Climate change feedbacks

Climate change feedbacks are natural processes that impact how much global temperatures will increase for a given amount of greenhouse gas emissions. Positive feedbacks amplify global warming while negative feedbacks diminish it. Feedbacks influence both the amount of greenhouse gases in the atmosphere and the amount of temperature change that happens in response. While emissions are the forcing that causes climate change, feedbacks combine to control climate sensitivity to that forcing.
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. 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. Accordingly, anthropogenic climate change alone cannot cause a runaway greenhouse effect.
Feedbacks can be divided into physical feedbacks and partially biological feedbacks. Physical feedbacks include decreased surface reflectivity and increased water vapor in the atmosphere. Water vapor is not only a powerful greenhouse gas, it also influences feedbacks in the distribution of clouds and temperatures in the atmosphere. Biological feedbacks are mostly associated with changes to the rate at which plant matter accumulates as part of the carbon cycle. The carbon cycle absorbs more than half of CO₂ emissions every year into plants and into the ocean. Over the long term the percentage will be reduced as carbon sinks become saturated and higher temperatures lead to effects like drought and wildfires.
Feedback strengths and relationships are estimated through global climate models, with their estimates calibrated against observational data whenever possible. Some feedbacks rapidly impact climate sensitivity, while the feedback response from ice sheets is drawn out over several centuries. 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. Carbon cycle uncertainty is driven by the large rates at which is both absorbed into plants and released when biomass burns or decays. For instance, permafrost thaw produces both and methane emissions in ways that are difficult to model. 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.

Definition and terminology

The Planck response is the additional thermal radiation objects emit as they get warmer. Whether Planck response is a climate change feedback depends on the context. In climate science the Planck response can be treated as an intrinsic part of warming that is separate from radiative feedbacks and carbon cycle feedbacks. However, the Planck response is included when calculating climate sensitivity.
A feedback that amplifies an initial change is called a positive feedback while a feedback that reduces an initial change is called a negative feedback. 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.
The initial change that triggers a feedback may be externally forced, or may arise through the climate system's internal variability. External forcing refers to "a forcing agent outside the climate system causing a change in the climate system" that may push the climate system in the direction of warming or cooling. External forcings may be human-caused or natural.

Physical feedbacks

Planck response (negative)

Planck response is "the most fundamental feedback in the climate system". As the temperature of a black body increases, the emission of infrared radiation increases with the fourth power of its absolute temperature according to the Stefan–Boltzmann law. This increases the amount of outgoing radiation back into space as the Earth warms. It is a strong stabilizing response and has sometimes been called the "no-feedback response" because it is an intensive property of a thermodynamic system when considered to be purely a function of temperature. Although Earth has an effective emissivity less than unity, the ideal black body radiation emerges as a separable quantity when investigating perturbations to the planet's outgoing radiation.
The Planck "feedback" or Planck response is the comparable radiative response obtained from analysis of practical observations or global climate models. Its expected strength has been most simply estimated from the derivative of the Stefan-Boltzmann equation as −4σT³ = −3.8 W/m²·K. Accounting from GCM applications has sometimes yielded a reduced strength, as caused by extensive properties of the stratosphere and similar residual artifacts subsequently identified as being absent from such models.
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 framework]. Ideally the Planck response strength obtained from GCMs, indirect measurements, and black body estimates will further converge as analysis methods continue to mature.

Water vapor feedback (positive)

According to Clausius–Clapeyron relation, saturation vapor pressure is higher in a warmer atmosphere, and so the absolute amount of water vapor will increase as the atmosphere warms. It is sometimes also called the specific humidity feedback, because relative humidity stays practically constant over the oceans, but it decreases over land. This occurs because land experiences faster warming than the ocean, and a decline in RH has been observed after the year 2000.
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. 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 W/m²·K. This is very similar to model estimates, which are at 1.77 ± 0.20 W/m²·K Either value effectively doubles the warming that would otherwise occur from CO₂ increases alone. Like with the other physical feedbacks, this is already accounted for in the warming projections under climate change scenarios.

Lapse rate (negative)

The lapse rate is the rate at which an atmospheric variable, normally temperature in Earth's atmosphere, falls with altitude. It is therefore a quantification of temperature, related to radiation, as a function of altitude, and is not a separate phenomenon in this context. The lapse rate feedback is generally a negative feedback. However, it is in fact a positive feedback in polar regions where it strongly contributed to polar amplified warming, one of the biggest consequences of climate change. This is because in regions with strong inversions, such as the polar regions, the lapse rate feedback can be positive because the surface warms faster than higher altitudes, resulting in inefficient longwave cooling.
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.

Surface albedo feedback (positive)

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.
The presence of ice cover and sea ice makes the North Pole and the South Pole colder than they would have been without it. During glacial periods, additional ice increases the reflectivity and thus lowers absorption of solar radiation, cooling the planet. But when warming occurs and the ice melts, darker land or open water takes its place and this causes more warming, which in turn causes more melting. In both cases, a self-reinforcing cycle continues until an equilibrium is found. Consequently, recent Arctic sea ice decline is a key reason behind the Arctic warming nearly four times faster than the global average since 1979, in a phenomenon known as Arctic amplification. Conversely, the high stability of ice cover in Antarctica, where the East Antarctic ice sheet rises nearly 4 km above the sea level, means that it has experienced very little net warming over the past seven decades.

As of 2021, the total surface feedback strength is estimated at 0.35 W/m²·K. On its own, Arctic sea ice decline between 1979 and 2011 was responsible for 0.21 of radiative forcing. This is equivalent to a quarter of impact from emissions over the same period. The combined change in all sea ice cover between 1992 and 2018 is equivalent to 10% of all the anthropogenic greenhouse gas emissions. 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.
When CMIP5 models estimate a total loss of Arctic sea ice cover from June to September, it increases the global temperatures by, with a range of 0.16–0.21 °C, while the regional temperatures would increase by over. These calculations include second-order effects such as the impact from ice loss on regional lapse rate, water vapor and cloud feedbacks, and do not cause "additional" warming on top of the existing model projections.