Tipping points in the climate system

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Possible tipping elements in the climate system.
Interactions of climate tipping points (bottom) with associated tipping points in the socioeconomic system (top) on different time scales.[1]

A tipping point in the climate system is a critical threshold that, when exceeded, leads to large and often irreversible changes in the state of the system. The term 'tipping point' is used by climate scientists to identify vulnerable features of the climate system.[2][3] If they 'tip', they are likely to have severe impacts on human society.[4][5]

The Intergovernmental Panel on Climate Change began considering the possibility of tipping points 20 years ago. At that time the IPCC concluded they would only be likely in the event of unmitigated global warming of 4 degrees C or more above preindustrial times. Tipping points are now considered to have significant probability at today's warming level of just over 1 degree C, with high probability above 2 degrees C of global warming.[6]

Large-scale components of the Earth system that may pass a tipping point are called tipping elements.[7] At least 15 different elements of the climate system, such as the Greenland and Antarctic ice sheets, have been identified as possible tipping points.[8][9] A danger is that if the tipping point in one system is crossed, this could lead to a cascade of other tipping points.[10] If a cascade occurs, this could cause a hothouse Earth in which global average temperatures would be higher than at any period in the past 1.2 million years.[11]

Tipping points are not necessarily abrupt. For example, with a temperature rise between 1.5 and 2 degrees Celsius, large parts of the Greenland ice sheet are likely to melt – but the melting process is set to take millennia.[12] However, a 2021 study of the Antarctic ice sheet has shown that tipping into full ice sheet retreat could occur by 2025.[13]

Definition[edit]

The 2021 IPCC Sixth Assessment Report says that a tipping point in the climate system is a "critical threshold beyond which a system reorganizes, often abruptly and/or irreversibly".[14] It can be brought about by a small disturbance causing a disproportionately large change in the system. Tipping points also require self-reinforcing feedbacks[9] and lead to changes in the climate system which are irreversible on a human timescale. For any particular climate component, the shift from one state to a new state may take many decades or centuries - although palaeoclimate data and global climate models, suggest that the "climate system may abruptly 'tip' from one regime to another in a comparatively short time."[15]

The Special Report on the Ocean and Cryosphere in a Changing Climate released by the Intergovernmental Panel on Climate Change (IPCC) in 2019 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. Tipping points are also used when referring to impact: the term can imply that an impact tipping point is (about to be) reached in a natural or human system".[16]

The term 'tipping point' has become a foundational concept in climate change science discussions and is used by climate scientists and the news media as a metaphor for "drastic, irreversible and dangerous climate change".[17] [18]

Geological record[edit]

The geologic record of temperature and greenhouse gas concentration allows climate scientists to gather information on climate feedbacks that lead to different climate states. A key finding is that when the concentration of carbon dioxide in the atmosphere goes up, the average global temperature goes up with it.[19] In the last 100 million years, global temperatures have peaked twice, tipping the climate into a hothouse state. During the Cretaceous period, roughly 92 million years ago, CO2 levels were around 1,000 ppm.[20] The climate was so hot that crocodile-like reptiles lived in what is now the Canadian Arctic, and forests thrived near the South Pole. The second hothouse period was the Paleocene-Eocene Thermal Maximum (PETM) 55-56 million years ago. Records suggest that during the PETM, the average global temperature rose between 5 and 8 °C; there was no ice at the poles, allowing palm trees and crocodiles to live above the Arctic Circle.[21]

The geologic record fails to provide clarity as to whether past temperature changes have taken only a few decades or many millennia. In March 2020, researchers showed that larger ecosystems can 'collapse' faster than previously thought, the Amazon rainforest for example (to a savanna) within ~50 years and the coral reefs of the Caribbean within ~15 years once a mode of 'collapse' is triggered, which in case of Amazonia they estimate could be as early as in 2021.[22][23][24][25]

However, as recently as three million years ago, atmospheric concentrations of CO2 matched today's levels. At that time, average global temperatures were 3C higher than they are now and sea levels were 5-to-25 metres higher.[26]

Combining this historical information with the understanding of current climate change resulted in the finding published in 2018 in Proceedings of the National Academy of Sciences that "a 2 °C warming could activate important tipping elements, raising the temperature further to activate other tipping elements in a domino-like cascade that could take the Earth System to even higher temperatures".[11][27]

Tipping elements[edit]

Scientists have identified a large set of elements in the climate system which may have a tipping point.[28][9] It is possible that some tipping points are close to being crossed or have already been crossed, like the ice sheets in West Antarctic and Greenland, warm-water coral reefs, and the Amazon rainforest.[29][30]

Shutdown of the Atlantic Meridional Overturning Circulation[edit]

The Atlantic Meridional Overturning Circulation (AMOC), also known as the Gulf Stream System, is a large system of ocean currents.[31][32] It is driven by differences in the density of water; colder and more salty water is heavier than warmer fresh water.[32] The AMOC acts as a conveyor belt, sending warm surface water from the tropics north, and carrying cold fresh water back south.[31] As warm water flows northwards, some evaporates which increases salinity. It also cools when it mixes with fresh water from melting ice in Greenland. Cold, salty water is more dense and slowly begins to sink.[33] Several kilometres below the surface, cold, dense water begins to move south. Increased rainfall and the melting of continental ice due to global warming is diluting surface sea water and warming it up. The lighter water is less able to sink, slowing down the circulation.[34]

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,[35] but it may do so before 2300 if emissions are very high. A weakening of 24% to 39% is expected depending on greenhouse emissions, even without tipping behaviour.[36] If the AMOC does shut down, a new stable state could emerge that lasts for thousands of years, possibly triggering other tipping points.[34]

A 2021 study found early-warning signals in a set of AMOC indices,[37] suggesting that the AMOC "may be nearing a shutdown".[38]

West Antarctic ice sheet disintegration[edit]

The West Antarctic Ice Sheet (WAIS) is one of three regions making up Antarctica. In places it is more than 4 kilometres thick and sits on bedrock that largely lies below sea level.[39] As such, it is in contact with ocean heat, as well as warmer air which makes it vulnerable to rapid 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 land ice into the ocean - with the potential to raise sea levels by around 3.3 metres.[40]

Ice loss from WAIS is accelerating.[41] The palaeo 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.[42] 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.[13]

Greenland ice sheet disintegration[edit]

The Greenland ice sheet is the second largest mass of ice in the world, and is three times the size of Texas.[43] It holds enough water, which if it melted, could raise global sea levels by 7.2 metres.[44] Due to global warming, the ice sheet is melting at an accelerating rate adding around 0.7 mm to global sea levels every year.[45] 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 the breaking off, or 'calving', of icebergs from its edge.[46]

Snowfall in Greenland is no longer able to compensate for the loss of ice due to this melting, such that the disintegration of the ice sheet may now be inevitable.[47] Melting would not occur abruptly, but would be irreversible over millennia.[48][49]

Amazon rainforest dieback[edit]

The Amazon rainforest is the largest tropical rainforest in the world. It is twice the size of India and spans nine countries in South America.[50] It generates around half of its own rainfall by recycling moisture through evaporation and transpiration as air moves across the forest.[51]

Deforestation of the Amazon began when colonists began establishing farms in the forest in the 1960s. They generally slashed and burned the trees in order to cultivate crops. However, soils in the Amazon are only productive for a short period after the land is cleared, so farmers would simply move and clear more land.[52] Other colonists cleared land to raise cattle, leading to further deforestation and environmental damage.[53] Heatwaves and drought have now become a factor driving additional tree deaths. This indicates that the Amazon is experiencing climatic conditions beyond its adaptative limits.[54]

In 2021, the first long-term study of greenhouse gases in the Amazon rainforest found that in the 2010s the rainforest released more carbon dioxide than it absorbed.[55] The forest had previously been a carbon sink, but is now emitting a billion tonnes of carbon dioxide a year. Deforestation has led to fewer trees which means more severe droughts and heatwaves develop leading to more tree deaths and more fires.[56][57]

Permafrost and methane hydrates[edit]

Permafrost is ground containing soil and/or organic material bound together by ice and which has remained frozen for at least two years.[58] It covers around a quarter of the non-glaciated land in the northern hemisphere – mainly in Siberia, Alaska, northern Canada and the Tibetan plateau – and can be as much as a kilometre thick.[59] Subsea permafrost up to 100 metres thick also occurs on the sea floor under part of the Arctic Ocean.[58] This frozen ground holds vast amounts of carbon, derived 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 currently in the Earth's atmosphere.[60]

As the climate warms and the permafrost begins to thaw, carbon dioxide and methane are released into the atmosphere. Research conducted by the US National Oceanic and Atmospheric Administration (NOAA) in 2019 found that thawing permafrost across the Arctic “could be releasing an estimated 300-600m tonnes of net carbon per year to the atmosphere”.[61] In a Special Report on the Ocean and Cryosphere in a Changing Climate, the IPCC says there is “high confidence” in projections of “widespread disappearance of Arctic near-surface permafrost this century" which is "projected to release 10s to 100s of billions of tonnes [or gigatonnes, GtC], up to as much as 240 GtC, of permafrost carbon as CO2 and methane into the atmosphere".[62]

Warming in the Arctic allows the frozen permafrost to thaw, releasing locked up carbon dioxide and methane into the atmosphere.[63] In June 2019, satellite images from around the Arctic showed burning fires that are farther north and of greater magnitude than at any time in the 16-year satellite record, and some of the fires appear to have ignited peat soils.[64] Peat is an accumulation of partially decayed vegetation and is an efficient carbon sink.[65] Scientists are concerned because the long-lasting peat fires release their stored carbon back to the atmosphere, contributing to further warming. The fires in June 2019, for example, released as much carbon dioxide as Sweden's annual greenhouse gas emissions.[66]

Coral reef die-off[edit]

Around 500 million people around the world depend on coral reefs for food, income, tourism and coastal protection.[67] 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.[68] A sustained ocean temperature spike of 1 °C (1.8 °F) above average is enough to cause bleaching.[69] Under continued heat stress, corals expel the tiny colourful algae which live in their tissues leaving behind a white skeleton. The algae, known as zooxanthellae, have a symbiotic relationship with coral such that without them, the corals slowly die.[70]

Between 1979 - 2010, 35 coral reef bleaching events were identified at a variety of locations.[71] Some bleaching events are relatively localised, but the frequency and severity of mass-bleaching events affecting coral over hundreds and sometimes thousands of kilometres has been increasing over the last few decades.[72] Mass bleaching events occurred in 1998, 2010, and between 2014–2017. This three year event affected more than 70 percent of the world's coral reefs, leaving two thirds of the Great Barrier Reef dead or severely bleached. Scientific American reports that the world has lost around 50% of coral reefs in the past 30 years.[73] The Intergovernmental Panel on Climate Change (IPCC) states that by the time temperatures have risen to 1.5C above pre-industrial times, between 70% and 90% of coral reefs that exist today will have disappeared; and that if the world warms by 2 °C, "coral reefs will be vanishingly rare".[74]

West African monsoon shift[edit]

The West African Monsoon (WAM) system brings rainfall to West Africa and is the main source of rainfall in the agriculturally based region of the Sahel, an area of semi-arid grassland between the Sahara desert to the north and tropical rainforests to the south. The monsoon is a complex system in which land, ocean and atmosphere are connected is such a way that the wind direction reverses with the seasons.[75]

However, the monsoon is notoriously unreliable. Between the late 1960s and 1980s, the average rainfall declined by more than 30% plunging the region into an extended drought. This led to a famine that killed tens of thousands of people and triggered an international aid effort.[76] Research has shown the drought was largely due to changes in the surface temperatures of the global oceans, in particular, warming of the tropical oceans in response to rising greenhouse gases combined with cooling in the North Atlantic as a result of air pollution from northern hemisphere countries.[77]

The El Niño–Southern Oscillation[edit]

The possibility that El Niño–Southern Oscillation (ENSO) is a tipping element has been debated, but remains uncertain.[78] 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 Indonesia, India and Brazil, and increased flooding in Peru. In 2015/2016, this caused food shortages affecting over 60 million people.[79] El Niño-induced droughts may increase the likelihood of forest fires in the Amazon.[80]

The threshold for tipping is estimated between 3.5 and 7 °C of global warming.[81] 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.[78] So far, there is no definitive evidence indicating changes in ENSO behaviour.[82]

Arctic sea ice[edit]

The IPCC finds that Arctic sea ice loss does not represent a tipping point because “projected losses are potentially reversible”.[83] This depends on the time scale. Arctic sea ice has been melting rapidly for several decades. Some climate scientists describe the Arctic as a tipping element.[84][85][86]

Tipping point effects[edit]

If the climate tips into a state where tipping points begin to cascade, coastal storms will have greater impact, hundreds of millions of people will be displaced by rising sea levels, there will be food and water shortages, and people will die from unhealthy heat levels and generally unlivable conditions.[87] Climatologist Michael E. Mann, believes a global temperature increase of 3 degrees Celsius or more has the potential to trigger collapse of the current societal organization and set the stage for massive unrest and global conflict[88][89] – bearing in mind the IPCC describes a high probability that tipping points will occur at temperatures above 2 degrees C of global warming.[90]

If cascading tipping points lead to climate temperature increases of 4–5 °C, this will make swaths of the planet around the equator uninhabitable, and lead to sea levels up to 60 metres (197 ft) higher than they are today.[91] Humans cannot survive if the air is too moist and hot, and billions of people may die.[citation needed] Hans Joachim Schellnhuber, Director of the Potsdam Institute for Climate Impact Research says if the world warms by this amount, it could only sustain about one billion people.[92]

A 2021 meta study, conducted by Simon Dietz, James Rising, Thomas Stoerk, and Gernot Wagner, 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 (SCC) by about 25%, with a 10% chance of tipping points more than doubling the SCC.[93] Effects like these have been popularized in books like The Uninhabitable Earth and The End of Nature.

Mathematical theory[edit]

Tipping point behaviour in the climate can be described in mathematical terms. Tipping points are then seen as any type of bifurcation with hysteresis,[94][95] which is the dependence of the state of a system on its history. For instance, depending on how warm or cold it was in the past, there can be differing amounts of ice on the poles at the same concentration of greenhouse gases or temperature.[96] In a 2012 study inspired by "mathematical and statistical approaches to climate modelling and prediction", the authors identify three types of tipping points in open systems such as the climate system—bifurcation, noise-induced and rate-dependent.[15]

Types[edit]

Bifurcation-induced tipping[edit]

This occurs when a particular parameter in the climate, which is observed to be consistently moving in a given direction over a period of time, eventually passes through a critical level - at which point a dangerous bifurcation, or fork takes place - and what was a stable state loses its stability or simply disappears.[97] The Atlantic Meridional Overturning Circulation (AMOC) is like a conveyor belt driven by thermohaline circulation. Slow changes to the bifurcation parameters in this system — the salinity, temperature and density of the water - have caused circulation to slow down by about 15% in the last 70 years or so. If it reaches a critical point where it stops completely, this would be an example of bifurcation induced tipping.[98][99]

Noise-induced tipping[edit]

This refers to transitions from one state to another due to random fluctuations or internal variability of the system. Noise-induced transitions show none of the early warning signals which occur with bifurcations. This means they are fundamentally unpredictable as there is no systematic change in the underlying parameters. Because they are unpredictable, such occurrences are often described as a ‘one-in-x-year’ event.[100] An example is the Dansgaard–Oeschger events during the last glacial period, with 25 occurrences of sudden climate fluctuations over a 500 year period.[101]

Rate-induced tipping[edit]

This aspect of tipping assumes that there is a unique, stable state for any fixed aspect or parameter of the climate and that, if left undisturbed, there will only be small responses to a ‘small’ stimulus. However, when changes in one of the system parameters begin to occur more rapidly, a very large 'excitable' response may appear. In the case of peatlands, for instance, after years of relative stability, the rate-induced tipping point leads to an "explosive release of soil carbon from peatlands into the atmosphere" - sometimes known as "compost bomb instability".[102][103]

Early warning signals[edit]

For tipping points that occur because of a bifurcation, it may be possible to detect whether they are getting closer to a tipping point, as the system is getting 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, changes may also be detected in the skewness and kurtosis of time series of relevant variables, with asymmetries in the distributions of anomalies indicating that tipping may be close.[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 paleorecord, 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] 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]

However because the temperature is increasing so quickly there may be no warning.[111]: 1-66 

Cascading tipping points[edit]

Crossing a threshold in one part of the climate system may trigger another tipping element to tip into a new state. These are called cascading tipping points.[112] 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 poses a multiplier threat because it holds roughly twice as much carbon as the amount currently circulating in the atmosphere.[113] If this is released into the atmosphere, the world will have to cope with emissions generated by the planet itself as well as those generated by human use of fossil fuels.[114]

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.[115] The authors of the study said that the science of tipping points is complex such that there is great uncertainty as to how they might unfold, but nevertheless, argue that the possibility of cascading tipping points represents “an existential threat to civilisation”.[116] In July 2021, Nature Geoscience published a review illustrating how cascading interactions in the Earth system have led to abrupt changes in climate, ecological and social systems during the past 30,000 years. The authors point out that "the geological record shows that abrupt changes can occur on timescales short enough to challenge the capacity of human societies to adapt to environmental pressures".[108]

Public concern[edit]

In April and May 2021, Ipsos Mori conducted an opinion survey in the G20 nations on behalf of the Global Commons Alliance (GCA). The results, published in August 2021, found 73% of those surveyed believe "Because of human activities, the Earth is close to ‘tipping points’ in nature where climate or nature may change suddenly, or may be more difficult to stabilise in the future".[117]: 34  People in poorer countries such as Indonesia, Turkey, and Brazil were significantly more aware of the risk of triggering tipping points than those in wealthier countries such as the United States, Japan, Great Britain and Australia.[117]

Runaway greenhouse effect[edit]

The runaway greenhouse effect is used in astronomical circles to refer to a greenhouse effect that is so extreme that oceans boil away and render a planet uninhabitable, an irreversible climate state that happened on Venus. The IPCC Fifth Assessment Report states that "a 'runaway greenhouse effect' —analogous to Venus— appears to have virtually no chance of being induced by anthropogenic activities."[118] 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.[119]

See also[edit]

References[edit]

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