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GlossaryUnit 2: Atmosphere // Section 9: Feedbacks in the Atmosphere
Understanding how rising GHG levels may affect Earth's energy balance is complicated because of feedbacks in Earth's climate system. Feedbacks are interactions between climate variables such as temperature, precipitation, and vegetation and elements that control the greenhouse effect, such as clouds and albedo. Positive feedbacks amplify temperature change by making the greenhouse effect stronger or by reducing albedo, so they make the climate system more sensitive to the properties that trigger them. Negative feedbacks have a dampening effect on temperature change, making the climate system less sensitive to the factors that trigger them.
Feedbacks can be very complex processes and may take place over short or long time spans. Important feedbacks in Earth's atmosphere include:
- Water vapor feedback (positive). The atmosphere can hold increasing amounts of water vapor as the temperature rises, because the pressure of water vapor in equilibrium with liquid water increases exponentially with temperature. The presence of more water vapor as temperature rises increases the greenhouse effect, as well as the absorption of solar radiation, which further raises temperature. This is the strongest and best-understood feedback mechanism in the atmosphere, because it is based on the straightforward fact that warm air can hold more water vapor than cool air.
- Cloud feedback on terrestrial radiation (positive). Because warmer temperatures increase water vapor amounts, they can increase cloudiness and further raise temperature. This is a very strong feedback that is not well understood. It is hard to know whether or how much cloudiness will increase as temperature does, because cloudiness depends more on upward air motion than on temperature or water vapor levels directly. (For details on how clouds form, see Section 5, "Vertical Motion in the Atmosphere.")
- Cloud feedback on solar radiation (negative). As temperature increases and atmospheric water vapor levels rise, cloudiness may increase. Greater cloudiness raises Earth's albedo, reflecting an increasing fraction of solar radiation back into space and decreasing temperature, although some cloud types are more reflective than others (Fig. 20). This is another very strong feedback that is not well understood because it is hard to know whether or how much cloudiness will increase with temperature. Also, as noted in the previous example, clouds can also absorb infrared radiation, raising temperatures.
Figure 20. Effects of cirrus and cumulus clouds on Earth's energy balance
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Source: © National Aeronautics and Space Administration. Earth Observatory.
- Vegetation feedback on solar radiation (negative). As temperatures rise, deserts may expand, increasing Earth's albedo and decreasing temperature. This is a very complex feedback. It is uncertain whether deserts will expand, or conversely, whether higher CO2 levels might stimulate higher plant growth levels and increase vegetation instead of reducing it.
- Ice-albedo feedback on solar radiation (positive). Rising temperatures cause polar glaciers and floating ice sheets to recede, decreasing Earth's albedo and raising temperatures. This feedback is very strong at times when polar ice has expanded widely, such as at the peak of ice ages. It can work in both directions, helping ice sheets to advance as Earth cools and accelerating the retreat of ice sheets during warming periods. There is relatively little polar ice on land today, so this feedback is not likely to play a major role in near-term climate change. However, temperature increases large enough to melt most or all of the floating ice in the Arctic could sharply accelerate global climate change, because ocean water absorbs almost all of the incident solar radiation whereas ice reflects most sunlight.
Feedbacks cause much of the uncertainty in today's climate change models, and more research is needed to understand how these relationships work. A 2003 National Research Council study called for better measurement of many factors that affect climate feedbacks, including temperature, humidity, the distribution and properties of clouds, the extent of snow cover and sea ice, and atmospheric GHG concentrations (footnote 2).