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"Runaway climate change" redirects here. Not to be confused with Runaway greenhouse effect.

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

Positive tipping point in society

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

Meltwater pulse 1A was a period of abrupt sea level rise around 14,000 years ago. It may be an example of a tipping point.[6]

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)
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 Partial:30 GtC/0.1 °C Total:75 GtC/0.2 °C 0.4 to 2.0
Boreal Permafrost (collapse) 4.0 3.0 6.0 50 10 300 125 - 250 GtC/175 - 350 GtCe/0.2 - 0.4 ~
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 ~ ~ ~ ~
Regional impact tipping elements[18]
Proposed climate tipping element (and tipping point) Threshold ( °C) Timescale (years) Maximum Impact ( °C)
Estimated Minimum Maximum Estimated Minimum Maximum Global Regional
Boreal Permafrost (abrupt thaw) 1.5 1.0 2.3 200 100 300 Abrupt thaw adds 50% to gradual: 10GtC/14 GtCe/0.04°C per °C by 2100;25GtC/35 GtCe/0.11°C per °C by 2300 ~
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 ? +52 GtC/net -0.18 -0.5 to -2
Boreal Forest (northern expansion) 4.0 1.5 7.2 100 40 ? -6 GtC/net +0.14 0.5-1.0

Gulf Stream System

See also: Shutdown of thermohaline circulation
The Northern part of the Atlantic Meridional Overturning Circulation

The Atlantic Meridional Overturning Circulation (AMOC), also known as the Gulf Stream System, is a large system of ocean currents.[19][20] It is driven by differences in the density of water; colder and more salty water is heavier than warmer fresh water.[20] The AMOC acts as a conveyor belt, sending warm surface water from the tropics north, and carrying cold fresh water back south.[19] 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.[20] 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,[21] 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.[22] If the AMOC does shut down, a new stable state could emerge that lasts for thousands of years, possibly triggering other tipping points.[10]

A 2021 study found early-warning signals in a set of AMOC indices, suggesting that the AMOC may be close to tipping.[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]

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.[28] It holds enough water, that if completely melted, could raise sea levels globally by 7.2 metres.[29] Due to global warming, the ice sheet is melting at an accelerating rate, adding almost 1 mm to global sea levels every year.[30] 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.[31]

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.[32] 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.[32]

East Antarctic ice sheet disintegration

East Antarctic ice sheet is 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. [33] 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

See also: Deforestation of the Amazon rainforest and Climate change in Brazil

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.[34] 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.[35][36]

Permafrost thaw

See also: Gas hydrate stability zone and Permafrost carbon cycle

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.[37][10] Subsea permafrost up to 100 metres thick also occurs on the sea floor under part of the Arctic Ocean.[38] 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.[38] 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.[39][40]

Boreal forest biome shift

Extensive boreal forest biomes, colloquially known as taiga, could eventually experience irreversible transition to a different biome state. [41] 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]

Coral reef die-off

Main article: Coral bleaching
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.[42] 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.[43] A sustained ocean temperature spike of 1 °C above average is enough to cause bleaching.[44] 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.[45] 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.[46] 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.[47]

Arctic sea ice

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.[48][49] 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. 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]

North Atlantic Current

Some climate models indicate that the deep convection in Labrador-Irminger Seas, which is the principal element of the North Atlantic Current, could collapse under certain global warming pathways, which would result in some cooling over Europe and have a substantial effect on precipitation patterns. A 2021 study found that this collapse occurs in only four CMIP6 models out of 35 analyzed. However, only 11 models out of 35 can simulate North Atlantic Current with a high degree of accuracy, and this includes all four models which simulate collapse of the current. As the result, the study estimated the risk of an abrupt cooling event over Europe caused by the collapse of the current at 36.4%, which is lower than 45.5% chance estimated by the previous generation of models. [50]

Sahel greening

Main article: African humid period

Some simulations of global warming and increased carbon dioxide concentrations have shown a substantial increase in precipitation in the Sahel/Sahara.[51] This and the increased plant growth directly induced by carbon dioxide[52] could lead to an expansion of vegetation into present-day desert, although it would be less extensive than during the mid-Holocene[51] and perhaps accompanied by a northward shift of the desert, i.e. a drying of northernmost Africa.[53] Such a precipitation increase may also reduce the amount of dust originating in Northern Africa,[54] 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.[55]

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.[56] In addition, the end of the 20th century drying trend may be due to global warming.[57] On the other hand, West Africa and parts of East Africa may become drier during given seasons and months.[58] [57] Currently, the Sahel is becoming greener but precipitation has not fully recovered to levels reached in the mid-20th century.[53]

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,[59] in particular through increased inter-hemispheric temperature gradients.[60] The direct effect of heat on plants may be detrimental.[61] Non-linear increases in vegetation cover are also possible.[60] One study in 2003 showed that vegetation intrusions in the Sahara can occur within decades after strong rises in atmospheric carbon dioxide[62] but would not cover more than about 45% of the Sahara.[63] That climate study also indicated that vegetation expansion can only occur if grazing or other perturbations to vegetation growth do not hamper it.[64] On the other hand, increased irrigation and other measures to increase vegetation growth such as the Great Green Wall could enhance it.[61]

Equatorial Stratocumulus clouds

In 2019, a study had found that in a large eddy simulation model, equatorial stratocumulus clouds, which cover 20% of the low-latitude oceans, could break up and scatter when CO2 levels rise above 1,200 ppm (almost three times higher than the current levels, and over 4 times greater than the preindustrial levels). The study estimated that this would cause a surface warming of about 8 degrees Celsius globally and 10 degrees in the subtropics (in addition to at least 4 degrees of warming already expected from CO2 concentrations around 1,200 ppm), and that these clouds would not reform until the CO2 concentrations drop to a much lower level. [65] It was suggested that this finding could help explain past episodes of unusually rapid warming such as Paleocene-Eocene Thermal Maximum [66] However, because large eddy simulation models are simpler and smaller-scale than the general circulation models used for climate projections, with limited representation of atmospheric processes like subsidence, this finding is currently considered speculative. Additionally, CO2 concentrations would only reach 1,200 ppm if the world follows Representative Concentration Pathway 8.5, which represents the highest possible greenhouse gas emission scenario and involves a massive expansion of coal infrastructure. In that case, 1,200 ppm would be passed shortly after 2100. [67]

In 2020, further work from the same authors revealed that this tipping point cannot be stopped with solar geoengineering: in a hypothetical scenario where very high CO2 emissions continue for a long time but are offset with extensive solar geoengineering, the break-up of stratocumulus clouds is simply delayed until CO2 concentrations hit 1,700 ppm, at which point it would still cause around 5 degrees of unavoidable warming. [68]

Formerly considered tipping elements

The possibility that the El Niño–Southern Oscillation (ENSO) is a tipping element had attracted attention in the past.[69] 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.[70] El Niño-induced droughts may increase the likelihood of forest fires in the Amazon.[71] The threshold for tipping was estimated to be between 3.5 and 7 °C of global warming in 2016.[72] 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.[69] So far, there is no definitive evidence indicating changes in ENSO behaviour, [71], 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." [73] 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. [74] However, more recent research has demonstrated that warming tends to strengthen the Indian monsoon, [75] and it is projected to strengthen in the future. [76]

Mathematical theory

Illustration of three types of tipping point; (a), (b) noise-, (c), (d) bifurcation- and (e), (f) rate-induced. (a), (c), (e) example time-series (coloured lines) through the tipping point with black solid lines indicating stable climate states (e.g. low or high rainfall) and dashed lines represent the boundary between stable states. (b), (d), (f) stability landscapes provide an understanding for the different types of tipping point. The valleys represent different climate states the system can occupy with hill tops separating the stable states.

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.[77][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][78] 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.[79][80]

Many types of bifurcations show hysteresis,[81] 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.[82]

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.[83][84] 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.[85][86]

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.[83][87] 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.[87] Quality limitations of paleodata further complicate the development of EWSs.[87] They have been developed for detecting tipping due to drought in forests in California,[88] and melting of the Pine Island Glacier in West Antarctica,[86] 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.[89]

Human-induced changes in the climate system may be too fast for early warning signals to become evident, especially in systems with inertia.[90]

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.[91] An example is the Dansgaard–Oeschger events during the last ice age, with 25 occurrences of sudden climate fluctuations over a 500 year period.[92]

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".[93][94] 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.[95]

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.[96] 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.[97] 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.[98]

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.[98][99] 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”.[100]

Impacts

See also: Effects of climate change

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.[101] 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.[87]

A 2021 meta study 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.[102]

Runaway greenhouse effect

A runaway greenhouse effect is a tipping point so extreme that oceans evaporate[103] and the water vapour escapes to space, an irreversible climate state that happened on Venus.[104] A runaway greenhouse effect has virtually no chance of being caused by people.[105]

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.[106]

See also

References

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