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.

Physical feedbacks include decreased surface reflectivity (from diminished snow and ice cover) and increased water vapor in the atmosphere.

[8]: 676  Over the long term the percentage will be reduced as carbon sinks become saturated and higher temperatures lead to effects like drought and wildfires.

[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.

[13] The initial change that triggers a feedback may be externally forced, or may arise through the climate system's internal variability.

[14][15] External forcings may be human-caused (for example, greenhouse gas emissions or land use change) or natural (for example, volcanic eruptions).

Its expected strength has been most simply estimated from the derivative of the Stefan-Boltzmann equation as −4σT3 = −3.8 W/m2/K (watts per square meter per degree of warming).

[17][19] 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.

[19] 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.

[20] Ideally the Planck response strength obtained from GCMs, indirect measurements, and black body estimates will further converge as analysis methods continue to mature.

[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.

[28] Albedo is the measure of how strongly the planetary surface can reflect solar radiation, which prevents its absorption and thus has a cooling effect.

[31] During glacial periods, additional ice increases the reflectivity and thus lowers absorption of solar radiation, cooling the planet.

These calculations include second-order effects such as the impact from ice loss on regional lapse rate, water vapor and cloud feedbacks,[43] and do not cause "additional" warming on top of the existing model projections.

[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.

Models with the strongest cloud feedback have the highest climate sensitivity, which means that they simulate much stronger warming in response to a doubling of CO2 (or equivalent greenhouse gas) concentrations than the rest.

[9][10] Around 2020, a small fraction of models was found to simulate so much warming as the result that they had contradicted paleoclimate evidence from fossils,[49][50] and their output was effectively excluded from the climate sensitivity estimate of the IPCC Sixth Assessment Report.

[8]: 698  The Amazon rainforest is a well-known example due to its enormous size and importance, and because the damage it experiences from climate change is exacerbated by the ongoing deforestation.

[8]: 677 Following Le Chatelier's principle, the chemical equilibrium of the Earth's carbon cycle will shift in response to anthropogenic CO2 emissions.

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".

Additionally, plants require less water as the atmospheric CO2 concentrations increase, because they lose less moisture to evapotranspiration through open stomata (the pores in leaves through which CO2 is absorbed).

However, increased droughts in certain regions can still limit plant growth, and the warming beyond optimum conditions has a consistently negative impact.

[69] I.e. long-term warming changes the balance in the methane-related microbial community within freshwater ecosystems so they produce more methane while proportionately less is oxidised to carbon dioxide.

[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.

[81][82] Current research shows that hydrates react very slowly to warming, and that it's very difficult for methane to reach the atmosphere after dissociation on the seafloor.

The ASR and OLR terms in this expression encompass many temperature-dependent properties and complex interactions that govern system behavior.

[86] 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.

Collectively the feedbacks may be 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:

[20] 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 instance, uncertainty over carbon cycle feedbacks may affect targets for reducing greenhouse gas emissions (climate change mitigation).

The relative magnitude of the top 6 climate change feedbacks and what they influence. Positive feedbacks amplify the global warming response to greenhouse gas emissions and negative feedbacks reduce it. [ 1 ] In this chart, the horizontal lengths of the red and blue bars indicate the strength of respective feedbacks.
Climate change occurs because the amount of thermal radiation absorbed by different parts of the Earth's environment currently exceeds the amount radiated away to space. [ 16 ] As the warming increases, outgoing radiation to space increases quickly due to the Planck response, which eventually helps to stabilize the Earth at some higher temperature level [ 17 ]
Atmospheric gases only absorb some wavelengths of energy but are transparent to others. The absorption patterns of water vapor (blue peaks) and carbon dioxide (pink peaks) overlap in some wavelengths. [ 21 ]
Lapse rate (green) is a negative feedback everywhere on Earth besides the polar latitudes . The net climate feedback (black) becomes less negative if it were excluded (orange) [ 24 ]
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, so their presence increases local and global temperatures, which helps to spur more melting
Details of how clouds interact with shortwave and longwave radiation at different atmospheric heights [ 45 ]
This diagram of the fast carbon cycle shows the movement of carbon between land, atmosphere, soil and oceans in billions of tons of carbon per year. Yellow numbers are natural fluxes, red are human contributions in billions of tons of carbon per year. White numbers indicate stored carbon.
The impulse response following a 100 GtC injection of CO 2 into Earth's atmosphere. [ 60 ] The majority of excess carbon is removed by ocean and land sinks in less than a few centuries, while a substantial portion persists.
Increase in global leaf area between 1982 and 2015, which was primarily caused by the CO2 fertilization effect [ 65 ]
Methane climate feedbacks in natural ecosystems.
Nine probable scenarios of greenhouse gas emissions from permafrost thaw during the 21st century, which show a limited, moderate and intense CO 2 and CH 4 emission response to low, medium and high-emission Representative Concentration Pathways . The vertical bar uses emissions of selected large countries as a comparison: the right-hand side of the scale shows their cumulative emissions since the start of the Industrial Revolution , while the left-hand side shows each country's cumulative emissions for the rest of the 21st century if they remained unchanged from their 2019 levels. [ 72 ]
The loss of albedo from major ice areas on Earth adds to warming: the values shown are for the initial warming of 1.5 °C (2.7 °F). [ 43 ] Total ice sheet loss requires multiple millennia: the others can be lost in a century or two [ 58 ] [ 59 ]
diagram showing five historical estimates of equilibrium climate sensitivity by the IPCC
Historical estimates of climate sensitivity from the IPCC assessments. The first three reports gave a qualitative likely range, and the next three had formally quantified it, by adding >66% likely range (dark blue). [ 87 ] [ 4 ] : 96 This uncertainty primarily depends on feedbacks. [ 9 ] [ 10 ]