Interactions within the climate system often involve complex, nonlinear relationships. All components of the climate system are intimately linked or coupled with all other components, such that changes in one subsystem may involve compen satory changes throughout the entire climate system. These changes may amplify the initial disturbance (anomaly) or dampen it. Interactions that tend to amplify the disturbance are termed positive feedback mechanisms or processes; they operate in such a way that the system is increasingly destabilized. Interactions that tend to dampen the initial disturbance are termed negative feedback mechanisms or processes; they provide a stabilizing influence on the system, tending to preserve the status quo (Prentice and Sarnthein, 1993).
Growth of continental ice sheets provides an example of positive feedback mechanisms. Whatever the initial perturbation of the climate system that led to continental ice-sheet growth in the past (see Section 2.6) once snow and ice persisted year round, the higher continental albedo would have resulted in lower global radiation receipts, hence lower temperature, and a more favorable environment for ice-sheet growth. Clearly, at some point other factors (such as precipitation starvation and bedrock depression) must have come into play as the ice sheet grew in size to reverse this trend toward increasing glacierization of the planet (Budd and Smith, 1981).
Changes in atmospheric C02 concentration also may induce positive feedbacks. As C02 levels increase, there will be an increase in the absorption of longwave (infrared) terrestrial radiation by C02; concomitantly, there will be an increase in longwave absorption by water vapor, resulting from enhanced earth surface and atmospheric infra-red emissions. Lower tropospheric temperatures will thus increase (the "greenhouse effect") though the magnitude of this increase remains controversial (Schneider, 1993; Lindzen, 1993). As atmospheric temperatures increase, the temperature of the upper layers of the ocean may also increase, causing C02 in solution to be released to the atmosphere, thereby reinforcing the trend toward higher temperatures. This (rather simplistic) example of a physical-biochemical feedback is sometimes referred to as the "runaway greenhouse effect." That such an eventuality will occur due to the anthropogenic production of excess C02 is unlikely. It might be argued that as temperatures increase there would be more evaporation from the oceans, increased cloudiness (higher global albedo), and hence a decrease in energy to the system. In addition, higher temperatures at high latitudes, associated with increased poleward advection of moisture, might be accompanied by more snowfall, resulting in higher continental albedo (and/or a shorter snow-free period) and hence lower overall global energy receipts. Such mechanisms are examples of negative feedbacks, whereby the system tends to become stabilized after an initial perturbation.
Interactions between different parts of the climate system that are brought about by a process within the system are considered internal mechanisms of climatic variation. They involve initiation by an internal factor, such as the upwelling of cool deep-ocean water or an unusually persistent snow cover over an extensive area of the land surface, which may be amplified by other components of the climate system and eventually lead to an adjustment in the atmospheric circulation. These adjustments within the climate system may in turn alter, and perhaps eliminate, the original factor that initiated the climatic variation. Generally, such mechanisms are stochastic in nature, so that the climatic consequences are not predictable over timescales much longer than the timescale of the initiating process. By contrast, there are factors external to the climate system that may bring about ("force") adjustments in climate, but those changes have no influence on the initiating factor (Mitchell, 1976). Changes in solar output and/or spectral characteristics, changes in the Earth's orbital parameters, and changes in atmospheric turbidity due to explosive volcanic eruptions are examples of external factors that may cause changes in the climate system but are not affected by those changes (Robock, 1978). Some of these mechanisms of climate variation are deterministic (predictable) as they vary in a known way. This is particularly the case with the Earth's orbital variations, which have been calculated accurately both for periods back in time and into the future (Berger and Loutre, 1991; Berger et al., 1991). There is therefore an element of predictability in the consequent climatic changes, though these may, in turn, depend on the particular internal conditions of the climate system prevailing at the time of the external forcing.
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