Climate change feedback

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Photo shows what appears to be permafrost thaw ponds in Hudson Bay, Canada, near Greenland. (2008) Global warming will increase permafrost and peatland thaw, which can result in collapse of plateau surfaces. Permafrost thaw ponds in Hudson Bay Canada near Greenland.jpg
Photo shows what appears to be permafrost thaw ponds in Hudson Bay, Canada, near Greenland. (2008) Global warming will increase permafrost and peatland thaw, which can result in collapse of plateau surfaces.

Climate change feedback is important in the understanding of global warming because feedback processes may amplify or diminish the effect of each climate forcing, and so play an important part in determining the climate sensitivity and future climate state. Feedback in general is the process in which changing one quantity changes a second quantity, and the change in the second quantity in turn changes the first. Positive feedback amplifies the change in the first quantity while negative feedback reduces it. [2]

Global warming rise in the average temperature of the Earths climate system and its related effects

Global warming is a long-term rise in the average temperature of the Earth's climate system, an aspect of climate change shown by temperature measurements and by multiple effects of the warming. Though earlier geological periods also experienced episodes of warming, the term commonly refers to the observed and continuing increase in average air and ocean temperatures since 1900 caused mainly by emissions of greenhouse gasses in the modern industrial economy. In the modern context the terms global warming and climate change are commonly used interchangeably, but climate change includes both global warming and its effects, such as changes to precipitation and impacts that differ by region. Many of the observed warming changes since the 1950s are unprecedented in the instrumental temperature record, and in historical and paleoclimate proxy records of climate change over thousands to millions of years.

Climate sensitivity is the temperature change in response to changes of the radiative forcing, for instance due to increased levels of CO
2
. Although climate sensitivity is usually used in the context of radiative forcing by carbon dioxide (CO2), it is thought of as a general property of the climate system: the change in surface air temperature following a unit change in radiative forcing, and the climate sensitivity parameter is therefore be expressed in units of °C/(W/m2). For this to be useful, the measure must be independent of the nature of the forcing (e.g. from greenhouse gases or solar variation); which is true approximately. When climate sensitivity is expressed for a doubling of CO2, its units are °C or equivalently K (Kelvin).

Climate state

Climate state describes a state of climate on Earth and similar terrestrial planets based on a thermal energy budget, such as the greenhouse or icehouse climate state.

Contents

The term "forcing" means a change which may "push" the climate system in the direction of warming or cooling. [3] An example of a climate forcing is increased atmospheric concentrations of greenhouse gases. By definition, forcings are external to the climate system while feedbacks are internal; in essence, feedbacks represent the internal processes of the system. Some feedbacks may act in relative isolation to the rest of the climate system; others may be tightly coupled; hence it may be difficult to tell just how much a particular process contributes. [4]

Greenhouse gas gas in an atmosphere that absorbs and emits radiation within the thermal infrared range

A greenhouse gas is a gas that absorbs and emits radiant energy within the thermal infrared range. Greenhouse gases cause the greenhouse effect. The primary greenhouse gases in Earth's atmosphere are water vapor, carbon dioxide, methane, nitrous oxide and ozone. Without greenhouse gases, the average temperature of Earth's surface would be about −18 °C (0 °F), rather than the present average of 15 °C (59 °F). The atmospheres of Venus, Mars and Titan also contain greenhouse gases.

Forcings and feedbacks together determine how much and how fast the climate changes. The main positive feedback in global warming is the tendency of warming to increase the amount of water vapor in the atmosphere, which in turn leads to further warming. [5] The main negative feedback comes from the Stefan–Boltzmann law, the amount of heat radiated from the Earth into space changes with the fourth power of the temperature of Earth's surface and atmosphere. Observations and modelling studies indicate that there is a net positive feedback to warming. [6] Large positive feedbacks can lead to effects that are abrupt or irreversible, depending upon the rate and magnitude of the climate change." [7]

Stefan–Boltzmann law

The Stefan–Boltzmann law describes the power radiated from a black body in terms of its temperature. Specifically, the Stefan–Boltzmann law states that the total energy radiated per unit surface area of a black body across all wavelengths per unit time is directly proportional to the fourth power of the black body's thermodynamic temperature T:

Tipping points in the climate system

A tipping point in the climate system is a threshold that, when exceeded, can lead to large changes in the state of the system. Potential tipping points have been identified in the physical climate system, in impacted ecosystems, and sometimes in both. For instance, feedback from the global carbon cycle is believed to be a driver for the transition between glacial and interglacial periods, with orbital forcing providing the initial trigger. Earth's geologic temperature record includes many more examples of geologically rapid transitions between different climate states.

Positive

Carbon cycle feedbacks

There have been predictions, and some evidence, that global warming might cause loss of carbon from terrestrial ecosystems, leading to an increase of atmospheric CO
2
levels. Several climate models indicate that global warming through the 21st century could be accelerated by the response of the terrestrial carbon cycle to such warming. [8] All 11 models in the C4MIP study found that a larger fraction of anthropogenic CO2 will stay airborne if climate change is accounted for. By the end of the twenty-first century, this additional CO2 varied between 20 and 200 ppm for the two extreme models, the majority of the models lying between 50 and 100 ppm. The higher CO2 levels led to an additional climate warming ranging between 0.1° and 1.5 °C. However, there was still a large uncertainty on the magnitude of these sensitivities. Eight models attributed most of the changes to the land, while three attributed it to the ocean. [9] The strongest feedbacks in these cases are due to increased respiration of carbon from soils throughout the high latitude boreal forests of the Northern Hemisphere. One model in particular (HadCM3) indicates a secondary carbon cycle feedback due to the loss of much of the Amazon Rainforest in response to significantly reduced precipitation over tropical South America. [10] While models disagree on the strength of any terrestrial carbon cycle feedback, they each suggest any such feedback would accelerate global warming.

C4MIP (more fully, Coupled Climate Carbon Cycle Model Intercomparison Project) is a joint project between the International Geosphere-Biosphere Programme (IGBP)and the World Climate Research Programme (WCRP). It is a model intercomparison project along the lines of the Atmospheric Model Intercomparison Project, but for global climate models that include an interactive carbon cycle.

Taiga the worlds largest land biome, characterized by coniferous forests

Taiga, also known as boreal forest or snow forest, is a biome characterized by coniferous forests consisting mostly of pines, spruces, and larches.

HadCM3 is a coupled atmosphere-ocean general circulation model (AOGCM) developed at the Hadley Centre in the United Kingdom. It was one of the major models used in the IPCC Third Assessment Report in 2001.

Observations show that soils in the U.K have been losing carbon at the rate of four million tonnes a year for the past 25 years [11] according to a paper in Nature by Bellamy et al. in September 2005, who note that these results are unlikely to be explained by land use changes. Results such as this rely on a dense sampling network and thus are not available on a global scale. Extrapolating to all of the United Kingdom, they estimate annual losses of 13 million tons per year. This is as much as the annual reductions in carbon dioxide emissions achieved by the UK under the Kyoto Treaty (12.7 million tons of carbon per year). [12]

It has also been suggested (by Chris Freeman) that the release of dissolved organic carbon (DOC) from peat bogs into water courses (from which it would in turn enter the atmosphere) constitutes a positive feedback for global warming. The carbon currently stored in peatlands (390–455 gigatonnes, one-third of the total land-based carbon store) is over half the amount of carbon already in the atmosphere. [13] DOC levels in water courses are observably rising; Freeman's hypothesis is that, not elevated temperatures, but elevated levels of atmospheric CO2 are responsible, through stimulation of primary productivity. [14] [15]

Professor Chris Freeman is a British environmental scientist at the University of Wales, Bangor. Freeman is Professor of Aquatic Biogeochemistry in the College of Natural Sciences in Bangor. Freeman's research focuses on carbon cycling, with an emphasis on peatland carbon storage and dissolved organic carbon dynamics. His work is best known for its description of a mechanism known as the "peatland enzymic latch" and observation of a rising trend in aquatic dissolved organic carbon concentrations. His work has been recognised with awards from the American Society for Limnology and Oceanography and the Royal Society.

Dissolved organic carbon (DOC) is the fraction of total organic carbon operationally defined as that which can pass through a filter size that typically ranges in size from 0.22 and 0.7 micrometers. The fraction remaining on the filter is called particulate organic carbon (POC).

Peat accumulation of partially decayed vegetation

Peat, also known as turf, is an accumulation of partially decayed vegetation or organic matter. It is unique to natural areas called peatlands, bogs, mires, moors, or muskegs. The peatland ecosystem is the most efficient carbon sink on the planet, because peatland plants capture CO2 naturally released from the peat, maintaining an equilibrium. In natural peatlands, the "annual rate of biomass production is greater than the rate of decomposition", but it takes "thousands of years for peatlands to develop the deposits of 1.5 to 2.3 m [4.9 to 7.5 ft], which is the average depth of the boreal [northern] peatlands". Sphagnum moss, also called peat moss, is one of the most common components in peat, although many other plants can contribute. The biological features of Sphagnum mosses act to create a habitat aiding peat formation, a phenomenon termed 'habitat manipulation'. Soils consisting primarily of peat are known as histosols. Peat forms in wetland conditions, where flooding obstructs the flow of oxygen from the atmosphere, slowing the rate of decomposition.

Tree deaths are believed to be increasing as a result of climate change, which is a positive feedback effect. [16]

Arctic methane release

Warming is also the triggering variable for the release of carbon (potentially as methane) in the arctic. [17] Methane released from thawing permafrost such as the frozen peat bogs in Siberia, and from methane clathrate on the sea floor, creates a positive feedback. [18] [19]

Methane release from melting permafrost peat bogs

Western Siberia is the world's largest peat bog, a one million square kilometer region of permafrost peat bog that was formed 11,000 years ago at the end of the last ice age. The melting of its permafrost is likely to lead to the release, over decades, of large quantities of methane. As much as 70,000 million tonnes of methane, an extremely effective greenhouse gas, might be released over the next few decades, creating an additional source of greenhouse gas emissions. [20] Similar melting has been observed in eastern Siberia. [21] Lawrence et al. (2008) suggest that a rapid melting of Arctic sea ice may start a feedback loop that rapidly melts Arctic permafrost, triggering further warming. [22] [23]

Methane release from hydrates

Methane clathrate, also called methane hydrate, is a form of water ice that contains a large amount of methane within its crystal structure. Extremely large deposits of methane clathrate have been found under sediments on the sea and ocean floors of Earth. The sudden release of large amounts of natural gas from methane clathrate deposits, in a runaway global warming event, has been hypothesized as a cause of past and possibly future climate changes. The release of this trapped methane is a potential major outcome of a rise in temperature; it is thought that this might increase the global temperature by an additional 5° in itself, as methane is much more powerful as a greenhouse gas than carbon dioxide. The theory also predicts this will greatly affect available oxygen content of the atmosphere. This theory has been proposed to explain the most severe mass extinction event on earth known as the Permian–Triassic extinction event, and also the Paleocene-Eocene Thermal Maximum climate change event. In 2008, a research expedition for the American Geophysical Union detected levels of methane up to 100 times above normal in the Siberian Arctic, likely being released by methane clathrates being released by holes in a frozen 'lid' of seabed permafrost, around the outfall of the Lena River and the area between the Laptev Sea and East Siberian Sea. [24] [25] [26]

Abrupt increases in atmospheric methane

Literature assessments by the Intergovernmental Panel on Climate Change (IPCC) and the US Climate Change Science Program (CCSP) have considered the possibility of future projected climate change leading to a rapid increase in atmospheric methane. The IPCC Third Assessment Report, published in 2001, looked at possible rapid increases in methane due either to reductions in the atmospheric chemical sink or from the release of buried methane reservoirs. In both cases, it was judged that such a release would be "exceptionally unlikely" [27] (less than a 1% chance, based on expert judgement). [28] The CCSP assessment, published in 2008, concluded that an abrupt release of methane into the atmosphere appeared "very unlikely" [29] (less than 10% probability, based on expert judgement). [30] The CCSP assessment, however, noted that climate change would "very likely" (greater than 90% probability, based on expert judgement) accelerate the pace of persistent emissions from both hydrate sources and wetlands. [29]

Decomposition

Organic matter stored in permafrost generates heat as it decomposes in response to the permafrost melting. [31]

Peat decomposition

Peat, occurring naturally in peat bogs, is a store of carbon significant on a global scale. When peat dries it decomposes, and may additionally burn. Water table adjustment due to global warming may cause significant excursions of carbon from peat bogs. [32] This may be released as methane, which can exacerbate the feedback effect, due to its high global warming potential.

Rainforest drying

Rainforests, most notably tropical rainforests, are particularly vulnerable to global warming. There are a number of effects which may occur, but two are particularly concerning. Firstly, the drier vegetation may cause total collapse of the rainforest ecosystem. [33] For example, the Amazon rainforest would tend to be replaced by caatinga ecosystems. Further, even tropical rainforests ecosystems which do not collapse entirely may lose significant proportions of their stored carbon as a result of drying, due to changes in vegetation. [34]

Forest fires

The IPCC Fourth Assessment Report predicts that many mid-latitude regions, such as Mediterranean Europe, will experience decreased rainfall and an increased risk of drought, which in turn would allow forest fires to occur on larger scale, and more regularly. This releases more stored carbon into the atmosphere than the carbon cycle can naturally re-absorb, as well as reducing the overall forest area on the planet, creating a positive feedback loop. Part of that feedback loop is more rapid growth of replacement forests and a northward migration of forests as northern latitudes become more suitable climates for sustaining forests. There is a question of whether the burning of renewable fuels such as forests should be counted as contributing to global warming. [35] [36] [37] Cook & Vizy also found that forest fires were likely in the Amazon Rainforest, eventually resulting in a transition to Caatinga vegetation in the Eastern Amazon region.[ citation needed ]

Desertification

Desertification is a consequence of global warming in some environments. [38] Desert soils contain little humus, and support little vegetation. As a result, transition to desert ecosystems is typically associated with excursions of carbon.

Modelling results

The global warming projections contained in the IPCC's Fourth Assessment Report (AR4) include carbon cycle feedbacks. [39] Authors of AR4, however, noted that scientific understanding of carbon cycle feedbacks was poor. [40] Projections in AR4 were based on a range of greenhouse gas emissions scenarios, and suggested warming between the late 20th and late 21st century of 1.1 to 6.4 °C. [39] This is the "likely" range (greater than 66% probability), based on the expert judgement of the IPCC's authors. Authors noted that the lower end of the "likely" range appeared to be better constrained than the upper end of the "likely" range, in part due to carbon cycle feedbacks. [39] The American Meteorological Society has commented that more research is needed to model the effects of carbon cycle feedbacks in climate change projections. [41]

Isaken et al. (2010) [42] considered how future methane release from the Arctic might contribute to global warming. Their study suggested that if global methane emissions were to increase by a factor of 2.5 to 5.2 above (then) current emissions, the indirect contribution to radiative forcing would be about 250% and 400% respectively, of the forcing that can be directly attributed to methane. This amplification of methane warming is due to projected changes in atmospheric chemistry.

Schaefer et al. (2011) [43] considered how carbon released from permafrost might contribute to global warming. Their study projected changes in permafrost based on a medium greenhouse gas emissions scenario (SRES A1B). According to the study, by 2200, the permafrost feedback might contribute 190 (+/- 64) gigatons of carbon cumulatively to the atmosphere. Schaefer et al. (2011) commented that this estimate may be low.

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. [44] 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. 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 melting permafrost), then they may also underestimate the extent of emissions reductions necessary to meet a concentration or temperature target.

Cloud feedback

Warming is expected to change the distribution and type of clouds. 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. High clouds tend to trap more heat and therefore have a positive feedback, low clouds normally reflect more sunlight so they have a negative feedback. These details were poorly observed before the advent of satellite data and are difficult to represent in climate models. [45]

Gas release

Release of gases of biological origin may be affected by global warming, but research into such effects is at an early stage. Some of these gases, such as nitrous oxide released from peat, directly affect climate. [46] Others, such as dimethyl sulfide released from oceans, have indirect effects. [47]

Ice-albedo feedback

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 ice-albedo feedback. Sea Ice MeltPonds.png
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 ice-albedo feedback.

When 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. During times of global cooling, additional ice increases the reflectivity which reduces the absorption of solar radiation which results in more cooling in a continuing cycle. [48] Considered a faster feedback mechanism. [49]

1870-2009 Northern hemisphere sea ice extent in million square kilometers. Blue shading indicates the pre-satellite era; data then is less reliable. In particular, the near-constant level extent in Autumn up to 1940 reflects lack of data rather than a real lack of variation. Seaice-1870-part-2009.png
1870–2009 Northern hemisphere sea ice extent in million square kilometers. Blue shading indicates the pre-satellite era; data then is less reliable. In particular, the near-constant level extent in Autumn up to 1940 reflects lack of data rather than a real lack of variation.

Albedo change is also the main reason why IPCC predict polar temperatures in the northern hemisphere to rise up to twice as much as those of the rest of the world, in a process known as polar amplification. In September 2007, the Arctic sea ice area reached about half the size of the average summer minimum area between 1979 and 2000. [50] [51] Also in September 2007, Arctic sea ice retreated far enough for the Northwest Passage to become navigable to shipping for the first time in recorded history. [52] The record losses of 2007 and 2008 may, however, be temporary. [53] Mark Serreze of the US National Snow and Ice Data Center views 2030 as a "reasonable estimate" for when the summertime Arctic ice cap might be ice-free. [54] The polar amplification of global warming is not predicted to occur in the southern hemisphere. [55] The Antarctic sea ice reached its greatest extent on record since the beginning of observation in 1979, [56] but the gain in ice in the south is exceeded by the loss in the north. The trend for global sea ice, northern hemisphere and southern hemisphere combined is clearly a decline. [57]

Ice loss may have internal feedback processes, as melting of ice over land can cause eustatic sea level rise, potentially causing instability of ice shelves and inundating coastal ice masses, such as glacier tongues. Further, a potential feedback cycle exists due to earthquakes caused by isostatic rebound further destabilising ice shelves, glaciers and ice caps.

The ice-albedo in some sub-arctic forests is also changing, as stands of larch (which shed their needles in winter, allowing sunlight to reflect off the snow in spring and fall) are being replaced by spruce trees (which retain their dark needles all year). [58]

Water vapor feedback

If the atmospheres are warmed, the saturation vapor 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] Climate models incorporate this feedback. Water vapor feedback is strongly positive, with most evidence supporting a magnitude of 1.5 to 2.0 W/m2/K, sufficient to roughly double the warming that would otherwise occur. [59] Water vapor feedback is considered a faster feedback mechanism. [49]

Negative

Blackbody radiation

As the temperature of a black body increases, the emission of infrared radiation back into space increases with the fourth power of its absolute temperature according to Stefan–Boltzmann law. [60] This increases the amount of outgoing radiation as the Earth warms. The impact of this negative feedback effect is included in global climate models summarized by the IPCC. This is also called the Planck feedback.

Carbon cycle

Le Chatelier's principle

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". [61] 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.

Chemical weathering

Chemical weathering over the geological long term acts to remove CO2 from the atmosphere. With current global warming, weathering is increasing, demonstrating significant feedbacks between climate and Earth surface. [62] Biosequestration also captures and stores CO2 by biological processes. The formation of shells by organisms in the ocean, over a very long time, removes CO2 from the oceans. [63] The complete conversion of CO2 to limestone takes thousands to hundreds of thousands of years. [64]

Net Primary Productivity

Net primary productivity changes in response to increased CO2, as plants photosynthesis increased in response to increasing concentrations. However, this effect is swamped by other changes in the biosphere due to global warming. [65]

Lapse rate

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. 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. [66] [67]


See also

Notes

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Permafrost

In geology, permafrost is ground, including rock or (cryotic) soil, at or below the freezing point of water 0 °C (32 °F) for two or more years. Most permafrost is located in high latitudes, but at lower latitudes alpine permafrost occurs at higher elevations. Ground ice is not always present, as may be in the case of non-porous bedrock, but it frequently occurs and it may be in amounts exceeding the potential hydraulic saturation of the ground material. Permafrost accounts for 0.022% of total water on Earth and the permafrost region covers 24% of exposed land in the Northern Hemisphere. It also occurs subsea on the continental shelves of the continents surrounding the Arctic Ocean, portions of which were exposed during the last glacial period.

Radiative forcing

Radiative forcing or climate forcing is the difference between insolation (sunlight) absorbed by the Earth and energy radiated back to space. The influences that cause changes to the Earth’s climate system altering Earth’s radiative equilibrium, forcing temperatures to rise or fall, are called climate forcings. Positive radiative forcing means Earth receives more incoming energy from sunlight than it radiates to space. This net gain of energy will cause warming. Conversely, negative radiative forcing means that Earth loses more energy to space than it receives from the sun, which produces cooling.

This article serves as a glossary of climate change terms. It lists terms that are related to global warming.

Clathrate gun hypothesis hypothesis that climate change can trigger a release of methane buried in seabeds which leads to runaway warming

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The effects of global warming in the Arctic, or climate change in the Arctic include rising air and water temperatures, loss of sea ice, and melting of the Greenland ice sheet with a related cold temperature anomaly, observed since the 1970s. Related impacts include ocean circulation changes, increased input of freshwater, and ocean acidification. Indirect effects through potential climate teleconnections to mid latitudes may result in a greater frequency of extreme weather events, ecological, biological and phenology changes, biological migrations and extinctions, natural resource stresses and as well as human health, displacement and security issues. Potential methane releases from the region, especially through the thawing of permafrost and methane clathrates, may occur. Presently, the Arctic is warming twice as fast compared to the rest of the world. The pronounced warming signal, the amplified response of the Arctic to global warming, is often seen as a leading indicator of global warming. The melting of Greenland's ice sheet is linked to polar amplification. According to a study published in 2016, about 0.5 °C of the warming in the Arctic has been attributed to reductions in sulfate aerosols in Europe since 1980.

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Arctic geoengineering

Temperatures in the Arctic region have tended to increase more rapidly than the global average. Projections of sea ice loss that are adjusted to take account of recent rapid Arctic shrinkage suggest that the Arctic will likely be free of summer sea ice sometime between 2059 and 2078. Various climate engineering schemes have been suggested to reduce the chance of significant and irreversible effects such as Arctic methane release.

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Atmospheric methane

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Long-term effects of global warming

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The permafrost carbon cycle is a sub-cycle of the larger global carbon cycle. Permafrost is defined as subsurface material that remains below 0o C for at least two consecutive years. Because permafrost soils remain frozen for long periods of time, they store large amounts of carbon and other nutrients within their frozen framework during that time. Permafrost represents a large carbon reservoir that is seldom considered when determining global terrestrial carbon reservoirs. Recent and ongoing scientific research however, is changing this view.

Soil carbon feedback

The soil carbon feedback concerns releases of carbon from soils, in response to global warming, known as a positive climate feedback. Soil carbon contains three times as much carbon as Earth's atmosphere, which makes them crucial to understand related carbon fluxes. However, other studies suggest that soils contain twice as much carbon than contained in the atmosphere. Measurements imply that 4 °C of warming increases annual soil respiration by up to 37%. Climate models do not account for effects of biochemical heat release associated with microbial decomposition.

References