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Examples of a positive and a negative feedback from everyday life.
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A positive feedback is a process in which an initial change will bring about an additional change in the same direction. An example of a simple positive feedback in everyday life is the growth of an interest-earning savings account. As interest is accrued the principal will begin to grow (assuming money is not withdrawn). As the principal grows, even more interest will be accrued, quickening the rate of principal growth.
There are also negative feedbacks, processes in which an initial change will bring about an additional change in the opposite direction. An example of a simple negative feedback is your body's cooling mechanism. When your body temperature rises, you begin to sweat. The evaporation of this sweat from your skin cools your body and your temperature returns to normal.
It is positive, rather than negative feedbacks that contribute to abrupt climate changes. In positive feedbacks, a small initial perturbation can yield a large change. Negative feedbacks, on the other hand, stabilize the system by bringing it back to its original state.
What are some examples of positive feedbacks in the climate system?
Ice-albedo feedback
Satellite image of southern Greenland. The ice sheet and sea ice (which is most visible along the east coast) are more reflective than either the land exposed along the coast line or the ocean water.
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Ice has a higher albedo (or reflectivity) than vegetation, soil, or water. As ice expands, more solar radiation is reflected to space, less is absorbed by the surface, and temperatures decrease. Cooler temperatures lead to more ice growth, more reflection of solar radiation back to space, and even cooler temperatures - a positive feedback. But positive ice-albedo feedbacks can work in the opposite direction as well. Once ice begins to melt and uncover land or water, more solar radiation will be absorbed by the surface, raising temperatures and causing even more ice to melt. This positive feedback might act more quickly over the oceans than over land because sea ice can melt faster than large continental ice sheets.
Vegetation feedbacks
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Climate is an important factor in determining the vegetation that can grow in a particular area, but vegetation also affects climate through changes to the hydrologic cycle. This positive feedback can help to sustain either rainforests or deserts. Images courtesy of NASA and USGS.
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Climate strongly influences what types of vegetation grow in a certain area. But, vegetation can also affect climate regionally by altering the ratio of evaporation to precipitation. In this way, a positive feedback can arise. As bare soil is colonized by trees and shrubs, for example, evaporation of water into the atmosphere is increased for two reasons. First, plants have a lower albedo (or reflectivity) than bare soil. This lower albedo increases the amount of solar radiation absorbed at the surface, which increases the amount of energy available for evaporation. Second, plants take water from the soil into their roots and lose this water through their leaves to the atmosphere, a process called transpiration.
These two processes are important because when soil water and surface water evaporate rather than running off to rivers and the oceans, moisture is recycled into the atmosphere where it can form more rain. Enhanced precipitation will sustain the colonizers and may promote the growth of additional plants. This positive feedback may also operate in the opposite direction if, for example, vegetation were to begin to die.
The ocean thermohaline circulation system is a slow, three-dimensional pattern of flow involving the surface and deep oceans around the world. An important feedback exists in this circulation related to the northward transport of salty waters in the Atlantic. Image courtesy of NASA.
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The thermohaline circulation is the part of the global ocean circulation that is driven by geographic differences in the density of sea water, which are controlled by temperature (thermal) and salinity (haline). In the North Atlantic this circulation transports warm and salty water from the tropics to the north. There, the water cools and releases heat to the atmosphere, warming the North Atlantic region. Once the water loses heat, it becomes cooler and more dense, sinking into the deep ocean. This deepwater flows slowly southward (~0.1 m/s) near the bottom of the ocean basins and gradually returns to the surface as a result of wind-driven upwelling near Antarctica and slow diffusive upwelling over the rest of the global ocean. It then joins near-surface currents to be returned to the areas of deepwater formation. This ocean circulation is sometimes referred to as the "ocean conveyor belt."
A key aspect of this global circulation circuit is the northward transport of salty waters to the North Atlantic where, with additional cooling, surface waters become dense enough to sink and form deepwater. Imagine that a flood of freshwater entered the North Atlantic, perhaps from the melting of land-based ice sheets. This would decrease the density of surface waters in the North Atlantic and would likely reduce deepwater formation. The "conveyor" circulation would slow, just as the conveyor belt at the grocery store would slow if an item became jammed in its descending path. Without a strong northward flow of salt, surface water densities in the North Atlantic would continue to decrease and deepwater formation would further weaken - a positive feedback. A positive feedback will also operate as a way of strengthening the circulation at times when the northward flux of salt is enhanced.
To see some of the climate effects of a freshwater flood in the North Atlantic, see this model of abrupt changes in the thermohaline circulation.
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