Tipping points in the climate system

The 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. It can also be associated with 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 stable 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. 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. == Comparison of tipping points ==

Scientists have identified many elements in the climate system which may have tipping points. Since then estimates for global warming thresholds have generally fallen, with some thought to be possible in the Paris Agreement range () by 2016. As of 2021 tipping points are considered to have significant probability at today's warming level of just over , with high probability above of global warming. As of September 2022, nine global core tipping elements and seven regional impact tipping elements have been identified. Out of those, one regional and three global climate elements are estimated to likely pass a tipping point if global warming reaches , namely Greenland ice sheet collapse, West Antarctic ice sheet collapse, tropical coral reef die off, and boreal permafrost abrupt thaw. Two further tipping points are forecast as likely if warming continues to approach : Barents sea ice abrupt loss, and the Labrador Sea subpolar gyre collapse. == Tipping points in the cryosphere ==

Greenland ice sheet disintegration The Greenland ice sheet is the second largest ice sheet in the world, and completely melting the water which it holds would raise sea levels globally by 7.2 metres (24 ft). Due to global warming, the ice sheet is currently melting at an accelerating rate, adding almost 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 it touches the sea, by calving (breaking off) icebergs from its margins. The Greenland ice sheet has a tipping point because of the melt-elevation feedback. Surface melting reduces the height of the ice sheet, and air at a lower altitude is warmer. The ice sheet is then exposed to warmer temperatures, accelerating its melt. A 2021 analysis of sub-glacial sediment at the bottom of a Greenland ice core finds that the Greenland ice sheet melted away at least once during the last million years, and therefore strongly suggests that its tipping point is below the maximum temperature increase over the preindustrial conditions observed over that period. There is some evidence that the Greenland ice sheet is losing stability, and getting close to a tipping point. 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). 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 of sea level rise over thousands of years. The paleo record suggests that during the past few hundred thousand years, the WAIS largely disappeared in response to similar levels of warming and emission scenarios projected for the next few centuries. Like with the other ice sheets, there is a counteracting negative feedback - greater warming also intensifies the effects of climate change on the water cycle, which result in an increased precipitation over the ice sheet in the form of snow during the winter, which would freeze on the surface, and this increase in the surface mass balance (SMB) counteracts some fraction of the ice loss. In the IPCC Fifth Assessment Report, it was suggested that this effect could potentially overpower increased ice loss under the higher levels of warming and result in small net ice gain, but by the time of the IPCC Sixth Assessment Report, improved modelling had proven that the glacier breakup would consistently accelerate at a faster rate. East Antarctic ice sheet disintegration The East Antarctic ice sheet is the largest and thickest ice sheet on Earth, with the maximum thickness of . A complete disintegration would raise the global sea levels by , but this may not occur until global warming of , while the loss of two-thirds of its volume may require at least of warming to trigger. 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. 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. Barents Sea warmed up to seven times faster than the global average. 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. 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. 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, 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. Altogether, it was estimated in 2023 that 49% of the world's glaciers would be lost by 2100 at of global warming, and 83% of glaciers would be lost at . 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 ~ and ~ to sea level rise, while the current likely trajectory of would result in the SLR contribution of ~ by 2100. Permafrost thaw , Canada, 2013 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. 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. While most thaw is gradual and will take centuries, abrupt thaw can occur in some places where permafrost is rich in large ice masses, which once melted cause the ground to slump or form 'thermokarst' lakes over years to decades. These processes can become self-sustaining, leading to localised tipping dynamics, and could increase greenhouse gas emissions by around 40%. Because and methane are both greenhouse gases, they act as a self-reinforcing feedback on permafrost thaw, but are unlikely to lead to a global tipping point or runaway warming process. == Tipping points related to ocean current collapse ==

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. It is driven by differences in the density of water; colder and more salty water is heavier than warmer fresh water. 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. Another 2021 study found early-warning signals in a set of AMOC indices, suggesting that the AMOC may be close to tipping. 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. 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. In October 2024, 44 climate scientists published an open letter, claiming that according to scientific studies in the past few years, the risk of AMOC collapse has been greatly underestimated, it can occur in the next few decades, with devastating impacts especially for Nordic countries. An August 2025 study concluded that the collapse of AMOC could start as early as the 2060s. North subpolar gyre Southern Ocean overturning circulation == Tipping points in terrestrial systems ==

to warn that the Amazonia is in the midst of a tipping point crisis. 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. However, when forest is lost via climate change (from droughts and wildfires) or deforestation, there will be less rain in downwind regions, increasing tree stress and mortality there. Eventually, if enough forest is lost a threshold can be reached beyond which large parts of the remaining rainforest may die off and transform into drier degraded forest or savanna landscapes, particularly in the drier south and east. In 2022, a study reported that the rainforest has been losing resilience since the early 2000s. Resilience is measured by recovery-time from short-term perturbations, with delayed return to equilibrium of the rainforest termed as critical slowing down. The observed loss of resilience reinforces the theory that the rainforest could be approaching a critical transition, although it cannot determine exactly when or if a tipping point will be reached. 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. 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. These trends were first detected in the Canadian boreal forests in the early 2010s, and summer warming had also been shown to increase water stress and reduce tree growth in dry areas of the southern boreal forest in central Alaska and portions of far eastern Russia. In Siberia, the taiga is converting from predominantly needle-shedding larch trees to evergreen conifers in response to a warming climate. Subsequent research in Canada found that even in the forests where biomass trends did not change, there was a substantial shift towards the deciduous broad-leaved trees with higher drought tolerance over the past 65 years. A Landsat analysis of 100,000 undisturbed sites found that the areas with low tree cover became greener in response to warming, but tree mortality (browning) became the dominant response as the proportion of existing tree cover increased. A 2018 study of the seven tree species dominant in the Eastern Canadian forests found that while warming alone increases their growth by around 13% on average, water availability is much more important than temperature. Also, further warming of up to would result in substantial declines unless matched by increases in precipitation. 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. 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. 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 or 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. Sahel greening 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.Currently, the Sahel is becoming greener but precipitation has not fully recovered to levels reached in the mid-20th century. A study from 2022 concluded: "Clearly the existence of a future tipping threshold for the WAM (West African Monsoon) and Sahel remains uncertain as does its sign but given multiple past abrupt shifts, known weaknesses in current models, and huge regional impacts but modest global climate feedback, we retain the Sahel/WAM as a potential regional impact tipping element (low confidence)." This and the increased plant growth directly induced by carbon dioxide could lead to an expansion of vegetation into present-day desert, although it might be accompanied by a northward shift of the desert, i.e. a drying of northernmost Africa. Vulnerable stores of tropical peat carbon: Cuvette Centrale peatland == 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. 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 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 above pre-industrial times, "Coral reefs... are projected to decline by a further 70–90%"; and that if the world warms by , they will become extremely rare. Break-up of equatorial stratocumulus clouds 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. 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. 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. 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". 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"). == Formerly considered tipping elements ==

. The threshold for tipping was estimated to be between and of global warming in 2016. Consequently, the 2022 assessment no longer includes it in the list of likely tipping elements. However, more recent research has demonstrated that warming tends to strengthen the Indian monsoon, and it is projected to strengthen in the future. Methane hydrate deposits in the Arctic were once thought to be vulnerable to a rapid dissociation which would have a large impact on global temperatures, in a dramatic scenario known as a clathrate gun hypothesis. Later research found that it takes millennia for methane hydrates to respond to warming, IPCC Sixth Assessment Report states "It is very unlikely that gas clathrates (mostly methane) in deeper terrestrial permafrost and subsea clathrates will lead to a detectable departure from the emissions trajectory during this century." == 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. 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. 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. Many types of bifurcations show 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. 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. 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 paleo record, 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. and melting of the Pine Island Glacier in West Antarctica, 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. An example is the Dansgaard–Oeschger events during the last ice age, with 25 occurrences of sudden climate fluctuations over a 500-year period. 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. 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. == Potential impacts ==

Tipping points can have very severe impacts. A collapse of the Atlantic Overturning Circulation would cause over 10 °C of cooling in parts of Europe, cause drying in Europe, Central America, West Africa, and southern Asia, and lead to about of sea level rise in the North Atlantic. The impacts of AMOC collapse would have serious implications for food security, with one projection showing reduced yields of key crops across most world regions, with for example arable agriculture becoming economically infeasible in Britain. These impacts could happen simultaneously in the case of cascading tipping points. Decisions taken over the next decade could influence the climate of the planet for tens to hundreds of thousands of years and potentially even lead to conditions which are inhospitable to current human societies. The report also states that there is a possibility of a cascade of tipping points being triggered even if the goal outlined in the Paris Agreement to limit warming to 1.5–2.0 °C (2.7–3.6 °F) is achieved. == Geological timescales ==

was a period of abrupt sea level rise around 14,000 years ago. It may be an example of a tipping point. and the water vapour escapes to space, an irreversible climate state that happened on Venus. A runaway greenhouse effect has virtually no chance of being caused by people. Venus-like conditions on the Earth require a large long-term forcing that is unlikely to occur until the sun brightens by a ten of percents, which will take 600–700 million years. ==See also==