in their behavior. Consider, for instance, the relationship between planetary albedo and warming. Albedo, as you may recall from Chapter Four is a value representing the reflectivity of a given surface. Albedo ranges from 0 to 1, with higher values representing greater reflectivity. Albedo is associated with one of the most well-documented positive feedback mechanisms in the global climate. As the planet warms, the area of the planet covered by snow and ice tends to decrease. Snow and ice, being white and highly reflective, have a fairly high albedo when compared with either open water or bare land. As more ice melts, then, the planetary (and local) albedo decreases. This results in more radiation being absorbed, leading to increased warming and further melting. It’s easy to see that unchecked, this process could facilitate runaway climate warming, which each small increase in temperature encouraging further, larger increases. This positive feedback is left out of more basic climate models, which lack the formal structure to account for such nuanced behavior.
Perhaps the most significant set of positive feedback mechanisms associated with the long-term behavior of the global climate are those that influence the capacity of the oceans to act as a carbon sink. The planetary oceans are the largest carbon sinks and reservoirs in the global climate system, containing 93% of the planet’s exchangeable carbon. The ocean and the atmosphere exchange something on the order of 100 gigatonnes (Gt) of carbon (mostly as CO2) each year via diffusion (a mechanism known as the “solubility pump”) and the exchange of
- At least past a certain tipping point. Very small amounts of warming can (and have) produced expanding sea ice, especially in the Antarctic. The explanation for this involves the capacity of air of different temperatures to bear moisture. Antarctica, historically the coldest place on Earth, is often so cold that snowfall is limited by the temperature related lack of humidity. As the Antarctic continent has warmed slightly, its capacity for storing moisture has increased, leading to higher levels of precipitation in some locations. This effect is, however, both highly localized and transient. Continued warming will rapidly undo the gains associated with this phenomenon.
- Feely et. al. (2007)
- That is, 93% of the carbon that can be passed between the three active carbon reservoirs (land, ocean, and atmosphere), and thus is not sequestered (e.g. by being locked up in carbon-based minerals in the Earth’s mantle).