Feedback

When a warming trend results in effects that induce further warming, the process is referred to as a positive feedback; when the warming results in effects that act to reduce the original warming, the process is referred to as a negative feedback. The main positive feedback involves the tendency of warming to increase the amount of water vapor in the atmosphere. The main negative feedback is the effect of temperature on emission of infrared radiation: as the temperature of a body increases, the emitted radiation increases with the fourth power of its absolute temperature. Water vapor feedback If the atmosphere is warmed the saturation vapour pressure increases, and the amount of water vapor in the atmosphere will tend to increase. Since water vapor is a greenhouse gas the increase in water vapor content makes the atmosphere warm further; this warming causes the atmosphere to hold still more water vapor (a positive feedback), and so on until other processes stop the feedback loop. The result is a much larger greenhouse effect than that due to CO2 alone. Although this feedback process causes an increase in the absolute moisture content of the air, the relative humidity stays nearly constant or even decreases slightly because the air is warmer.[45] Clouds Feedback effects due to clouds are an area of ongoing research. Seen from below, clouds emit infrared radiation back to the surface, and so exert a warming effect; seen from above, clouds reflect sunlight and emit infrared radiation to space, and so exert a cooling effect. Whether the net effect is warming or cooling depends on details such as the type and altitude of the cloud, details that have been difficult to represent in climate models.[45] Lapse rate A subtler feedback process relates to changes in the lapse rate as the atmosphere warms. The atmosphere's temperature decreases with height in the troposphere. Since emission of infrared radiation varies with the fourth power of temperature, longwave radiation emitted from the upper atmosphere is less than that emitted from the lower atmosphere. Most of the radiation emitted from the upper atmosphere escapes to space, while most of the radiation emitted from the lower atmosphere is re-absorbed by the surface or the atmosphere. Thus, the strength of the greenhouse effect depends on the atmosphere's rate of temperature decrease with height: if the rate of temperature decrease is greater the greenhouse effect will be stronger, and if the rate of temperature decrease is smaller then the greenhouse effect will be weaker. Both theory and climate models indicate that with increased greenhouse gas content the rate of temperature decrease with height will be reduced, producing a negative lapse rate feedback that weakens the greenhouse effect. Measurements of the rate of temperature change with height are very sensitive to small errors in observations, making it difficult to establish whether the models agree with observations.[46] Ice-albedo feedback When global temperatures increase, ice near the poles melts at an increasing rate. As the ice melts, land or open water takes its place. Both land and open water are on average less reflective than ice, and thus absorb more solar radiation. This causes more warming, which in turn causes more melting, and this cycle continues.[47] Rapid Arctic shrinkage is already occurring, with 2007 being the lowest ever recorded sea ice area. Some models suggest that tipping points exist, leading to a potentially rapid collapse of sea ice cover in the Arctic.[48] Arctic methane release Warming is also the triggering variable for the release of methane from sources both on land and on the deep ocean floor, making both of these possible feedback effects. Thawing permafrost, such as the frozen peat bogs in Siberia, creates a positive feedback due to the potentially rapid release of CO2 and CH4.[49][unreliable source?] Methane discharge from permafrost is presently under intensive study.[citation needed] Reduced absorption of CO2 by the oceans Ocean ecosystems' ability to sequester carbon are expected to decline as the oceans warm. This is because warming reduces the nutrient levels of the mesopelagic zone (about 200 to 1000 m depth), which limits the growth of diatoms in favor of smaller phytoplankton that are poorer biological pumps of carbon.[50]