Dynamical Feedbacks

A DYNAMICAL FEEDBACK is a relationship within a system in which the effects caused by one part of the system on another part eventually come back to affect the first part. The easiest such relationship to envision might be a tug of war, in which each side has to pull harder, causing the other side to pull harder, causing the first side to pull harder still. In the study of global warming and climate change, these feedbacks are especially critical.

In a more general sense, a feedback is a relationship found in most complex systems: the output of that system is returned to the input. Negative feedback reduces that output (cancels out some of the effect), while positive feedback increases it. In climate change, as in many complex systems, both negative and positive feedbacks exist; just as with credits and debits in a bank account, the net effect is determined by the strength and magnitude of those multiple gains and losses.

Where global warming is concerned, positive feedbacks are easy to identify: they are all relationships where global warming encourages a trend, which itself contributes to further warming. An oft-cited hypothetical from Earth science classes is the use of chlorofluorocarbon-emitting air conditioning: as summers get hotter, such air conditioners run longer, releasing more CFCs into the air contributing further to the greenhouse effect,and the next summer is hotter still. There are plenty of observable expamples of dynamical feedback. In recent years, there have been more forest fires than usual. Global warming and dry weather contribute to this. In turn, the fires impact the climate, not only through deforestation, but by releasing large amounts of carbon dioxide.

Similarly, warmer temperatures have meant the thawing of permafrost, in the high-latitude soil that has been below the freezing point for at least two years (and often much longer). In western Siberia, areas of permafrost peat bogs that have remained frozen for thousands of years—since the last ice age—have begun to thaw, releasing considerable amounts of methane, which will worsen the greenhouse effect. Methane hydrate—ice containing dissolved methane—exists in enormous deposits on the cold ocean floor, and warming sufficient to melt it would release a great deal of methane all at once—at, it is believed, a much faster rate than has been seen in the past, even from industrial pollution. Methane is a greenhouse gas with an effect on global warming five times that of carbon dioxide.

The carbon cycle, too, can work as a positive dynamical feedback for global warming. As anthropogenic causes increase the total amount of carbon passing through the carbon cycle, more of it remains free in the atmosphere, as the carbon load exceeds what the other phases of the cycle can take in. Various models have found that global warming increases the amount of anthropogenic carbon dioxide remaining in the atmosphere, which, in turn, accelerates global warming; the models disagree only on the extent to which this happens.

The ice-albedo feedback is another clear example. As temperatures near the Earth's polar regions increase, more and more ice melts—either permanently or for the summer—leaving land and water in its place. Ice is a highly reflective surface, that reflects more sunlight back into space does than land or water—and the difference in reflectivity is relevantly high. Over time, as the polar ice retreats, more Sunlight is retained by the Earth's surface, and with it, more heat. Though popular discussion of the polar ice caps melting with regards to global warming focuses on rising sea levels, many models implicate the ice-albedo feedback in their predictions of future catastrophe and major climate change.

Perhaps the greatest feedback is the evaporative feedback. Just like carbon dioxide and methane, water vapor is a greenhouse gas. Higher temperatures lead to greater evaporation of the Earth's surface water. At the same time, the amount of moisture the air can hold increases exponentially with the temperature—as anyone has noticed in a southern summer, the hotter it is, the more humidity or the more water vapor can be held in the air. So globally, as temperatures rise, not only is more water vapor produced, but less of it is forced to condense or precipitate, and the absolute water vapor content of the atmosphere increases—and with it, the temperature rises again.

SEE ALSO: Antarctic Ice Sheets; Biogeochemical Feedbacks; Climate Feedbacks; Evaporation Feedbacks; Ice Albedo Feedback.

BIBLIOGRAPHY. Ken O. Buesseler, et al., "Revisiting Carbon Flux Through the Ocean's Twilight Zone," Science (v.316/5824, 2007); James P. Kennett, Kevin G. Cannariato, Ingrid L. Hendy, and Richard J. Behl, Methane Hydrates in Quaternary Climate Change: The Clathrate Gun Hypothesis (American Geophysical Union, 2002); Gerald A. Meehl ,et al., "How Much More Global Warming and Sea Level Rise?" Science (v.307/5716, 2005); Margaret Torn and John Harte, "Missing Feedbacks, Asymmetric Uncertainties, and The Underestimation of Future Warming," Geophysical Research Letters (v.33/10, 2006).

Bill Kte'pi Independent Scholar

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