greenhouse gases. The effectiveness of this blanket depends on the quantity and types of greenhouse gases in the atmosphere. All other things being equal, the more greenhouse gases in the atmosphere, the warmer the earth's climate will be.

Herein lies the positive feedback associated with CFCs. The more CFCs in the atmosphere, the warmer the climate will become. The warmer the climate, the more CFCs will be needed to run air conditioning units, refrigerators, and freezers. Also, as the protective ozone layer is depleted by CFCs, more people will be compelled to use sunblocks (sunscreen) to protect their skin from the damaging effects of solar ultraviolet radiation that passes in increasing amounts through the weakened layer of protective ozone. Some European countries market CFC-powered aerosol sprays of sunblock, the use of which depletes the stratospheric ozone layer further and increases the need for the product. Recently enacted CFC emmision control agreements are intended to limit such positive feedbacks (MacCracken 1987).

Feedback between organisms and their environment may be an important aspect of the global changes that occurred at the Vendian-Cambrian boundary. One of the effects of this paleoecological feedback is a series of extinctions that occurred during the Vendian.

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