Climate change feedbacks
Climate change feedbacks are natural processes which impact how much global temperatures will increase for a given amount of
While the overall sum of feedbacks is negative, it is becoming less negative as greenhouse gas emissions continue. This means that warming is slower than it would be in the absence of feedbacks, but that warming will accelerate if emissions continue at current levels.[4]: 95–96 Net feedbacks will stay negative largely because of increased thermal radiation as the planet warms, which is an effect that is several times larger than any other singular feedback.[4]: 96 Accordingly, anthropogenic climate change alone cannot cause a runaway greenhouse effect.[5][6]
Feedbacks can be divided into physical feedbacks and partially biological feedbacks. Physical feedbacks include decreased
Feedback strengths and relationships are estimated through global climate models, with their estimates calibrated against observational data whenever possible.[4]: 967 Some feedbacks rapidly impact climate sensitivity, while the feedback response from ice sheets is drawn out over several centuries.[7]: 967 Feedbacks can also result in localized differences, such as polar amplification resulting from feedbacks that include reduced snow and ice cover. While basic relationships are well understood, feedback uncertainty exists in certain areas, particularly regarding cloud feedbacks.[9][10] Carbon cycle uncertainty is driven by the large rates at which CO2 is both absorbed into plants and released when biomass burns or decays. For instance, permafrost thaw produces both CO2 and methane emissions in ways that are difficult to model.[8]: 677 Climate change scenarios use models to estimate how Earth will respond to greenhouse gas emissions over time, including how feedbacks will change as the planet warms.[11]
Definition and terminology
The
A feedback that amplifies an initial change is called a positive feedback[12] while a feedback that reduces an initial change is called a negative feedback.[12] Climate change feedbacks are in the context of global warming, so positive feedbacks enhance warming and negative feedbacks diminish it. Naming a feedback positive or negative does not imply that the feedback is good or bad.[13]
The initial change that triggers a feedback may be
[14]: 2222 External forcing refers to "a forcing agent outside the climate system causing a change in the climate system"[14]: 2229 that may push the climate system in the direction of warming or cooling.[15][16] External forcings may be human-caused (for example, greenhouse gas emissions or land use change) or natural (for example, volcanic eruptions).[14]: 2229Physical feedbacks
Planck response (negative)
Planck response is "the most fundamental feedback in the climate system".
The Planck "feedback" or
Most extensive "grey body" properties of Earth that influence the outgoing radiation are usually postulated to be encompassed by the other GCM feedback components, and to be distributed in accordance with a particular forcing-feedback formulation of the climate system.[21] Ideally the Planck response strength obtained from GCMs, indirect measurements, and black body estimates will further converge as analysis methods continue to mature.[20]
Water vapor feedback (positive)
According to
Since water vapor is a greenhouse gas, the increase in water vapor content makes the atmosphere warm further, which allows the atmosphere to hold still more water vapor. Thus, a positive feedback loop is formed, which continues until the negative feedbacks bring the system to equilibrium.[7]: 969 Increases in atmospheric water vapor have been detected from satellites, and calculations based on these observations place this feedback strength at 1.85 ± 0.32 m2/K. This is very similar to model estimates, which are at 1.77 ± 0.20 m2/K[7]: 969 Either value effectively doubles the warming that would otherwise occur from CO2 increases alone.[24] Like with the other physical feedbacks, this is already accounted for in the warming projections under climate change scenarios.[25]
Lapse rate (negative)
The lapse rate is the rate at which an atmospheric variable, normally
The atmosphere's temperature decreases with height in the troposphere. Since emission of infrared radiation varies with temperature, longwave radiation escaping to space from the relatively cold upper atmosphere is less than that emitted toward the ground from the lower atmosphere. Thus, the strength of the greenhouse effect depends on the atmosphere's rate of temperature decrease with height. Both theory and climate models indicate that global warming will reduce the rate of temperature decrease with height, producing a negative lapse rate feedback that weakens the greenhouse effect.[30]
Surface albedo feedback (positive)
Albedo is the measure of how strongly the planetary surface can reflect solar radiation, which prevents its absorption and thus has a cooling effect. Brighter and more reflective surfaces have a high albedo and darker surfaces have a low albedo, so they heat up more. The most reflective surfaces are ice and snow, so surface albedo changes are overwhelmingly associated with what is known as the ice-albedo feedback. A minority of the effect is also associated with changes in physical oceanography, soil moisture and vegetation cover.[7]: 970
The presence of ice cover and
As of 2021, the total surface feedback strength is estimated at 0.35 [0.10 to 0.60] W m2/K.[4]: 95 On its own, Arctic sea ice decline between 1979 and 2011 was responsible for 0.21 (W/m2) of radiative forcing. This is equivalent to a quarter of impact from CO2 emissions over the same period.[36] The combined change in all sea ice cover between 1992 and 2018 is equivalent to 10% of all the anthropogenic greenhouse gas emissions.[43] Ice-albedo feedback strength is not constant and depends on the rate of ice loss - models project that under high warming, its strength peaks around 2100 and declines afterwards, as most easily melted ice would already be lost by then.[44]
When
Cloud feedback (positive)
Seen from below, clouds emit infrared radiation back to the surface, which has a warming effect; seen from above, clouds reflect sunlight and emit infrared radiation to space, leading to a cooling effect. Low clouds are bright and very reflective, so they lead to strong cooling, while high clouds are too thin and transparent to effectively reflect sunlight, so they cause overall warming.[48] As a whole, clouds have a substantial cooling effect.[7]: 1022 However, climate change is expected to alter the distribution of cloud types in a way which collectively reduces their cooling and thus accelerates overall warming.[7]: 975 While changes to clouds act as a negative feedback in some latitudes,[26] they represent a clear positive feedback on a global scale.[4]: 95
As of 2021, cloud feedback strength is estimated at 0.42 [–0.10 to 0.94] W m2/K.
Thus, estimates of cloud feedback differ sharply between climate models. Models with the strongest cloud feedback have the highest
Biogeophysical and biogeochemical feedbacks
CO2 feedbacks (mostly negative)
There are positive and negative climate feedbacks from Earth's carbon cycle. Negative feedbacks are large, and play a great role in the studies of
Altogether,
Role of oceans
Following Le Chatelier's principle, the chemical equilibrium of the Earth's carbon cycle will shift in response to anthropogenic CO2 emissions. The primary driver of this is the ocean, which absorbs anthropogenic CO2 via the so-called solubility pump. At present this accounts for only about one third of the current emissions, but ultimately most (~75%) of the CO2 emitted by human activities will dissolve in the ocean over a period of centuries: "A better approximation of the lifetime of fossil fuel CO2 for public discussion might be 300 years, plus 25% that lasts forever".[63] However, the rate at which the ocean will take it up in the future is less certain, and will be affected by stratification induced by warming and, potentially, changes in the ocean's thermohaline circulation. It is believed that the single largest factor in determining the total strength of the global carbon sink is the state of the Southern Ocean - particularly of the Southern Ocean overturning circulation.[5]
Chemical weathering
Primary production through photosynthesis
Non-CO2 (unclear)
Release of gases of biological origin would be affected by global warming, and this includes climate-relevant gases such as
On the other hand, changes in emissions of compounds such sea salt, dimethyl sulphide, dust, ozone and a range of biogenic volatile organic compounds are expected to be negative overall. As of 2021, all of these non-CO2 feedbacks are believed to practically cancel each other out, but there is only low confidence, and the combined feedbacks could be up to 0.25 W m2/K in either direction.[7]: 967
Permafrost (positive)
Permafrost is not included in the estimates above, as it is difficult to model, and the estimates of its role is strongly time-dependent as its carbon pools are depleted at different rates under different warming levels.[7]: 967 Instead, it is treated as a separate process that will contribute to near-term warming, with the best estimates shown below.
Altogether, it is expected that cumulative greenhouse gas emissions from permafrost thaw will be smaller than the cumulative anthropogenic emissions, yet still substantial on a global scale, with some experts comparing them to emissions caused by deforestation.[74] The IPCC Sixth Assessment Report estimates that carbon dioxide and methane released from permafrost could amount to the equivalent of 14–175 billion tonnes of carbon dioxide per 1 °C (1.8 °F) of warming.[75]: 1237 For comparison, by 2019, annual anthropogenic emissions of carbon dioxide alone stood around 40 billion tonnes.[75]: 1237 A major review published in the year 2022 concluded that if the goal of preventing 2 °C (3.6 °F) of warming was realized, then the average annual permafrost emissions throughout the 21st century would be equivalent to the year 2019 annual emissions of Russia. Under RCP4.5, a scenario considered close to the current trajectory and where the warming stays slightly below 3 °C (5.4 °F), annual permafrost emissions would be comparable to year 2019 emissions of Western Europe or the United States, while under the scenario of high global warming and worst-case permafrost feedback response, they would approach year 2019 emissions of China.[74]
Fewer studies have attempted to describe the impact directly in terms of warming. A 2018 paper estimated that if global warming was limited to 2 °C (3.6 °F), gradual permafrost thaw would add around 0.09 °C (0.16 °F) to global temperatures by 2100,[76] while a 2022 review concluded that every 1 °C (1.8 °F) of global warming would cause 0.04 °C (0.072 °F) and 0.11 °C (0.20 °F) from abrupt thaw by the year 2100 and 2300. Around 4 °C (7.2 °F) of global warming, abrupt (around 50 years) and widespread collapse of permafrost areas could occur, resulting in an additional warming of 0.2–0.4 °C (0.36–0.72 °F).[77][78]Long-term feedbacks
Ice sheets
The Earth's two remaining ice sheets, the Greenland ice sheet and the Antarctic ice sheet, cover the world's largest island and an entire continent, and both of them are also around 2 km (1 mi) thick on average.[79][80] Due to this immense size, their response to warming is measured in thousands of years and is believed to occur in two stages.[7]: 977
The first stage would be the effect from ice melt on thermohaline circulation. Because meltwater is completely fresh, it makes it harder for the surface layer of water to sink beneath the lower layers, and this disrupts the exchange of oxygen, nutrients and heat between the layers. This would act as a negative feedback - sometimes estimated as a cooling effect of 0.2 °C (0.36 °F) over a 1000-year average, though the research on these timescales has been limited.[7]: 977 An even longer-term effect is the ice-albedo feedback from ice sheets reaching their ultimate state in response to whatever the long-term temperature change would be. Unless the warming is reversed entirely, this feedback would be positive.[7]: 977
The total loss of the Greenland Ice Sheet is estimated to add 0.13 °C (0.23 °F) to global warming (with a range of 0.04–0.06 °C), while the loss of the West Antarctic Ice Sheet adds 0.05 °C (0.090 °F) (0.04–0.06 °C), and East Antarctic ice sheet 0.6 °C (1.1 °F)[45] Total loss of the Greenland ice sheet would also increase regional temperatures in the Arctic by between 0.5 °C (0.90 °F) and 3 °C (5.4 °F), while the regional temperature in Antarctica is likely to go up by 1 °C (1.8 °F) after the loss of the West Antarctic ice sheet and 2 °C (3.6 °F) after the loss of the East Antarctic ice sheet.[60][61]
These estimates assume that global warming stays at an average of 1.5 °C (2.7 °F). Because of the logarithmic growth of the greenhouse effect,[4]: 80 the impact from ice loss would be larger at the slightly lower warming level of 2020s, but it would become lower if the warming proceeds towards higher levels.[45] While Greenland and the West Antartic ice sheet are likely committed to melting entirely if the long-term warming is around 1.5 °C (2.7 °F), the East Antarctic ice sheet would not be at risk of complete disappearance until the very high global warming of 5–10 °C (9.0–18.0 °F)[60][61]
Methane hydrates
Methane hydrates or
Mathematical formulation of global energy imbalance
Earth is a
where ASR is the absorbed
In order to diagnose that behavior around a relatively stable equilibrium state, one may consider a perturbation to EEI as indicated by the symbol Δ. Such a perturbation is induced by a radiative forcing (ΔF) which can be natural or man-made. Responses within the system to either return back towards the stable state, or to move further away from the stable state are called feedbacks λΔT:
- .
Collectively the feedbacks are approximated by the linearized parameter λ and the perturbed temperature ΔT because all components of λ (assumed to be first-order to act independently and additively) are also functions of temperature, albeit to varying extents, by definition for a thermodynamic system:
- .
Some feedback components having significant influence on EEI are: = water vapor, = clouds, = surface albedo, = carbon cycle, = Planck response, and = lapse rate. All quantities are understood to be global averages, while T is usually translated to temperature at the surface because of its direct relevance to humans and much other life.[21]
The negative Planck response, being an especially strong function of temperature, is sometimes factored out to give an expression in terms of the relative feedback gains gi from other components:
- .
For example for the water vapor feedback.
Within the context of modern numerical climate modelling and analysis, the linearized formulation has limited use. One such use is to diagnose the relative strengths of different feedback mechanisms. An estimate of climate sensitivity to a forcing is then obtained for the case where the net feedback remains negative and the system reaches a new equilibrium state (ΔEEI=0) after some time has passed:[19]: 19–20
- .
Implications for climate policy
Uncertainty over climate change feedbacks has implications for climate policy. For instance, uncertainty over carbon cycle feedbacks may affect targets for reducing greenhouse gas emissions (climate change mitigation).[90] Emissions targets are often based on a target stabilization level of atmospheric greenhouse gas concentrations, or on a target for limiting global warming to a particular magnitude. Both of these targets (concentrations or temperatures) require an understanding of future changes in the carbon cycle.[8]: 678
If models incorrectly project future changes in the carbon cycle, then concentration or temperature targets could be missed. For example, if models underestimate the amount of carbon released into the atmosphere due to positive feedbacks (e.g., due to thawing permafrost), then they may also underestimate the extent of emissions reductions necessary to meet a concentration or temperature target.[8]: 678 [91]
See also
- Climate variability and change
- Climate inertia
- Complex system
- Effects of climate change
- Parametrization (climate)
- Tipping points in the climate system
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