Tipping points in the climate system

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

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.

In climate science, a tipping point is a critical threshold that, when exceeded, leads to large and often irreversible changes in the climate system. If tipping points are exceeded, they are likely to have severe impacts on human society. Tipping points are often, but not necessarily abrupt. For example, with average temperature rise of between 1 and 4 °C, the Greenland ice sheet pass a tipping point and be completely lost, but the melt would take place over over millennia.

The Intergovernmental Panel on Climate Change (IPCC) 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 °C or more above preindustrial times. Tipping points are now considered to have significant probability at today's warming level of just over 1 °C, with high probability above 2 °C of global warming. The geological record shows a large set of abrupt chances, that indicate tipping behaviour.

Large-scale components of the Earth system that may pass a tipping point are called tipping elements. Tipping elements include the West Antarctic and Greenland ice sheets, the Amazon rainforest, coral reefs and the major current in the Atlantic Ocean. A danger is that if the tipping point in one system is crossed, this could lead to a cascade of other tipping points, leading to severe impacts.

Definition

The 2021 IPCC Sixth Assessment Report defines a tipping point as a "critical threshold beyond which a system reorganizes, often abruptly and/or irreversibly". 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 require self-reinforcing feedbacks, which could lead to changes in the climate system 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. In ecosystems and in social systems, a tipping point can trigger a regime shift, a major systems reorganisation into a new stable state.

The Special Report on the Ocean and Cryosphere in a Changing Climate released by the 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".

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".

Geological record

The geological record shows that there have been abrupt changes in elements of the climate system that point towards the existance of tipping points. For instance, The Dansgaard–Oeschger events during the last glacial 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 levels 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 saw an abrupt termination into a drier state.

Tipping elements

Scientists have identified a large set of elements in the climate system which may have a tipping point. The Intergovernmental Panel on Climate Change (IPCC) 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 °C or more above preindustrial times. Tipping points are now considered to have significant probability at today's warming level of just over 1 °C, with high probability above 2 °C of global warming. 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.

Shutdown of the Atlantic Meridional Overturning Circulation

See also: Shutdown of thermohaline circulation

The Atlantic Meridional Overturning Circulation (AMOC), also known as the Gulf Stream System, is a large system of ocean currents. It is driven by differences in the density of water; colder and more salty water is heavier than warmer fresh water. The AMOC acts as a conveyor belt, sending warm surface water from the tropics north, and carrying cold fresh water back south. As warm water flows northwards, some evaporates which increases salinity. It also cools when it is exposed to cooler temperatures and sea ice. Cold, salty water is more dense and slowly begins to sink. 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.

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, 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. If the AMOC does shut down, a new stable state could emerge that lasts for thousands of years, possibly triggering other tipping points.

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

West Antarctic ice sheet disintegration

The West Antarctic Ice Sheet (WAIS) is a large ice sheet in Antarctica; in places it is more than 4 kilometres thick. It sits on bedrock mostly below sea level. 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.

Ice loss from the WAIS is accelerating. 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. 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.

Greenland ice sheet disintegration

The Greenland ice sheet is the second largest ice sheet in the world, and is three times the size of Texas. It holds enough water, which if it melted, could raise sea levels globally by 7.2 metres. Due to global warming, the ice sheet is melting at an accelerating rate adding around 1 mm to global sea levels every year. 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.

The Greenland ice sheet has a tipping point because of the melt-elevation feedback. Surface melting reduced the height of the ice sheet. As air becomes warmer at a lower latitude, the ice sheet is then exposed to warmer temperatures, accelerating the melt. The threshold for the Greeland 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. There is some evidence that the Greenland ice sheet is losing stability, and getting close to a tipping point.

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 the size of 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. When forest is lost via climate change (droughts and fires) or deforestation, there will be less rain and more trees may die. Eventually, large parts of the rainforest may die off and transform into a dry savanna landscape.

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. 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. In 2022, a study reported that resilience of the Amazon rainforest has been waning since the early 2000s. Resiliency is measured by recovery-time from short-term perturbations. The delayed return to a state of equilibrium of the rainforest is termed a critical slowing down (CSD). The observed CSD in the 2022 study reinforces the theory that the rainforest is approaching a critical transition.

Permafrost

See also: Gas hydrate stability zone and Permafrost carbon cycle

Permafrost is ground containing soil and/or organic material bound together by ice and which has remained frozen for at least two years. It covers large fractions of land – mainly in Siberia, Alaska, northern Canada and the Tibetan plateau – and can be up to a kilometre thick. Subsea permafrost up to 100 metres thick also occurs on the sea floor under part of the Arctic Ocean. 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.

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.

Coral reef die-off

Main article: Coral bleaching

Around 500 million people around the world depend on coral reefs for food, income, tourism and coastal protection. 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. A sustained ocean temperature spike of 1 °C (1.8 °F) above average is enough to cause bleaching. 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. 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. 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 have disappeared; and that if the world warms by 2 °C, "coral reefs will be vanishingly rare".

The El Niño–Southern Oscillation

The possibility that El Niño–Southern Oscillation (ENSO) is a tipping element has been debated, but remains uncertain. 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. El Niño-induced droughts may increase the likelihood of forest fires in the Amazon.

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

Arctic sea ice

The IPCC finds that Arctic sea ice loss does not represent a tipping point because “projected losses are potentially reversible”. Sea ice coverage and global temperatures are found to vary directly without the exponential increase in loss characteristic of a tipping point. However, this is dependent on the time scale. Arctic sea ice has been melting rapidly for several decades, and loss of arctic sea ice during the summer months is being viewed as particularly concerning by many climate scientists. Despite the lack of a "textbook" tipping point, sea ice loss will have a warming effect on global climate due to the ice-albedo feedback.

Mathematical theory

Tipping point behaviour in the climate can be described in mathematical terms. Tipping points are then seen as any type of bifurcation with hysteresis, 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. 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.

Types

Bifurcation-induced tipping

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. 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.

Noise-induced tipping

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. An example is the Dansgaard–Oeschger events during the last glacial period, with 25 occurrences of sudden climate fluctuations over a 500 year period.

Rate-induced tipping

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".

Early warning signals

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. 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.

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. 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. Quality limitations of paleodata further complicate the development of EWSs. They have been developed for detecting tipping due to drought in forests in California, the Pine Island Glacier in West Antarctica, 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.

However because the temperature is increasing so quickly there may be no warning.

Cascading tipping points

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. 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. Thawing permafrost poses a multiplier threat because it holds roughly twice as much carbon as the amount currently circulating in the atmosphere. 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.

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. 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”. 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".

Tipping point effects and concern

See also: Effects of climate change

Tipping points can have very severe impacts. They can execarbate 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 the relocation of extensive population centres. A collapse of the Atlantic Overturning Circulation would alter European radically, and lead to a meter of sea level rise in the North Atlantic. These impacts could happen simultaneously in the case of cascading tipping points.

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 (SCC) by about 25%, with a 10% chance of tipping points more than doubling the SCC. The social cost of carbon reflects the economic damage from carbon emissions.

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". 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.

Runaway greenhouse effect

Main article: Runaway greenhouse effect

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." 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.

Social tipping points and climate models

Tipping points in human behaviour can have both positive and negative effects, contrasting with the normally negative connotation associated with climate tipping points. Some positive tipping points in societal behaviour can drive positive climate action. There is a strong link between human behaviour and environmental stability, which is not easily accounted for in climate models, such as the ongoing Lake Chad crisis. The interaction of the socio-economic and regional changes induced by the climate in the Lake Chad region produces behaviors in society that change the environmental course of the region. For example, a lack of sustainable resource usage creates deep societal instabilities which prevent positive climate action from being enacted. These responses can be included in climate models to improve the accuracy of climate predictions.

Local environmental issues have the ability to affect regions across the globe. This effect of ecosystems or social systems at a distance is called telecoupling and can be realized when crop-producing regions experience a drought that causes a food shortage elsewhere.

Climate models that allow human behaviour to change the state of the system are called Integrated Assessment Models (IAM). Current models, such as DICE, FUND, and REMIND do not account for the societal changes that could be caused by social tipping points which would drastically change the results. Because of the deeply intertwined relationship between the environment and humanity, accurately modeling social tipping points is necessary for predicting the future of Earth’s climate and is an active area of research.

See also

• Greenhouse and icehouse Earth
• Climate sensitivity
• Planetary boundaries
• Climate engineering
• World Scientists' Warning to Humanity