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Possible tipping elements in the climate system

In climate science, a tipping point is a critical threshold that, when crossed, leads to large and often irreversible changes in the climate system.^[1] If tipping points are crossed, they are likely to have severe impacts on human society.^[2]^[3] Tipping behaviour is found across the climate system, in ecosystems, ice sheets, and the circulation of the ocean and atmosphere.^[3]

Tipping points are often, but not necessarily, abrupt. For example, with average global warming somewhere between 1 and 4 °C, the Greenland ice sheet passes a tipping point and is doomed, but the melt would take place over millennia.^[4] There are multiple types of tipping points: some can be crossed due to the magnitude of environmental change, others due to the rate, and a third category due to internal variability of the system or "noise".^[5]

Tipping points are possible at today's global warming of just over 1 °C above preindustrial times, and highly probable above 2 °C of global warming.^[3] The geological record shows many abrupt changes that suggest tipping points may have been crossed in ancient times.^[6] It is possible that some tipping points are close to being crossed or have already been crossed, like those of the West Antarctic and Greenland ice sheets, the Amazon rainforest and warm-water coral reefs.^[7] A danger is that if the tipping point in one system is crossed, this could cause a cascade of other tipping points, leading to severe impacts.^[8]

Definition

The sixth report from the United Nations Intergovernmental Panel on Climate Change (IPCC), released in 2021, defines a tipping point as a "critical threshold beyond which a system reorganizes, often abruptly and/or irreversibly".^[9] It can be brought about by a small disturbance causing a disproportionately large change in the system. One set of definitions of "tipping points" also requires self-reinforcing feedbacks, which could lead to changes in the climate system irreversible on a human timescale.^[10] For any particular climate component, the shift from one state to a new stable state may take many decades or centuries.^[10] In ecosystems and in social systems, a tipping point can trigger a regime shift, a major systems reorganisation into a new stable state.^[11]

The 2019 IPCC Special Report on the Ocean and Cryosphere in a Changing Climate defines a tipping point as: "A level of change in system properties beyond which a system reorganises, often in a non-linear manner, and does not return to the initial state even if the drivers of the change are abated. For the climate system, the term refers to a critical threshold at which global or regional climate changes from one stable state to another stable state.".^[12]

Geological record

The geological record shows that there have been abrupt changes in the climate system that indicate ancient tipping points. For instance, the Dansgaard–Oeschger events during the last ice age were periods of abrupt warming (within decades) in Greenland and Europe, that may have involved the abrupt changes in major ocean currents. During the deglaciation in the early Holocene, sea level rise was not smooth, but rose abruptly during meltwater pulses. The monsoon in North Africa saw abrupt changes on decadal timescales during the African humid period. This period, spanning from 15,000 to 5,000 years ago, also ended suddenly in a drier state.^[6]

Tipping elements

Scientists have identified many elements in the climate system which may have tipping points.^[13]^[10] in the early 2000s the IPCC began considering the possibility of tipping points. At that time the IPCC concluded they would only be likely in the event of global warming of 4 °C or more above preindustrial times. As of 2021 tipping points are considered to have significant probability at today's warming level of just over 1 °C, with high probability above 2 °C of global warming,^[3] and it is possible that some tipping points are close to being crossed or have already been crossed, like those of the ice sheets in West Antarctic and Greenland, warm-water coral reefs, and the Amazon rainforest.^[14]^[15] As of September 2022, nine core tipping elements and seven regional impact tipping elements were identified. Out of those, one regional and three core climate elements are estimated to likely pass a tipping point if global warming reaches 1.5 °C, namely the Greenland ice sheet collapse, the West Antarctic ice sheet collapse, tropical coral reef die off, and the boreal permafrost abrupt thaw. Two further tipping points are forecast as likely if warming continues to approach 2 °C : Barents sea ice abrupt loss, and the Labrador sea subpolar gyre collapse.^[16]^[17] ^[18]

Global core tipping elements^[18]
Proposed climate tipping element (and tipping point)	Threshold ( °C)			Timescale (years)			Maximum Impact ( °C)
Proposed climate tipping element (and tipping point)	Estimated	Minimum	Maximum	Estimated	Minimum	Maximum	Global	Regional
Greenland Ice Sheet (collapse)	1.5	0.8	3.0	10k	1k	15k	0.13	0.5 to 3.0
West Antarctic Ice Sheet (collapse)	1.5	1.0	3.0	2k	500	13k	0.05	1.0
Labrador-Irminger Seas/SPG Convection (collapse)	1.8	1.1	3.8	10	5	50	-0.5	-3.0
East Antarctic Subglacial Basins (collapse)	3.0	2.0	6.0	2k	500	10k	0.05	?
Amazon Rainforest (dieback)	3.5	2.0	6.0	100	50	200	0.1 (partial) 0.2 (total)^{[T1 1]}	0.4 to 2.0
Boreal Permafrost (collapse)	4.0	3.0	6.0	50	10	300	0.2 - 0.4^{[T1 2]}	~
Atlantic Meridional Overturning Circulation (collapse)	4	1.4	8	50	15	300	-0.5	-4 to -10
Arctic Winter Sea Ice (collapse)	6.3	4.5	8.7	20	10	100	0.6	0.6 to 1.2
East Antarctic Ice Sheet (collapse)	7.5	5.0	10.0	?	10k	?	0.6	2.0
Low-latitude Coral Reefs (dieoff)	1.5	1.0	2.0	10	~	~	~	~

^ The paper also provides the same estimate in terms of equivalent emissions: partial dieback would be equivalent to the emissions of 30 billion tonnes of carbon, while total dieback would be equivalent to 75 billion tonnes of carbon
^ The paper also provides the same estimate in terms of emissions: between 125 and 250 billion tonnes of carbon and between 175 and 350 billion tonnes of carbon equivalent.

Regional impact tipping elements^[18]
Proposed climate tipping element (and tipping point)	Threshold ( °C)			Timescale (years)			Maximum Impact ( °C)
Proposed climate tipping element (and tipping point)	Estimated	Minimum	Maximum	Estimated	Minimum	Maximum	Global	Regional
Boreal Permafrost (abrupt thaw)	1.5	1.0	2.3	200	100	300	0.04 per °C by 2100;0.11 per °C by 2300^{[T2 1]}	~
Barents Sea Ice (abrupt loss)	1.6	1.5	1.7	25	?	?	~	+
Mountain Glaciers (loss)	2.0	1.5	3.0	200	50	1k	0.08	+
Sahel and W.African Monsoon (greening)	2.8	2	3.5	50	10	500	~	+
Boreal Forest (southern dieoff)	4.0	1.5	5.0	100	50	?	net -0.18^{[T2 2]}	-0.5 to -2
Boreal Forest (northern expansion)	4.0	1.5	7.2	100	40	?	net +0.14^{[T2 3]}	0.5-1.0

^ The paper clarifies that this represents a 50% increase of gradual permafrost thaw: it also provides the same estimate in terms of emissions per each degree of warming: 10 billion tonnes of carbon and 14 billion tonnes of carbon equivalent by 2100, and 25/35 billion tonnes of carbon/carbon equivalent by 2300
^ The loss of these forests would be equivalent to the emissions of 52 billion tons of carbon, but this would be more than offset by the area's albedo effect increasing and reflecting more sunlight.
^ Extra forest growth here would absorb around 6 billion tons of carbon, but because this area receives a lot of sunlight, this is very minor when compared to reduced albedo, as this vegetation absorbs more heat than the snow-covered ground it moves into.

Greenland ice sheet disintegration

The Greenland ice sheet is the second largest ice sheet in the world, and is three times the size of the American state of Texas.^[19] It holds enough water, that if completely melted, could raise sea levels globally by 7.2 metres.^[20] Due to global warming, the ice sheet is melting at an accelerating rate, adding almost 1 mm to global sea levels every year.^[21] Around half of the ice loss occurs via surface melting, and the remainder occurs at the base of the ice sheet where the ice sheet touches the sea, by calving (breaking off) icebergs from its margins.^[22]

The Greenland ice sheet has a tipping point because of the melt-elevation feedback. Surface melting reduces the height of the ice sheet. As air at a lower altitude is warmer, the ice sheet is then exposed to warmer temperatures, accelerating the melt.^[23] The threshold for the Greenland ice sheet to tip is between 1 and 4 °C of global warming, beyond which complete ice loss becomes inevitable. The melt would take place over millennia, and the rate of melt depends on the amount of global warming.^[4] There is some evidence that the Greenland ice sheet is losing stability, and getting close to a tipping point.^[23]

West Antarctic ice sheet disintegration

The West Antarctic Ice Sheet (WAIS) is a large ice sheet in Antarctica; in places more than 4 kilometres thick. It sits on bedrock mostly below sea level.^[24] As such, it is in contact with the heat from the ocean which makes it vulnerable to fast and irreversible ice loss. A tipping point could be reached if thinning or collapse of the WAIS's ice shelves triggers a feedback loop that leads to rapid and irreversible loss of its ice into the ocean. If completely melted, the ice sheet would contribute around 3.3 metres of sea level rise.^[10]

Ice loss from the WAIS is accelerating.^[25] The paleo record suggests that during the past few hundred thousand years, the WAIS largely disappeared in response to similar levels of warming and CO₂ emission scenarios projected for the next few centuries.^[26] A 2021 study of ocean floor sediments in the Antarctic's iceberg alley has shown that that tipping has occurred in the past on several occasions; and that tipping can be sudden and full ice sheet retreat can take as little as ten years.^[27]

North Subpolar Gyre

Section 'Climate Change' not found

East Antarctic ice sheet disintegration

East Antarctic ice sheet is the largest and thickest ice sheet on Earth, with the maximum thickness of 4,800 m. A complete disintegration would raise the global sea levels by 53.3 m, but this may not occur until global warming of 10 degrees Celsius, while the loss of two-thirds of its volume may require at least 6 degrees of warming to trigger. ^[28] Its melt would also occur over a longer timescale than the loss of any other ice on the planet, taking no less than 10,000 years to finish. However, the subglacial basin portions of the East Antarctic ice sheet may be vulnerable to tipping at lower levels of warming. ^[18] The Wilkes Basin is of particular concern, as it holds enough ice to raise sea levels by about 3 to 4 metres. ^[1]

Amazon rainforest dieback

The Amazon rainforest is the largest tropical rainforest in the world. It is twice as big as India and spans nine countries in South America. It produces around half of its own rainfall by recycling moisture through evaporation and transpiration as air moves across the forest.^[10] When forest is lost via climate change (droughts and fires) or deforestation, there will be less rain and more trees will die. Eventually, large parts of the rainforest may die off and transform into a dry savanna landscape.^[29] In 2022, a study reported that the rainforest has been losing resilience since the early 2000s. Resiliency is measured by recovery-time from short-term perturbations. This delayed return to equilibrium of the rainforest is termed critical slowing down. The observed loss of resilience reinforces the theory that the rainforest is approaching a critical transition.^[30]^[31]

Permafrost thaw

Perennially frozen ground, or permafrost, covers large fractions of land – mainly in Siberia, Alaska, northern Canada and the Tibetan plateau – and can be up to a kilometre thick.^[32]^[10] Subsea permafrost up to 100 metres thick also occurs on the sea floor under part of the Arctic Ocean.^[33] This frozen ground holds vast amounts of carbon from plants and animals that died and decomposed over thousands of years. Scientists believe there is nearly twice as much carbon in permafrost than is present in Earth's atmosphere.^[33] As the climate warms and the permafrost begins to thaw, carbon dioxide and methane are released into the atmosphere. With higher temperatures, microbes become active and decompose the biological material in the permafrost. This could happen rapidly, or over longer timespans, and the loss would be irreversible. Because CO₂ and methane are both greenhouse gases, they act as a self-reinforcing feedback on permafrost melt.^[34]^[35]

Atlantic Meridional Overturning Circulation

The Atlantic Meridional Overturning Circulation (AMOC), also known as the Gulf Stream System, is a large system of ocean currents.^[36]^[37] It is driven by differences in the density of water; colder and more salty water is heavier than warmer fresh water.^[37] The AMOC acts as a conveyor belt, sending warm surface water from the tropics north, and carrying cold fresh water back south.^[36] As warm water flows northwards, some evaporates which increases salinity. It also cools when it is exposed to cooler air. Cold, salty water is more dense and slowly begins to sink. Several kilometres below the surface, cold, dense water begins to move south.^[37] Increased rainfall and the melting of ice due to global warming dilutes the salty surface water, and warming further decreases its density. The lighter water is less able to sink, slowing down the circulation.^[10]

Theory, simplified models, and reconstructions of abrupt changes in the past suggest the AMOC has a tipping point. If freshwater input from melting glaciers reaches a certain threshold, it could collapse into a state of reduced flow. Even after melting stops, the AMOC may not return to its current state. It is unlikely that the AMOC will tip in the 21st century,^[38] but it may do so before 2300 if greenhouse gas emissions are very high. A weakening of 24% to 39% is expected depending on greenhouse emissions, even without tipping behaviour.^[39] If the AMOC does shut down, a new stable state could emerge that lasts for thousands of years, possibly triggering other tipping points.^[10]

In 2021, a study which used a "primitive" finite-difference ocean model estimated that AMOC collapse could be invoked by a sufficiently fast increase in ice melt even if it never reached the common thresholds for tipping obtained from slower change. Thus, it implied that the AMOC collapse is more likely than what is usually estimated by the complex and large-scale climate models. ^[40] Another 2021 study found early-warning signals in a set of AMOC indices, suggesting that the AMOC may be close to tipping.^[41] However, it was contradicted by another study published in the same journal the following year, which found a "largely stable" AMOC which had so far not been affected by climate change beyond its own natural variability. ^[42] Two more studies published in 2022 have also suggested that the modelling approaches commonly used to evaluate AMOC appear to overestimate the risk of its collapse. ^[43] ^[44]

Arctic sea ice

Average decadal extent and area of the Arctic Ocean sea ice since the start of satellite observations.

Annual trend in the Arctic sea ice extent and area for the 2011-2022 time period.

Arctic sea ice was once identified as a potential tipping element. The loss of sunlight-reflecting sea ice during summer exposes the (dark) ocean, which would warm. Arctic sea ice cover is likely to melt entirely under even relatively low levels of warming, and it was hypothesized that this could eventually transfer enough heat to the ocean to prevent sea ice recovery even if the global warming is reversed. Modelling now shows that this heat transfer during the Arctic summer does not overcome the cooling and the formation of new ice during the Arctic winter. As such, the loss of Arctic ice during the summer is not a tipping point for as long as the Arctic winter remains cool enough to enable the formation of new Arctic sea ice.^[45]^[46] However, if the higher levels of warming prevent the formation of new Arctic ice even during winter, then this change may become irreversible. Consequently, Arctic Winter Sea Ice is included as a potential tipping point in a 2022 assessment.^[18]

Additionally, the same assessment argued that while the rest of the ice in the Arctic Ocean may recover from a total summertime loss during the winter, ice cover in the Barents Sea may not reform during the winter even below 2 degrees of warming.^[18] This is because the Barents Sea is already the fastest-warming part of the Arctic: in 2021-2022, while the warming within the Arctic Circle has been nearly four times faster than the global average since 1979,^[47]^[48] Barents Sea warmed up to seven times faster than the global average.^[49]^[50]This tipping point matters because of the decade-long history of research into the connections between the state of Barents-Kara Sea ice and the weather patterns elsewhere in Eurasia.^[51]^[52]^[53]^[54]^[55]

Coral reef die-off

Bleached coral with normal coral in the background

Around 500 million people around the world depend on coral reefs for food, income, tourism and coastal protection.^[56] Since the 1980s, this is being threatened by the increase in sea surface temperatures which is triggering mass bleaching of coral, especially in sub-tropical regions.^[57] A sustained ocean temperature spike of 1 °C above average is enough to cause bleaching.^[58] Under heat stress, corals expel the small colourful algae which live in their tissues, which causes them to turn white. The algae, known as zooxanthellae, have a symbiotic relationship with coral such that without them, the corals slowly die.^[59] After these zooxanthellae have disappeared, the corals are vulnerable to a transition towards a seaweed-dominated ecosystem, making it very difficult to shift back to a coral-dominated ecosystem.^[60] The IPCC estimates that by the time temperatures have risen to 1.5 °C above pre-industrial times, between 70% and 90% of coral reefs that exist today will disappear; and that if the world warms by 2 °C, they will become extremely rare.^[61]

Mountain glaciers

Observed glacier mass loss in the Hindu Kush Himalaya region since the 20th century.

Mountain glaciers are the largest repository of land-bound ice after the Greenland and the Antarctica ice sheets, and they are also undergoing melting as the result of climate change. A glacier tipping point is when it enters a disequilibrium state with the climate and will melt away unless the temperatures go down.^[62]^[63] Examples include glaciers of the North Cascade Range, where even in 2005 67% of the glaciers observed were in disequilibrium and will not survive the continuation of the present climate, ^[64] or the French Alps, where The Argentière and Mer de Glace glaciers are expected to disappear completely by end of the 21st century if current climate trends persist.^[65]

The absolute largest amount of glacier ice is located in the Hindu Kush Himalaya region, which is colloquially known as the Earth's "Third Pole" as the result. It is believed that one third of that ice will be lost by 2100 even if the warming is limited to 1.5 °C, while the "intermediate" and "severe" climate change scenarios (RCP 4.5 and 8.5) are likely to lead to the losses of 50% and >67% of the region's glaciers over the same timeframe. Glacier melt is projected to accelerate regional river flows until the amount of meltwater peaks around 2060, going into an irreversible decline afterwards. Since regional precipitation will continue to increase even as the glacier meltwater contribution declines, annual river flows are only expected to diminish in the western basins where contribution from the monsoon is low: however, irrigation and hydropower generation would still have to adjust to greater interannual variability and lower pre-monsoon flows in all of the region's rivers.^[66] ^[67]

Sahel greening

Some simulations of global warming and increased carbon dioxide concentrations have shown a substantial increase in precipitation in the Sahel/Sahara.^[68] This and the increased plant growth directly induced by carbon dioxide^[69] could lead to an expansion of vegetation into present-day desert, although it would be less extensive than during the mid-Holocene^[68] and perhaps accompanied by a northward shift of the desert, i.e. a drying of northernmost Africa.^[70] Such a precipitation increase may also reduce the amount of dust originating in Northern Africa,^[71] with effects on hurricane activity in the Atlantic and increased threats of hurricane strikes in the Caribbean, the Gulf of Mexico and the East Coast of the United States of America.^[72]

The Special Report on Global Warming of 1.5 °C and the IPCC Fifth Assessment Report indicate that global warming will likely result in increased precipitation across most of East Africa, parts of Central Africa and the principal wet season of West Africa, although there is significant uncertainty related to these projections especially for West Africa.^[73] In addition, the end of the 20th century drying trend may be due to global warming.^[74] On the other hand, West Africa^[75] and parts of East Africa may become drier during given seasons and months.^[75]^[74] Currently, the Sahel is becoming greener but precipitation has not fully recovered to levels reached in the mid-20th century.^[70]

Climate models have yielded equivocal results about the effects of anthropogenic global warming on the Sahara/Sahel precipitation. Human-caused climate change occurs through different mechanisms than the natural climate change that led to the AHP,^[76] in particular through increased inter-hemispheric temperature gradients.^[77] The direct effect of heat on plants may be detrimental.^[78] Non-linear increases in vegetation cover are also possible,^[77] with several climate models showing abrupt increases when global temperatures rise by 2–4 °C (3.6–7.2 °F).^[79] One study in 2003 showed that vegetation intrusions in the Sahara can occur within decades after strong rises in atmospheric carbon dioxide^[80] but would not cover more than about 45% of the Sahara.^[81] That climate study also indicated that vegetation expansion can only occur if grazing or other perturbations to vegetation growth do not hamper it.^[82] On the other hand, increased irrigation and other measures to increase vegetation growth such as the Great Green Wall could enhance it.^[78] A 2022 study indicated that while increased greenhouse gas concentrations by themselves are not sufficient to start an AHP if greenhouse gas-vegetation feedbacks are ignored, they lower the threshold for orbital changes to induce Sahara greening.^[83]

Boreal forest biome shift

Extensive boreal forest biomes, colloquially known as taiga, could eventually experience irreversible transition to a different biome state. ^[84] It is now believed that continued warming and permafrost thaw would entail both the transition of the southern edge of boreal forests to wetland or grassland, but also allow these forests to advance further north into the former tundra biome, with the net forest change uncertain and dependent on the extent of global warming and local dynamics.^[18]

Equatorial stratocumulus clouds

Section 'Possible climate tipping point' not found

Formerly considered tipping elements

The possibility that the El Niño–Southern Oscillation (ENSO) is a tipping element had attracted attention in the past.^[85] Normally strong winds blow west across the South Pacific Ocean from South America to Australia. Every two to seven years, the winds weaken due to pressure changes and the air and water in the middle of the Pacific warms up, causing changes in wind movement patterns around the globe. This known as El Niño and typically leads to droughts in India, Indonesia and Brazil, and increased flooding in Peru. In 2015/2016, this caused food shortages affecting over 60 million people.^[86] El Niño-induced droughts may increase the likelihood of forest fires in the Amazon.^[87] The threshold for tipping was estimated to be between 3.5 and 7 °C of global warming in 2016.^[88] After tipping, the system would be in a more permanent El Niño state, rather than oscillating between different states. This has happened in Earth's past, in the Pliocene, but the layout of the ocean was significantly different from now.^[85] So far, there is no definitive evidence indicating changes in ENSO behaviour,^[87] and the IPCC Sixth Assessment Report concluded that it is "virtually certain that the ENSO will remain the dominant mode of interannual variability in a warmer world." ^[89] Consequently, the 2022 assessment no longer includes it in the list of likely tipping elements. ^[18]

The Indian summer monsoon is another part of the climate system which was considered suspectible to irreversible collapse in the earlier research. ^[90] However, more recent research has demonstrated that warming tends to strengthen the Indian monsoon, ^[91] and it is projected to strengthen in the future. ^[92]

Methane hydrate deposits in the Arctic were once thought to be vulnerable to a rapid dissociation which would have a large impact on global temperatures, in a dramatic scenario known as a clathrate gun hypothesis. Later research found that it takes millennia for methane hydrates to respond to warming,^[93] while methane emissions from the seafloor rarely transfer from the water column into the atmosphere.^[94] ^[95]^[96] IPCC Sixth Assessment Report states "It is very unlikely that gas clathrates (mostly methane) in deeper terrestrial permafrost and subsea clathrates will lead to a detectable departure from the emissions trajectory during this century".^[97]

Mathematical theory

Tipping point behaviour in the climate can be described in mathematical terms. Three types of tipping points have been identified—bifurcation, noise-induced and rate-dependent.^[98]^[5]

Bifurcation-induced tipping

Bifurcation-induced tipping happens when a particular parameter in the climate (for instance a change in environmental conditions or forcing), passes a critical level – at which point a bifurcation takes place – and what was a stable state loses its stability or simply disappears.^[5]^[99] The Atlantic Meridional Overturning Circulation (AMOC) is an example of a tipping element that can show bifurcation-induced tipping. Slow changes to the bifurcation parameters in this system – the salinity and temperature of the water – may push the circulation towards collapse.^[100]^[101]

Many types of bifurcations show hysteresis,^[102] which is the dependence of the state of a system on its history. For instance, depending on how warm it was in the past, there can be differing amounts of ice on the poles at the same concentration of greenhouse gases or temperature.^[103]

Early warning signals

For tipping points that occur because of a bifurcation, it may be possible to detect whether a system is getting closer to a tipping point, as it becomes less resilient to perturbations on approach of the tipping threshold. These systems display critical slowing down, with an increased memory (rising autocorrelation) and variance. Depending on the nature of the tipping system, there may be other types of early warning signals.^[104]^[105] Abrupt change is not an early warning signal (EWS) for tipping points, as abrupt change can also occur if the changes are reversible to the control parameter.^[106]^[107]

These EWSs are often developed and tested using time series from the paleo record, like sediments, ice caps, and tree rings, where past examples of tipping can be observed.^[104]^[108] It is not always possible to say whether increased variance and autocorrelation is a precursor to tipping, or caused by internal variability, for instance in the case of the collapse of the AMOC.^[108] Quality limitations of paleodata further complicate the development of EWSs.^[108] They have been developed for detecting tipping due to drought in forests in California,^[109] and melting of the Pine Island Glacier in West Antarctica,^[107] among other systems. Using early warning signals (increased autocorrelation and variance of the melt rate time series), it has been suggested that the Greenland ice sheet is currently losing resilience, consistent with modelled early warning signals of the ice sheet.^[110]

Human-induced changes in the climate system may be too fast for early warning signals to become evident, especially in systems with inertia.^[111]

Noise-induced tipping

Noise-induced tipping is the transition from one state to another due to random fluctuations or internal variability of the system. Noise-induced transitions do not show any of the early warning signals which occur with bifurcations. This means they are unpredictable because the underlying potential does not change. Because they are unpredictable, such occurrences are often described as a ‘one-in-x-year’ event.^[112] An example is the Dansgaard–Oeschger events during the last ice age, with 25 occurrences of sudden climate fluctuations over a 500 year period.^[113]

Rate-induced tipping

Rate-induced tipping occurs when a change in the environment is faster than the force that restores the system to its stable state.^[5] In peatlands, for instance, after years of relative stability, rate-induced tipping can lead to an "explosive release of soil carbon from peatlands into the atmosphere" – sometimes known as "compost bomb instability".^[114]^[115] The AMOC may also show rate-induced tipping: if the rate of ice melt increases too fast, it may collapse, even before the ice melt reaches the critical value where the system would undergo a bifurcation.^[116]

Cascading tipping points

Crossing a threshold in one part of the climate system may trigger another tipping element to tip into a new state. Such sequences of thresholds are called cascading tipping points, an example of a domino effect.^[117] Ice loss in West Antarctica and Greenland will significantly alter ocean circulation. Sustained warming of the northern high latitudes as a result of this process could activate tipping elements in that region, such as permafrost degradation, and boreal forest dieback.^[1] Thawing permafrost is a threat multiplier because it holds roughly twice as much carbon as the amount currently circulating in the atmosphere.^[118] Loss of ice in Greenland likely destabilises the West Antarctic ice sheet via sea level rise, and vice-versa, especially if Greenland were to melt first as West Antarctica is particularly vulnerable to contact with warm sea water.^[119]

A 2021 study with three million computer simulations of a climate model showed that nearly one-third of those simulations resulted in domino effects, even when temperature increases were limited to 2 °C – the upper limit set by the Paris Agreement in 2015.^[119]^[120] The authors of the study said that the science of tipping points is so complex that there is great uncertainty as to how they might unfold, but nevertheless, argued that the possibility of cascading tipping points represents “an existential threat to civilisation”.^[121]

Impacts

Tipping points can have very severe impacts.^[1] They can exacerbate current dangerous impacts of climate change, or give rise to new impacts. Some potential tipping points would take place abruptly, such as disruptions to the Indian monsoon, with severe impacts on food security for hundred of millions. Other impacts would likely take place over longer timescales, such as the melt of the ice caps. The 10 m of sea level rise from the combined melt of Greenland and West Antarctica would require moving many cities inland. A collapse of the Atlantic Overturning Circulation would alter Europe radically, and lead to a metre of sea level rise in the North Atlantic.^[3] These impacts could happen simultaneously in the case of cascading tipping points.^[93] A review of abrupt changes over the last 30,000 years showed that tipping points can lead to a large set of cascading impacts in climate, ecological and social systems. For instance, the abrupt termination of the African humid period cascaded, and desertification and regime shifts led to the retreat of pastoral societies in North Africa and a change of dynasty in Egypt.^[108]

A 2021 meta analysis on the potential economic impact of tipping points found that they raise global risk; the medium estimate was that they increase the social cost of carbon by about 25%, with a 10% chance of tipping points more than doubling it. The social cost of carbon reflects the economic damage from carbon emissions.^[122] It included a detailed table of the estimated additional impact from each tipping point per every tonne of carbon under the "intermediate" Representative Concentration Pathway 4.5 Since it was written before the publication of the IPCC Sixth Assessment Report and the 2022 assessment of tipping points, it includes some tipping points they no longer consider plausible, like methane hydrates and the Indian summer monsoon, and excludes the more recently identified ones like the Labrador-Irminger circulation.

The SCC (2020 US dollars) and the percentage change in the SCC due to tipping points collectively and individually^[122]
Tipping point (TP)	Expected SCC (US$/tCO²)	Increase due to TP, %
None	52.03	—
Permafrost carbon	56.41	8.4
Ocean methane hydrates	58.85	13.1
Arctic sea ice/Surface Albedo Feedback	51.14	-1.7
Amazon	52.07	0.1
Greenland Ice Sheet	52.97	1.8
West Antarctic Ice Sheet	53.57	2.9
AMOC	51.28	-1.4
Indian summer monsoon	52.70	1.3
All TPs	64.80	24.5
Σ main effects, all TPs	—	24.5
All costly TPs	67.05	28.9
∑ main effects, costly TPs only	—	27.6

In 2022, this assessment was severely criticized by a group of scientists including Steve Keen and Timothy Lenton, who considered those values, as well as the paper's suggestion that tipping points activated at 6 °C of global warming would reduce global per capita consumption by around 1.4%, to be an enormous underestimate.^[123] The authors of the assessment responded to this criticism by noting that their assessment should be treated as the starting point in economic assessment of tipping points rather than the final word, and since most of the literature included in their meta-analysis lacks the ability to estimate nonmarket climate damages, their numbers are likely to be underestimates. They have also noted that if climate change is assumed to impact the economic growth rate rather than the base level of economic activity (which they consider unlikely, but still a possibility) then the social cost of carbon becomes nearly 100 times larger, while the impact of tipping points becomes around 3 times larger.^[124]

Runaway greenhouse effect

A runaway greenhouse effect is a tipping point so extreme that oceans evaporate^[125] and the water vapour escapes to space, an irreversible climate state that happened on Venus.^[126] A runaway greenhouse effect has virtually no chance of being caused by people.^[127]

Venus-like conditions on the Earth require a large long-term forcing that is unlikely to occur until the sun brightens by a few tens of percents, which will take a few billion years.^[128]

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External links

Global Systems Institute Tipping points research at Exeter University
The climate tipping points 2022 BBC radio documentary

[19] The paper also provides the same estimate in terms of equivalent emissions: partial dieback would be equivalent to the emissions of 30 billion tonnes of carbon, while total dieback would be equivalent to 75 billion tonnes of carbon

[20] The paper also provides the same estimate in terms of emissions: between 125 and 250 billion tonnes of carbon and between 175 and 350 billion tonnes of carbon equivalent.

[21] The paper clarifies that this represents a 50% increase of gradual permafrost thaw: it also provides the same estimate in terms of emissions per each degree of warming: 10 billion tonnes of carbon and 14 billion tonnes of carbon equivalent by 2100, and 25/35 billion tonnes of carbon/carbon equivalent by 2300

[22] The loss of these forests would be equivalent to the emissions of 52 billion tons of carbon, but this would be more than offset by the area's albedo effect increasing and reflecting more sunlight.

[23] Extra forest growth here would absorb around 6 billion tons of carbon, but because this area receives a lot of sunlight, this is very minor when compared to reduced albedo, as this vegetation absorbs more heat than the snow-covered ground it moves into.

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@@ Line 131: / Line 131: @@
 === Arctic sea ice ===
 {{Main|Arctic sea ice decline}}
+{{multiple image
+| total_width       = 450
+| width1            =
+| footer            =
+| image1            = NASA NH decadal ice extent 2022.png
+| alt1              = Average decadal extent and area of the Arctic Ocean sea ice since 1979.
+| caption1          = Average decadal extent and area of the Arctic Ocean sea ice since the start of satellite observations.
+| width2            =
+| image2            = NASA NH yearly ice extent 2022.png
+| alt2              = Annual trend in the Arctic sea ice extent and area for the 2011-2022 time period.
+| caption2          = Annual trend in the Arctic sea ice extent and area for the 2011-2022 time period.
+}}
 [[Arctic ice pack|Arctic sea ice]] was once identified as a potential tipping element. The loss of [[Ice–albedo feedback|sunlight-reflecting sea ice]] during summer exposes the (dark) ocean, which would warm. Arctic sea ice cover is likely to melt entirely under even relatively low levels of warming, and it was hypothesized that this could eventually transfer enough heat to the ocean to prevent sea ice recovery even if the global warming is reversed. Modelling now shows that this heat transfer during the [[Arctic summer]] does not overcome the cooling and the formation of new ice during the [[Arctic winter]]. As such, the loss of Arctic ice during the summer is not a tipping point for as long as the Arctic winter remains cool enough to enable the formation of new Arctic sea ice.<ref>{{Cite web |date=17 December 2021 |title=Does Arctic sea ice have a tipping point? |url=https://nsidc.org/cryosphere/icelights/2011/03/arctic-sea-ice-and-tipping-point |access-date=19 July 2022 |website=National Snow and Ice Data Center}}</ref><ref>{{Cite book |chapter= Technical Summary |last1=Arias |first1=Paola A. |last2 = Bellouin |first2= Nicolas |last3 = Coppola |first3 = Erika |last4 = Jones |first4 = Richard G. |last5 = Krinner |first5 = Gerhard |display-authors=4 |chapter-url= https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_TS.pdf |year= 2021 |title= IPCC AR6 WG1 |pages=76 }}</ref> However, if the higher levels of warming prevent the formation of new Arctic ice even during winter, then this change may become irreversible. Consequently, Arctic Winter Sea Ice is included as a potential tipping point in a 2022 assessment.<ref name="Explainer"/>
-Additionally, the same assessment argued that while the rest of the ice in the Arctic Ocean may recover from a total summertime loss during the winter, ice cover in the [[Barents Sea]] may not reform during the winter even below 2 degrees of warming.<ref name="Explainer"/> This is because the Barents Sea is already the fastest-warming part of the Arctic: in 2021-2022, while the warming within the [[Arctic Circle]] has been nearly four times faster than the global average since 1979,<ref name="Rantanen2022">{{Cite journal |last1=Rantanen |first1=Mika |last2=Karpechko |first2=Alexey Yu |last3=Lipponen |first3=Antti |last4=Nordling |first4=Kalle |last5=Hyvärinen |first5=Otto |last6=Ruosteenoja |first6=Kimmo |last7=Vihma |first7=Timo |last8=Laaksonen |first8=Ari |date=11 August 2022 |title=The Arctic has warmed nearly four times faster than the globe since 1979 |url=https://www.nature.com/articles/s43247-022-00498-3 |journal=Communications Earth & Environment |language=en |volume=3 |issue=1 |pages=1–10 |doi=10.1038/s43247-022-00498-3 |s2cid=251498876 |issn=2662-4435}}</ref><ref name="4X2021">{{cite web |date=2021-12-14 |title=The Arctic is warming four times faster than the rest of the world |url=https://www.science.org/content/article/arctic-warming-four-times-faster-rest-world |website=[[Science Magazine]] |language=en |access-date=6 October 2022}}</ref> Barents Sea warmed up to seven times faster than the global average.<ref name="Isaksen2022">{{Cite journal |last1=Isaksen |first1=Ketil |last2=Nordli |first2=Øyvind  |display-authors=etal |date=15 June 2022 |title=Exceptional warming over the Barents area |url=https://www.nature.com/articles/s41598-022-13568-5 |journal=Scientific Reports |language=en |volume=12 |doi=10.1038/s41598-022-13568-5 }}</ref><ref name="Carrington22">{{cite web |date=2022-06-15 |title=New data reveals extraordinary global heating in the Arctic |author=Damian Carrington |url=https://www.theguardian.com/environment/2022/jun/15/new-data-reveals-extraordinary-global-heating-in-the-arctic |website=[[The Guardian]] |language=en |access-date=7 October 2022}}</ref>This tipping point matters because of the decade-long history of research into the connections between patterns of Barents-[[Kara Sea|Kara]] Sea ice and the weather patterns elsewhere in Eurasia.<ref>{{cite journal |doi=10.1029/2009JD013568 |title=A link between reduced Barents-Kara sea ice and cold winter extremes over northern continents |year=2010 |last1=Petoukhov |first1=Vladimir |last2=Semenov |first2=Vladimir A. |journal=Journal of Geophysical Research |volume=115 |issue=D21 |pages=D21111 |bibcode=2010JGRD..11521111P|url=http://oceanrep.geomar.de/8738/1/2009JD013568-pip.pdf |doi-access=free }}</ref><ref>{{Cite journal |last1=He |first1=Shengping |last2=Gao |first2=Yongqi |last3=Furevik |first3=Tore |last4=Wang |first4=Huijun |last5=Li |first5=Fei |date=16 December 2017 |title=Teleconnection between sea ice in the Barents Sea in June and the Silk Road, Pacific–Japan and East Asian rainfall patterns in August |url=https://link.springer.com/article/10.1007/s00376-017-7029-y |journal=Advances in Atmospheric Sciences |volume=35 |pages=52–64 |doi=10.1007/s00376-017-7029-y }}</ref><ref>{{Cite journal |last1=Zhang |first1=Ruonan |last2=Screen |first2=James A. |date=16 June 2021 |title=Diverse Eurasian Winter Temperature Responses to Barents-Kara Sea Ice Anomalies of Different Magnitudes and Seasonality |url=https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2021GL092726|journal=Geophysical Research Letters |volume=48 |issue=13 |doi=10.1029/2021GL092726 }}</ref><ref>{{Cite journal |last1=Song |first1=Mirong |last2=Wang |first2=Zhao-Yin |last3=Zhu |first3=Zhu |last4=Liu |first4=Ji-Ping |date=August 2021 |title=Nonlinear changes in cold spell and heat wave arising from Arctic sea-ice loss |url=https://www.sciencedirect.com/science/article/pii/S1674927821001118  |journal=Advances in Climate Change Research |volume=12 |issue=4 |pages=553-562 |doi=10.1016/j.accre.2021.08.003}}</ref><ref>{{Cite journal |last1=Sun |first1=Jianqi |last2=Liu |first2=Sichang |last3=Cohen |first3=Judah |last4=Yu |first4=Shui |date=2 August 2022 |title=Influence and prediction value of Arctic sea ice for spring Eurasian extreme heat events |url=https://www.nature.com/articles/s43247-022-00503-9 |journal=Communications Earth & Environment |volume=3 |doi=10.1038/s43247-022-00503-9}}</ref>
+Additionally, the same assessment argued that while the rest of the ice in the Arctic Ocean may recover from a total summertime loss during the winter, ice cover in the [[Barents Sea]] may not reform during the winter even below 2 degrees of warming.<ref name="Explainer"/> This is because the Barents Sea is already the fastest-warming part of the Arctic: in 2021-2022, while the warming within the [[Arctic Circle]] has been nearly four times faster than the global average since 1979,<ref name="Rantanen2022">{{Cite journal |last1=Rantanen |first1=Mika |last2=Karpechko |first2=Alexey Yu |last3=Lipponen |first3=Antti |last4=Nordling |first4=Kalle |last5=Hyvärinen |first5=Otto |last6=Ruosteenoja |first6=Kimmo |last7=Vihma |first7=Timo |last8=Laaksonen |first8=Ari |date=11 August 2022 |title=The Arctic has warmed nearly four times faster than the globe since 1979 |url=https://www.nature.com/articles/s43247-022-00498-3 |journal=Communications Earth & Environment |language=en |volume=3 |issue=1 |pages=1–10 |doi=10.1038/s43247-022-00498-3 |s2cid=251498876 |issn=2662-4435}}</ref><ref name="4X2021">{{cite web |date=2021-12-14 |title=The Arctic is warming four times faster than the rest of the world |url=https://www.science.org/content/article/arctic-warming-four-times-faster-rest-world |website=[[Science Magazine]] |language=en |access-date=6 October 2022}}</ref> Barents Sea warmed up to seven times faster than the global average.<ref name="Isaksen2022">{{Cite journal |last1=Isaksen |first1=Ketil |last2=Nordli |first2=Øyvind  |display-authors=etal |date=15 June 2022 |title=Exceptional warming over the Barents area |url=https://www.nature.com/articles/s41598-022-13568-5 |journal=Scientific Reports |language=en |volume=12 |doi=10.1038/s41598-022-13568-5 }}</ref><ref name="Carrington22">{{cite web |date=2022-06-15 |title=New data reveals extraordinary global heating in the Arctic |author=Damian Carrington |url=https://www.theguardian.com/environment/2022/jun/15/new-data-reveals-extraordinary-global-heating-in-the-arctic |website=[[The Guardian]] |language=en |access-date=7 October 2022}}</ref>This tipping point matters because of the decade-long history of research into the connections between the state of Barents-[[Kara Sea|Kara]] Sea ice and the weather patterns elsewhere in Eurasia.<ref>{{cite journal |doi=10.1029/2009JD013568 |title=A link between reduced Barents-Kara sea ice and cold winter extremes over northern continents |year=2010 |last1=Petoukhov |first1=Vladimir |last2=Semenov |first2=Vladimir A. |journal=Journal of Geophysical Research |volume=115 |issue=D21 |pages=D21111 |bibcode=2010JGRD..11521111P|url=http://oceanrep.geomar.de/8738/1/2009JD013568-pip.pdf |doi-access=free }}</ref><ref>{{Cite journal |last1=He |first1=Shengping |last2=Gao |first2=Yongqi |last3=Furevik |first3=Tore |last4=Wang |first4=Huijun |last5=Li |first5=Fei |date=16 December 2017 |title=Teleconnection between sea ice in the Barents Sea in June and the Silk Road, Pacific–Japan and East Asian rainfall patterns in August |url=https://link.springer.com/article/10.1007/s00376-017-7029-y |journal=Advances in Atmospheric Sciences |volume=35 |pages=52–64 |doi=10.1007/s00376-017-7029-y }}</ref><ref>{{Cite journal |last1=Zhang |first1=Ruonan |last2=Screen |first2=James A. |date=16 June 2021 |title=Diverse Eurasian Winter Temperature Responses to Barents-Kara Sea Ice Anomalies of Different Magnitudes and Seasonality |url=https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2021GL092726|journal=Geophysical Research Letters |volume=48 |issue=13 |doi=10.1029/2021GL092726 }}</ref><ref>{{Cite journal |last1=Song |first1=Mirong |last2=Wang |first2=Zhao-Yin |last3=Zhu |first3=Zhu |last4=Liu |first4=Ji-Ping |date=August 2021 |title=Nonlinear changes in cold spell and heat wave arising from Arctic sea-ice loss |url=https://www.sciencedirect.com/science/article/pii/S1674927821001118  |journal=Advances in Climate Change Research |volume=12 |issue=4 |pages=553-562 |doi=10.1016/j.accre.2021.08.003}}</ref><ref>{{Cite journal |last1=Sun |first1=Jianqi |last2=Liu |first2=Sichang |last3=Cohen |first3=Judah |last4=Yu |first4=Shui |date=2 August 2022 |title=Influence and prediction value of Arctic sea ice for spring Eurasian extreme heat events |url=https://www.nature.com/articles/s43247-022-00503-9 |journal=Communications Earth & Environment |volume=3 |doi=10.1038/s43247-022-00503-9}}</ref>
 === Coral reef die-off ===
@@ Line 142: / Line 154: @@
 === Mountain glaciers ===
 {{Main|Retreat of glaciers since 1850}}
+[[File:HKH-Glacier-Mass-Change.png|thumb|Observed glacier mass loss in the Hindu Kush Himalaya region since the 20th century.]]
 Mountain [[glacier]]s are the largest repository of land-bound ice after the Greenland and the Antarctica ice sheets, and they are also undergoing melting as the result of climate change. A glacier tipping point is when it enters a disequilibrium state with the climate and will melt away unless the temperatures go down.<ref name=hubbard>{{cite book|last1=Hubbard|first1=Bryn|author2=Neil F. Glasser|title=Field Techniques in Glaciology and Glacial Geomorphology|date=May 20, 2005|publisher=Wiley|isbn=978-0470844274|pages=179–198|url=https://books.google.com/books?id=Y9MWQT0gdBMC&pg=PA179|access-date=November 23, 2020}}</ref><ref name=pelto2>{{cite journal|author=Pelto, M.S.|title=Forecasting temperate alpine glacier survival from accumulation zone observations|journal=The Cryosphere|year=2010|volume=4|issue=1|pages=67–75|doi=10.5194/tc-4-67-2010|bibcode=2010TCry....4...67P|url=https://tc.copernicus.org/articles/4/67/2010/|doi-access=free|access-date=November 23, 2020}}</ref> Examples include glaciers of the North [[Cascade Range]], where even in 2005 67% of the glaciers observed were in disequilibrium and will not survive the continuation of the present climate, <ref>{{cite web | author=Mauri S. Pelto |publisher=[[Nichols College]] | title=North Cascade Glacier Terminus Behavior | url=http://www.nichols.edu/departments/glacier/north%20cascade%20glacier%20retreat.htm | access-date=August 7, 2016}}</ref> or the French [[Alps]], where The Argentière and Mer de Glace glaciers are expected to disappear completely by end of the 21st century if current climate trends persist.<ref>{{cite web |last1=Vaughn |first1=Adam |title=Special report: How climate change is melting France's largest glacier |url=https://www.newscientist.com/article/mg24332483-600-special-report-how-climate-change-is-melting-frances-largest-glacier/ |publisher=New Scientist |access-date=February 3, 2021 |date=September 18, 2019}}</ref>
+The absolute largest amount of glacier ice is located in the [[Hindu Kush]] [[Himalaya]] region, which is colloquially known as the Earth's "Third Pole" as the result. It is believed that one third of that ice will be lost by 2100 even if the warming is limited to 1.5 °C, while the "intermediate" and "severe" climate change scenarios ([[Representative Concentration Pathway|RCP]] 4.5 and 8.5)  are likely to lead to the losses of 50% and >67% of the region's glaciers over the same timeframe. Glacier melt is projected to accelerate regional river flows until the amount of meltwater peaks around 2060, going into an irreversible decline afterwards. Since regional precipitation will continue to increase even as the glacier meltwater contribution declines, annual river flows are only expected to diminish in the western basins where contribution from the [[monsoon]] is low: however, [[irrigation]] and [[hydropower]] generation would still have to adjust to greater interannual variability and lower pre-monsoon flows in all of the region's rivers.<ref>{{cite web |date=4 February 2019 |author=Damian Carrington |title=A third of Himalayan ice cap doomed, finds report |url=https://www.theguardian.com/environment/2019/feb/04/a-third-of-himalayan-ice-cap-doomed-finds-shocking-report |access-date=20 October 2022}}</ref> <ref name=":HKH2019Cryo">{{Cite book |last1=Bolch |first1=Tobias |last2=Shea |first2=Joseph M. |last3=Liu |first3=Shiyin |last4=Azam |first4=Farooq M. |last5=Gao |first5=Yang |last6=Gruber |first6=Stephan |last7=Immerzeel |first7=Walter W. |last8=Kulkarni |first8=Anil |last9=Li |first9=Huilin |last10=Tahir |first10=Adnan A. |last11=Zhang |first11=Guoqing |last12=Zhang |first12=Yinsheng |title=The Hindu Kush Himalaya Assessment: Mountains, Climate Change, Sustainability and People |date=5 January 2019 |chapter=Status and Change of the Cryosphere in the Extended Hindu Kush Himalaya Region |chapter-url=https://link.springer.com/chapter/10.1007/978-3-319-92288-1_3 |pages=209-255 }}</ref>
 === Sahel greening ===
@@ Line 186: / Line 201: @@
 == Cascading tipping points ==
+[[File:Wunderling 2021 tipping cascade.png|thumb|A proposed tipping cascade with four tipping elements.]]
 Crossing a threshold in one part of the climate system may trigger another tipping element to tip into a new state. Such sequences of thresholds are called cascading tipping points, an example of a [[domino effect]].<ref>{{Cite journal |last1=Rocha |first1=Juan C. |last2=Peterson |first2=Garry |last3=Bodin |first3=Örjan |last4=Levin |first4=Simon |date=2018 |title=Cascading regime shifts within and across scales |journal=Science |volume=362 |issue=6421 |pages=1379–1383 |bibcode=2018Sci...362.1379R |doi=10.1126/science.aat7850 |issn=0036-8075 |pmid=30573623 |doi-access=free |s2cid=56582186}}</ref> Ice loss in West Antarctica and Greenland will significantly alter [[Ocean current|ocean circulation]]. Sustained warming of the northern high latitudes as a result of this process could activate tipping elements in that region, such as permafrost degradation, and boreal [[forest dieback]].<ref name="Lenton2019" /> Thawing permafrost is a threat multiplier because it holds roughly twice as much carbon as the amount currently circulating in the atmosphere.<ref>{{Cite web |title=The irreversible emissions of a permafrost 'tipping point' |url=https://www.weforum.org/agenda/2020/02/irreversible-emissions-permafrost-tipping-point/ |access-date=17 July 2022 |website=[[World Economic Forum]]}}</ref> Loss of ice in Greenland likely destabilises the West Antarctic ice sheet via sea level rise, and vice-versa, especially if Greenland were to melt first as West Antarctica is particularly vulnerable to contact with warm sea water.<ref name=":2">{{Cite journal |last1=Wunderling |first1=Nico |last2=Donges |first2=Jonathan F. |last3=Kurths |first3=Jürgen |last4=Winkelmann |first4=Ricarda |date=3 June 2021 |title=Interacting tipping elements increase risk of climate domino effects under global warming |url=https://esd.copernicus.org/articles/12/601/2021/ |journal=Earth System Dynamics |language=English |volume=12 |issue=2 |pages=601–619 |doi=10.5194/esd-12-601-2021 |issn=2190-4979}}</ref>