Biogeochemical Feedbacks

feedbacks ARE PRocESSES in a system that can either amplify or dampen the system's response to external influences. When the concentration of a certain variable in a subsystem affects the behavior of the entire system, then changes in inputs or concentrations of that variable can result in multiple, coupled responses. It is possible that some of the responses enhance, while others mitigate, the initial response of the system to the external forcing.

In the case of the Earth's system, if a change in the environment leads to additional and enhanced changes in that system, it is said to have resulted in a positive feedback. When a large external forcing results in an even larger response from the affected system, the phenomenon is commonly referred to as a vicious cycle of positive feedback loops. In contrast, if a change in the environment leads to a process that mitigates the change, and results in smaller response from the affected system, it is said to be a negative feedback. Some systems have the ability to regulate their environment and maintain a stable condition by using multiple, interrelated dynamic mechanisms, known as homeostasis. Feedback processes regulate the response of the Earth's system to natural or anthropogenic forcings.

Biogeochemical feedbacks operate in the coupled biosphere-pedosphere-hydrosphere-atmospheresys-tem. Biogeochemical feedbacks include changes in biological activity; atmospheric, water, or soil chemistry; and terrestrial and oceanic uptake of green house gases that affect, or are affected by, changes in atmospheric dynamics. Some of the most important biogeochemical feedbacks include: changes in rates of plant productivity and carbon sequestration by soils due to altered patterns of air temperature and precipitation; changes in temperature that lead to faster decomposition of organic matter stored in peat lands; and changes in atmospheric conditions that may lead to altered patterns of oceanic temperature and circulation that ultimately lead to changes in ocean carbon storage. By modifying the fluxes of greenhouse gases to/from the atmosphere, biosphere, pedosphere, and hydrosphere, feedback processes in the climate system affect the residence time of the gases in the different spheres.

A study released by the National Academy of Sciences in 2001 estimates that half of the anticipated increase in air temperature over the next few years to decades will be attributable to internal feedbacks within the climate system, and the other half to the direct response of external factors that force change in the climate system.

An example of a positive feedback loop that results in amplification of the response of a system to an external influence is one in which increases in atmospheric concentration of carbon dioxide (CO2) lead to increases in air temperature. This, in turn, results in higher rates of organic matter decomposition (respiration) that releases CO2 to the atmosphere—further increasing the concentration of CO2 in the atmosphere. In this case, a negative feedback loop would result in a chain of events leading to decreased concentrations of CO2 in the atmosphere. For example, an increase in CO2 in the atmosphere leads to increases in air temperature, which results in additional plant growth, which takes CO2 from the atmosphere (acts as a sink for atmospheric CO2), reducing atmospheric concentration of CO2.

Another important biogeochemical feedback to climate change is the changes in production of dimethyl sulfide gas (DMS, a common atmospheric aerosol around oceans) by phytoplankton in the upper layers of the ocean. Increased plankton activity during warm atmospheric conditions leads to greater release of DMS from the oceans to the atmosphere. Consequent increase in density and concentration of aerosols in the atmosphere reduces the size of cloud droplets, resulting in increased albedo (reflection of solar radiation by an object) that leads to increases in air temperature—a negative feedback loop.

Biogeochemical feedbacks can be manifested at different spatial and temporal scales and may have significant control of short- and long-term changes in weather and climate at the level of leaves, stands, biomes, or ecosystems. Moreover, biogeochemical feedbacks can be direct or indirect. Direct effects are those processes that influence chemical composition and dynamics of the atmosphere or oceans, and normal functioning of the biosphere. Some of the most prominent direct feedbacks are increased plant productivity due to increasing concentration of CO2 in the atmosphere, a phenomenon known as the CO2 fertilization effect. Indirect feedbacks are those mediated by ecosystem responses to climate changes that result because of increasing concentration of greenhouse gases. Some examples of this include the change in ocean circulation patterns on phytoplankton productivity, which, in turn, affect the amount of CO2 that biological processes in the ocean can sequester.

SEE ALSo: Albedo; Anthropogenic Forcing; Carbon Dioxide; Carbon Sinks; Climate Feedback.

BIBLIoGRAPHY. D.A. Lashof, et al., "Terrestrial Ecosystem Feedbacks to Global Climate Change," Annual Reviews of Energy and the Environment (v.22, 1997); Panel on Climate Change Feedbacks, Climate Research Committee and National Research Council, "Understanding Climate Change Feedbacks" (National Academy of Sciences, 2003).

ASMERET ASEFAW BERHE University of California, Berkeley

Teamrat A. Ghezzehei Lawrence Berkeley National Laboratory

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