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.
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.
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).