Feedback Mechanisms
The energy budget of the climate system, based largely on satellite data; the numbers inside the circles are globally averaged, and annually averaged flows in units that reflect the percentage of solar energy Earth receives in a year. The numbers in the boxes are the amounts of thermal energy stored in the atmosphere and surface reservoirs (the surface, in this case, is mainly the surface water of the oceans). The two large flows on the right represent a kind of energy recycling program that constitutes the greenhouse effect; heat emitted from Earth’s surface is absorbed by gases in the atmosphere and then re-radiated back to the surface.
Here 100 energy units = 5.56e24J/year, the total annual solar energy received averages 342 W/m^@ over the surface of the Earth
Incoming solar radiation: 100
Insolation Reflected by Clouds and Aerosols: 23
Insolation Reflected off Land Surface: 9
Insolation Absorbed by Surface: 49
Atmosphere Reservoir: 16.5
Surface Reservoir (30% Land, 70% Water): 271.2
Heat Transfer to Atmosphere: 133
Heat Lost to Space: 11
Heat returned to Surface (Greenhouse Effect): 95
Heat Radiated into Space from top of Atmosphere: 57
The view of the climate system depicted in the adjacent figure is one of stability — energy flows in and out, in perfect balance, so the temperature of the earth should stay the same. But if we can learn anything from studying Earth’s history, we learn that change is the rule and stability the exception. When change occurs, it almost always brings feedback mechanisms into play — they can accentuate and dampen change, and they are incredibly important to our climate system. There are many good examples of feedback mechanisms, but here are a few to illustrate the idea.
Ice — Albedo Feedback
Ice reflects sunlight better than almost any other material on Earth, and in reflecting sunlight, it lowers the amount of insolation absorbed by Earth, which makes it colder. If the Earth becomes colder, more ice may grow, covering more area and thus reflecting even more insolation, which in turn cools the Earth further. Thus cooling instigates ice expansion, which promotes additional cooling, and so on — this is clearly a cycle that feeds back on itself to encourage the initial change. Since this chain of events furthers the initial change that triggered the whole thing, it is called a positive feedback (but note that the change may not be good from our perspective). Positive feedback mechanisms tend to lead to runaway change — some small initial change is thus accentuated into a major change.
Weathering Feedback
Rocks exposed at the surface interact with water and the atmosphere and undergo a set of chemical and physical changes we call weathering. The chemical part of weathering often involves the consumption of carbonic acid (formed from water and carbon dioxide) in dissolving minerals in rocks. This process of weathering is thus a sink for atmospheric carbon dioxide, which is an important greenhouse gas. If you remove carbon dioxide from the atmosphere, you weaken the greenhouse effect and this leads to cooling of the Earth. Like many chemical reactions, this chemical weathering occurs more rapidly in hotter climates, which are associated with higher levels of carbon dioxide. So consider a scenario in which some warming occurs; this will encourage faster weathering, which will consume carbon dioxide, which will lead to cooling. In this case, the initial change triggered a set of processes that countered the initial change — this is called a negative feedback (even though it may have beneficial results) because it works in opposition to the change that triggered it.
Cloud Feedback
Another important negative feedback mechanism involves the formation of clouds. On the whole, clouds in today's climate have a slight net cooling effect — this is the balance of the increased albedo due to low clouds and the increased greenhouse effect caused by high cirrus clouds. As a general rule, as the atmosphere gets warmer, it can hold more water vapor, and with more water vapor, we expect more clouds, and the increased clouds will then tend to limit the warming that initiated the increased clouds — thus we have another negative feedback mechanism.
Positive and Negative Feedbacks — Yin and Yang
In Asian philosophy, yin and yang can be thought of as interacting, interconnected forces that are essential components of a dynamic system. In the Earth system, positive and negative feedbacks are a bit like yin and yang — they are essential components of the whole system that ultimately play an important role in maintaining a more or less stable state. Positive feedback mechanisms enhance or amplify some initial change, while negative feedback mechanisms stabilize a system and prevent it from getting into extreme states. In many respects, the history of Earth’s climate system can be seen as a bit of a battle between these two types of feedback, but in the end, the negative feedbacks win out and our climate is generally stable with a limited range of change (excepting, of course, a few extremes such as the Snowball Earth events back around 750 Myr ago).
Schematic illustration of two feedback mechanisms that are important in Earth’s climate system. Each feedback mechanism, as depicted above, may be triggered by either a warming or a cooling; in either case, they trigger an amplifying or countering effect. Note that the positive feedback mechanism, left to its own devices, could lead to runaway cooling and a completely frozen Earth, but positive feedbacks will generally activate negative feedback mechanisms that limit the runaway tendency of positive feedbacks
Positive Feedback Mechanism
Cooling → Ice Growth → Increase Albedo → Less Insolation Absorbed → Cooling
Warming → Ice Melthing → Decrease Albedo → More Insolation Absorbed → Warming
Negative Feedback Mechanism
Warming → Increased Weathering → Weaker Greenhouse → Cooling
Cooling → Decreased Weathering → Stronger Greenhouse → Warming