Feedback

When a warming trend results in effects that induce further warming, the process is referred to as a positive feedback; when the effects induce cooling, the process is referred to as a negative feedback. The primary positive feedback involves water vapor. The primary 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. This provides a powerful negative feedback which stabilizes the climate system over time.

Water vapor feedback

One of the most pronounced positive feedback effects relates to the evaporation of water. If the atmosphere is warmed, the saturation vapour pressure increases, and the quantity 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.

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.

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.

Aerial photograph showing a section of sea ice. The lighter blue areas are melt ponds and the darkest areas are open water, both have a lower albedo than the white sea ice. The melting ice contributes to the ice-albedo feedback.

Ice-albedo

Another important feedback process is ice-albedo feedback.^[45] 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. 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.

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. Methane discharge from permafrost is presently under intensive study.

Clathrate gun hypothesis

Warmer deep ocean temperatures could also release the greenhouse gas methane from the 'frozen' state of the vast deep ocean deposits of methane clathrate, according to the Clathrate Gun Hypothesis, albeit over millenial time-scales. A further release of methane from shallow cold water clathrates is also expected, and is predicted to be faster. Buffett and Archer predict a large release of methane in response to warming, and a large increase in methane stores if oxygen levels in the ocean fall. They offer a "global estimate of 3×10¹⁸ g of carbon (3000 Gton C) in clathrate and 2×10¹⁸ g (2000 Gton C) in methane bubbles. The predicted methane inventory decreases by 85% in response to 3 °C of warming. Conversely, the methane inventory increases by a factor of 2 if the O2 concentration of the deep ocean decreases by 40 μM or carbon rain increases by 50%

Sequestration

Ocean ecosystems' ability to sequester carbon are expected to decline as it warms. This is because the resulting low nutrient levels of the mesopelagic zone (about 200 to 1000 m depth) limits the growth of diatoms in favor of smaller phytoplankton that are poorer biological pumps of carbon.

Attributed and expected effects

lthough it is difficult to connect specific weather events to global warming, an increase in global temperatures may in turn cause broader changes, including glacial retreat, Arctic shrinkage, and worldwide sea level rise. Changes in the amount and pattern of precipitation may result in flooding and drought. There may also be changes in the frequency and intensity of extreme weather events. These changes are not likely to be reversible on timescales shorter than a thousand years. Other effects may include changes in agricultural yields, addition of new trade routes, reduced summer streamflows, species extinctions, and increases in the range of disease vectors.

Some effects on both the natural environment and human life are, at least in part, already being attributed to global warming. A 2001 report by the IPCC suggests that glacier retreat, ice shelf disruption such as that of the Larsen Ice Shelf, sea level rise, changes in rainfall patterns, and increased intensity and frequency of extreme weather events are attributable in part to global warming. Other expected effects include water scarcity in some regions and increased precipitation in others, changes in mountain snowpack, and adverse health effects from warmer temperatures.

Social and economic effects of global warming may be exacerbated by growing population densities in affected areas. Temperate regions are projected to experience some benefits, such as fewer deaths due to cold exposure. A summary of probable effects and recent understanding can be found in the report made for the IPCC Third Assessment Report by Working Group II. The newer IPCC Fourth Assessment Report summary reports that there is observational evidence for an increase in intense tropical cyclone activity in the North Atlantic Ocean since about 1970, in correlation with the increase in sea surface temperature (see Atlantic Multidecadal Oscillation), but that the detection of long-term trends is complicated by the quality of records prior to routine satellite observations. The summary also states that there is no clear trend in the annual worldwide number of tropical cyclones.

Additional anticipated effects include sea level rise of 0.18 to 0.59 meters (0.59 to 1.9 ft) in 2090-2100 relative to 1980-1999, repercussions to agriculture, possible slowing of the thermohaline circulation, reductions in the ozone layer, increasingly intense (but less frequent) hurricanes and extreme weather events, lowering of ocean pH, oxygen depletion in the oceans, and the spread of diseases such as malaria and dengue fever, as well as Lyme disease, hantavirus infections, bubonic plague, and cholera. One study predicts 18% to 35% of a sample of 1,103 animal and plant species would be extinct by 2050, based on future climate projections. However, few mechanistic studies have documented extinctions due to recent climate change and one study suggests that projected rates of extinction are uncertain.

Increased atmospheric CO₂ increases the amount of CO₂ dissolved in the oceans. CO₂ dissolved in the ocean reacts with water to form carbonic acid, resulting in ocean acidification. Ocean surface pH is estimated to have decreased from 8.25 near the beginning of the industrial era to 8.14 by 2004, and is projected to decrease by a further 0.14 to 0.5 units by 2100 as the ocean absorbs more CO₂. Since organisms and ecosystems are adapted to a narrow range of pH, this raises extinction concerns, directly driven by increased atmospheric CO₂, that could disrupt food webs and impact human societies that depend on marine ecosystem services.