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Feedback LoopsSystem feedback loops serve as "cause-effect" pathways to manage flows of activity in natural systems. These pathways connect the results of a change in one part of the system, out to other parts of the system and then back to the first part.
A classic example is the change in abundance of a predator and its prey. As the number of prey animals increases, the number of the predators also often increases. This puts more pressure on the prey species, which then declines. Eventually, the predator's food source (the prey) becomes in short supply and the number of predators falls. Negative feedback causes the initial change to be reduced, slowing down change and helping to regulate the system and keep it in balance. Positive feedback causes the initial change to be expanded, accelerating change and destabilizing the system so that it moves towards breakdown or a major new change in the whole system. Scientists can model and predict with increasing confidence the effects that rising concentrations of greenhouse gases will have on heat trapping and therefore on temperature and weather patterns. However, changes to climate affect the land surface, ice caps, and ocean currents. Changes to land and ocean conditions in turn affect the activity of living organisms. It is much harder to understand and predict, and almost impossible to model, many of these more complicated feedback loops in the climate system. Therefore we know much less about how feedback loops will ultimately affect global climate. This introduces a great deal of uncertainty into the science of predicting climate change.
Most of the poorly understood feedback loops recognized so far in the global climate system are positive feedback loops, leading some scientists to believe that the uncertainties in predicting climate change may understate the true problem. Once started, positive feedback loops can develop their own
momentum and may lead to climate "surprises" or what some scientists are now referring to as "runaway climate change." Click here for more explanation on radiation and the climate system. Examples of biological feedbacks related to higher atmospheric levels of greenhouse gases
Release of carbon from forest fires, which increase with hotter, drier conditions Trees are storehouses of carbon, which is absorbed during photosynthesis and used for growth. Forest fires have two important impacts on the carbon balance: first, the stored carbon is released back into the atmosphere, adding to overall emissions levels; and second, there are now fewer trees available to remove CO2 from the air through photosynthesis. For more information, link to: unep.no
Positive feedback Forest dieback and reduced growth due to stress caused by climate change, which reduces the amount of CO2 absorption by trees Trees live a long time, they reproduce slowly and many are quite sensitive to the climate in which they grow, all of which make it hard for them to adapt to relatively rapid changes in temperature and moisture. The boreal forest in particular, is expected to be vulnerable to climate change. As trees become stressed by drought and temperature increases, they are more prone to pests and dieback, as well as fires. Tree loss reduces capacity to trap carbon and large scale deforestation can also lead to lower rainfall as evaporation is reduced. 3 Positive feedback Release of methane and other greenhouse gases from wetlands, peatlands, permafrost and arctic sediments as temperatures increase Methane is a more powerful greenhouse gas than carbon dioxide. Wetlands and peatlands store carbon but are also a major source of methane, CO2 and other greenhouse gases. A warming climate is expected to disrupt this balance, with more gases being released by wetlands and peatlands than are stored. Permafrost and other sediments at high latitudes contain large amounts of methane hydrates (methane associated with water) resulting from the decomposition of organic matter in the sediments. Methane hydrates are also found beneath continental shelves. The behaviour of these compounds is not well understood but they are known to be very sensitive to temperature. Positive feedback Changes in soil nutrient cycling, including the carbon cycle, and activity of soil microorganisms Chemical and biological interactions in soil are complex and still not completely understood. Many factors, in addition to temperature, influence whether soil carbon is stored or released. However warm tropical soils store very little carbon and cool temperate soils store a lot more. When air and soil temperatures increase activity by soil microbes (bacteria, fungi and invertebrates), then CO2 emissions will increase. Just like people when they exercise, increased microbial activity increases respiration, which produces more carbon dioxide as a waste product. Uncertain, some positive feedback Increased growth in some plants due to increased amounts of CO2 in the atmosphere and warmer temperatures Plants absorb CO2 during photosynthesis and, under controlled conditions, some plants have grown better when exposed to higher concentrations of CO2. In CO2-enriched environment, some plants may grow more quickly, thus removing more carbon from the atmosphere. However, researchers at Stanford University in California found that despite higher CO2 levels, plant growth can actually drop if moisture and temperature levels creep up too. 4 Uncertain, possibly negative feedback Examples of climate system feedback loops related to higher atmospheric concentrations of greenhouse gases: A warmer atmosphere will contain more water vapour a powerful greenhouse gas 5 As the atmosphere warms, more water will evaporate from the ocean and from wet land surfaces. Thus, generally, a warm atmosphere will be wetter, with a higher water vapour content, yielding a positive feedback. Positive feedback Clouds both absorb thermal radiation and emit thermal radiation, so the effect depends on the physical properties of the clouds Clouds affect radiation in the atmosphere in two ways. They reduce the total amount of energy available by reflecting some solar radiation back to space. They also behave similarly to greenhouse gases, in that they absorb radiation from the Earth thereby reducing the amount of heat lost to space. Which effect dominates depends on the cloud's physical and optical properties. In general, low clouds tend to have a cooling effect while high clouds tend to warm the system. 6 Positive and negative feedback Reduced albedo from melting ice and snow "Albedo" is the proportion of light (short wave radiation) that is reflected by a surface. Light-coloured surfaces such as snow and ice reflect much more light than do vegetation or soil. As ice and snow cover shrinks due to warmer atmospheric temperatures, less light is reflected and more is absorbed and converted to long wave thermal energy, leading to increased warming. Positive feedback
Other essential global systems interact with greenhouse gases and can also create feedback loops. . The thinning of the ozone layer (caused by emissions of ozone-depleting substances such as CFCs) increases penetration by the sun's ultraviolet radiation. This harmful UV radiation increases the risk of skin cancer, but it also affects the ability of tiny organisms called phytoplankton in the ocean to live and absorb CO2 through photosynthesis. These biological processes in the ocean play an important role in regulating global climate balance by taking CO2 out of the atmosphere and storing it (an example of carbon sequestration). Global climate feedback loops are complicated and not well understood, which adds a degree of uncertainty to predictions of the size and impacts of climate change. Nevertheless, the potential for runaway climate change should be taken seriously because, once feedback loops are set in motion, they may be difficult or impossible to reverse.
3. Source: Houghton, John. 1997. Global Warming: The Complete Briefing, 2nd edition. New York: Cambridge University Press. |
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Example of feedback
How it works
Expected effect on climate change
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