Tipping points in the climate system
In
Tipping points are often, but not necessarily,
The geological record shows many abrupt changes that suggest tipping points may have been crossed in pre-historic times.[10]
Definition
The IPCC Sixth Assessment Report defines a tipping point as a "critical threshold beyond which a system reorganizes, often abruptly and/or irreversibly".[11] It can be brought about by a small disturbance causing a disproportionately large change in the system. It can also be associated with self-reinforcing feedbacks, which could lead to changes in the climate system irreversible on a human timescale.[12] For any particular climate component, the shift from one state to a new stable state may take many decades or centuries.[12]
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.".[13]
In ecosystems and in social systems, a tipping point can trigger a regime shift, a major systems reorganisation into a new stable state.[14] Such regime shifts need not be harmful. In the context of the climate crisis, the tipping point metaphor is sometimes used in a positive sense, such as to refer to shifts in public opinion in favor of action to mitigate climate change, or the potential for minor policy changes to rapidly accelerate the transition to a green economy.[15][16][17]
Comparison of tipping points
Scientists have identified many elements in the climate system which may have tipping points.[18][12] In the early 2000s the IPCC began considering the possibility of tipping points, originally referred to as "large-scale discontinuities". At that time the IPCC concluded they would only be likely in the event of global warming of 4 °C (7.2 °F) or more above preindustrial times, and another early assessment placed most tipping point thresholds at 3–5 °C (5.4–9.0 °F) above 1980–1999 average warming.[19] Since then estimates for global warming thresholds have generally fallen, with some thought to be possible in the Paris Agreement range (1.5–2 °C (2.7–3.6 °F)) by 2016.[20] As of 2021 tipping points are considered to have significant probability at today's warming level of just over 1 °C (1.8 °F), with high probability above 2 °C (3.6 °F) of global warming.[5] Some tipping points may be 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.[21][22]
As of September 2022, nine 'global core' tipping elements and seven 'regional impact' tipping elements have been identified.
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 | 10,000 | 1,000 | 15,000 | 0.13 | 0.5 to 3.0 | |
West Antarctic Ice Sheet (collapse) | 1.5 | 1.0 | 3.0 | 2,000 | 500 | 13,000 | 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 | 2,000 | 500 | 10,000 | 0.05 | ? | |
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 | ? | 10,000 | ? | 0.6 | 2.0 | |
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) | 4.0 | 1.4 | 8.0 | 50 | 15 | 300 | -0.5 | -4 to -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.
Proposed climate tipping element (and tipping point) | Threshold ( °C) | Timescale (years) | Maximum Impact ( °C) | ||||||
---|---|---|---|---|---|---|---|---|---|
Estimated | Minimum | Maximum | Estimated | Minimum | Maximum | Global | Regional | ||
Low-latitude Coral Reefs (dieoff) | 1.5 | 1.0 | 2.0 | 10 | ~ | ~ | ~ | ~ | |
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 | 1,000 | 0.08 | + | |
Sahel and W.African Monsoon (greening) | 2.8 | 2.0 | 3.5 | 50 | 10 | 500 | ~ | + | |
Boreal Forest (southern dieoff) |
4.0 | 1.4 | 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.
Tipping points in the cryosphere
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.[24] The water which it holds would, if completely melted, raise sea levels globally by 7.2 metres (24 ft).[25] Due to global warming, the ice sheet is melting at an accelerating rate, adding almost 1 mm to global sea levels every year.[26] Around half of the ice loss occurs via surface melting, and the remainder occurs at the base of the ice sheet where it touches the sea, by calving (breaking off) icebergs from its margins.[27]
The Greenland ice sheet has a tipping point because of the melt-elevation
West Antarctic ice sheet disintegration
The West Antarctic Ice Sheet (WAIS) is a large ice sheet in Antarctica; in places more than 4 kilometres (2.5 mi) thick. It sits on bedrock mostly below sea level, having formed a deep subglacial basin due to the weight of the ice sheet over millions of years.[31] 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 once the WAIS's grounding lines (the point at which ice no longer sits on rock and becomes floating ice shelves) retreat behind the edge of the subglacial basin, resulting in self-sustaining retreat in to the deeper basin - a process known as the 'Marine Ice Sheet Instability' (MISI).[32][33] Thinning and collapse of the WAIS's ice shelves is helping to accelerate this grounding line retreat. If completely melted, the WAIS would contribute around 3.3 metres (11 ft) of sea level rise over thousands of years.[12]
Ice loss from the WAIS is accelerating, and some outlet glaciers are estimated to be close to or possibly already beyond the point of self-sustaining retreat.[34][35][36] 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.[37]
Like with the other ice sheets, there is a counteracting negative feedback - greater warming also intensifies the
East Antarctic ice sheet disintegration
Arctic sea ice decline
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 °C (3.6 °F) of warming.[6] This is because the Barents Sea is already the fastest-warming part of the Arctic: in 2021-2022 it was found that while the warming within the Arctic Circle has already been nearly four times faster than the global average since 1979,[43][44] Barents Sea warmed up to seven times faster than the global average.[45][46] 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.[47][48][49][50][51]
Retreat of mountain glaciers
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.[53][54] 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,[55] 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.[56] Altogether, it was estimated in 2023 that 49% of the world's glaciers would be lost by 2100 at 1.5 °C (2.7 °F) of global warming, and 83% of glaciers would be lost at 4 °C (7.2 °F). This would amount to one quarter and nearly half of mountain glacier *mass* loss, respectively, as only the largest, most resilient glaciers would survive the century. This ice loss would also contribute ~9 cm (3+1⁄2 in) and ~15 cm (6 in) to sea level rise, while the current likely trajectory of 2.7 °C (4.9 °F) would result in the SLR contribution of ~11 cm (4+1⁄2 in) by 2100.[52]
The absolute largest amount of glacier ice is located in the
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.[60][12] Subsea permafrost up to 100 metres thick also occurs on the sea floor under part of the Arctic Ocean.[61] This frozen ground holds vast amounts of carbon from plants and animals that have 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.[61] 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.[62][63]
Atlantic Meridional Overturning Circulation (AMOC)
The Atlantic Meridional Overturning Circulation (AMOC), also known as the Gulf Stream System, is a large system of ocean currents.[64][65] It is driven by differences in the density of water; colder and more salty water is heavier than warmer fresh water.[65] The AMOC acts as a conveyor belt, sending warm surface water from the tropics north, and carrying cold fresh water back south.[64] 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.[65] 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.[12]
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,[66] 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.[67] If the AMOC does shut down, a new stable state could emerge that lasts for thousands of years, possibly triggering other tipping points.[12]
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.[68] Another 2021 study found early-warning signals in a set of AMOC indices, suggesting that the AMOC may be close to tipping.[69] 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.[70] 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.[71][72]
North Subpolar Gyre
Some
Southern Ocean overturning circulation
Southern ocean overturning circulation itself consists of two parts, the upper and the lower cell. The smaller upper cell is most strongly affected by
Tipping points in terrestrial systems
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.[12] 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.[91] 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.[92][93]
Boreal forest biome shift
During the last quarter of the twentieth century, the zone of latitude occupied by taiga experienced some of the greatest temperature increases on Earth. Winter temperatures have increased more than summer temperatures. In summer, the daily low temperature has increased more than the daily high temperature.[94] It has been hypothesised that the boreal environments have only a few states which are stable in the long term - a treeless tundra/steppe, a forest with >75% tree cover and an open woodland with ~20% and ~45% tree cover. Thus, continued climate change would be able to force at least some of the presently existing taiga forests into one of the two woodland states or even into a treeless steppe - but it could also shift tundra areas into woodland or forest states as they warm and become more suitable for tree growth.[95]
These trends were first detected in the Canadian boreal forests in the early 2010s,
A 2021 paper had confirmed that the boreal forests are much more strongly affected by climate change than the other forest types in Canada and projected that most of the eastern Canadian boreal forests would reach a tipping point around 2080 under the RCP 8.5 scenario, which represents the largest potential increase in anthropogenic emissions.[104] Another 2021 study projected that under the "moderate" SSP2-4.5 scenario, boreal forests would experience a 15% worldwide increase in biomass by the end of the century, but this would be more than offset by the 41% biomass decline in the tropics.[105] In 2022, the results of a 5-year warming experiment in North America had shown that the juveniles of tree species which currently dominate the southern margins of the boreal forests fare the worst in response to even 1.5 °C (2.7 °F) or 3.1 °C (5.6 °F) of warming and the associated reductions in precipitation. While the temperate species which would benefit from such conditions are also present in the southern boreal forests, they are both rare and have slower growth rates.[106]
Sahel greening
Some simulations of
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.[110]: 16–17 Currently, the Sahel is becoming greener but precipitation has not fully recovered to levels reached in the mid-20th century.[109]: 267
A study from 2022 concluded: "Clearly the existence of a future tipping threshold for the WAM (
Vulnerable stores of tropical peat carbon: Cuvette Centrale peatland
In 2017, it was discovered that 40% of the Cuvette Centrale wetlands are underlain with a dense layer of peat, which contains around 30 petagrams (billions of tons) of carbon. This amounts to 28% of all tropical peat carbon, equivalent to the carbon contained in all the forests of the Congo Basin. In other words, while this peatland only covers 4% of the Congo Basin area, its carbon content is equal to that of all trees in the other 96%.[111][112][113] It was then estimated that if all of that peat burned, the atmosphere would absorb the equivalent of 20 years of current United States carbon dioxide emissions, or three years of all anthropogenic CO2 emissions.[112][114]
This threat prompted the signing of Brazzaville Declaration in March 2018: an agreement between
Other tipping points
Coral reef die-off
Around 500 million people around the world depend on coral reefs for food, income, tourism and coastal protection.[118] 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.[119] A sustained ocean temperature spike of 1 °C (1.8 °F) above average is enough to cause bleaching.[120] 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.[121] 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.[122] The IPCC estimates that by the time temperatures have risen to 1.5 °C (2.7 °F) above pre-industrial times, Coral reefs... are projected to decline by a further 70–90% at 1.5 °C; and that if the world warms by 2 °C (3.6 °F), they will become extremely rare.[123]
Break-up of equatorial stratocumulus clouds
In 2019, a study employed a
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.[129] 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.[3] Thawing permafrost is a threat multiplier because it holds roughly twice as much carbon as the amount currently circulating in the atmosphere.[130] 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.[131]
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 (3.6 °F) – the upper limit set by the Paris Agreement in 2015.[131][132] 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".[133] A network model analysis suggested that temporary overshoots of climate change – increasing global temperature beyond Paris Agreement goals temporarily as often projected – can substantially increase risks of climate tipping cascades ("by up to 72% compared with non-overshoot scenarios").[134][135]
Formerly considered tipping elements
The possibility that the
The
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.[148][149]
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.[149][150] 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.[151][152]
Many types of bifurcations show hysteresis,[153] 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.[154]
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.[155][156] 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.[157][158]
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.[155][159] 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.[159] Quality limitations of paleodata further complicate the development of EWSs.[159] They have been developed for detecting tipping due to drought in forests in California,[160] and melting of the Pine Island Glacier in West Antarctica,[158] 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.[161]
Human-induced changes in the climate system may be too fast for early warning signals to become evident, especially in systems with inertia.[162]
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.[163] An example is the Dansgaard–Oeschger events during the last ice age, with 25 occurrences of sudden climate fluctuations over a 500 year period.[164]
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.
Potential impacts
Tipping points can have very severe impacts.
Scholars have described a threshold which, if crossed, could trigger multiple tipping points and self-reinforcing feedback loops that would prevent stabilization of the climate, causing much greater warming and sea-level rises and leading to severe disruption to ecosystems, society, and economies.
Geological timescales
The geological record shows that there have been abrupt changes in the
Runaway greenhouse effect
A
See also
- African humid period#Implications for future global warming
- Greenhouse and icehouse Earth
- Climate sensitivity
- Planetary boundaries
- World Scientists' Warning to Humanity
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External links
- Global tipping points and research on tipping points at the University of Exeter
- The climate tipping points - 2022 BBC radio documentary