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 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.
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.
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.
Downloaded Tuesday, 22-Jul-2014 07:34:22 EDT
Last Updated Wednesday, 20-Aug-2008 11:21:31 EDT by firstname.lastname@example.org
Please see the Paleoclimatology Contact Page or the NCDC Contact Page if you have questions or comments.