In climate science, a tipping point is a key level that, once reached, causes major, fast, and often permanent changes in the climate system. If tipping points are reached, they can lead to serious effects on human life and may speed up global warming. These tipping points are found in many parts of the climate system, such as ice sheets, mountain glaciers, ocean currents, ecosystems, and the atmosphere. Examples include the melting of permafrost, which releases methane, a strong greenhouse gas, or the melting of ice sheets and glaciers, which lowers Earth's ability to reflect sunlight, causing the planet to warm faster. Permafrost thawing is a big concern because it holds about twice as much carbon as is currently in the atmosphere.
Tipping points may or may not happen suddenly. For example, if global temperatures rise between 0.8°C (1.4°F) and 3°C (5.4°F), the Greenland ice sheet may reach a tipping point and begin to melt, but the melting would take thousands of years. Tipping points could already be near or have already been reached, such as those in the West Antarctic and Greenland ice sheets, the Amazon rainforest, and warm-water coral reefs. A 2022 study in the journal Science found that warming beyond 1.5°C could cause several tipping points, like the collapse of major ice sheets, sudden permafrost thaw, and coral reef death, which might lead to linked problems across the climate system.
A risk is that crossing a tipping point in one part of the climate system could cause other tipping points to be reached, leading to serious or even dangerous effects. For example, ice loss in West Antarctica and Greenland could greatly change ocean currents. Continued warming in northern regions might then trigger other tipping points, such as permafrost loss or the death of boreal forests.
Scientists have identified many parts of the climate system that may have tipping points. As of September 2022, nine global and seven regional tipping points are known. If global warming reaches 1.5°C (2.7°F), one regional and three global tipping points are likely to be reached. These include the collapse of the Greenland ice sheet, the collapse of the West Antarctic ice sheet, the death of tropical coral reefs, and the sudden thawing of boreal permafrost.
Tipping points occur in many systems, such as the cryosphere (areas with ice and snow), ocean currents, and land-based systems. In the cryosphere, tipping points include the breakdown of the Greenland, West Antarctic, and East Antarctic ice sheets, the loss of Arctic sea ice, the retreat of mountain glaciers, and permafrost thaw. In ocean systems, tipping points include changes to the Atlantic Meridional Overturning Circulation (AMOC), the North Subpolar Gyre, and the Southern Ocean’s circulation. In land systems, tipping points include the death of the Amazon rainforest, changes in boreal forests, greening in the Sahel region, and the loss of tropical peat carbon stores.
Definition
The IPCC Sixth Assessment Report explains that a tipping point is a "critical threshold beyond which a system changes, often suddenly and/or permanently." A small change can cause a much larger effect in the system. It can also involve feedback loops that make changes continue and grow, possibly leading to climate changes that cannot be reversed within human lifetimes. For any part of the climate system, moving from one state to a new, stable state may take many decades or even centuries.
The 2019 IPCC Special Report on the Ocean and Cryosphere in a Changing Climate describes a tipping point as: "A level of change in a system beyond which it changes in a way that is not straightforward, and it does not return to its original state even if the causes of the change are reduced. For the climate system, this term refers to a threshold where the climate shifts from one stable state to another stable state."
In ecosystems and social systems, a tipping point can cause a major change, known as a regime shift, where the system reorganizes into a new, stable state. These shifts are not always harmful. In the context of the climate crisis, the idea of a tipping point is sometimes used positively, such as when public opinion changes to support climate action or when small policy changes help speed up the move toward a greener economy.
Comparison of tipping points
Scientists have found many parts of the climate system that might reach tipping points, which are sudden changes that can cause major effects. In the early 2000s, the IPCC started studying these tipping points, which were first called large-scale discontinuities. At that time, the IPCC believed these changes would only happen if global temperatures rose by 4 °C (7.2 °F) or more above preindustrial levels. An earlier study suggested most tipping points would occur with warming of 3–5 °C (5.4–9.0 °F) compared to the 1980–1999 average. Later, estimates for when tipping points might occur decreased, with some possibly happening within the Paris Agreement goal of 1.5–2 °C (2.7–3.6 °F) by 2016. By 2021, scientists thought tipping points could happen at today’s warming level of just over 1 °C (1.8 °F), and with high chances of happening if warming reaches 2 °C (3.6 °F). Some tipping points, such as those involving ice sheets in West Antarctica and Greenland, warm-water coral reefs, and the Amazon rainforest, may already be close to or have already been crossed.
As of September 2022, scientists have identified nine global tipping elements and seven regional tipping elements. Of these, one regional and three global elements are likely to reach a tipping point if global warming reaches 1.5 °C (2.7 °F). These include the collapse of the Greenland ice sheet, the collapse of the West Antarctic ice sheet, the die-off of tropical coral reefs, and the sudden thawing of boreal permafrost. Two additional tipping points are expected to occur if warming continues toward 2 °C (3.6 °F): the sudden loss of sea ice in the Barents Sea and the collapse of the Labrador Sea subpolar gyre.
Tipping points in the cryosphere
The Greenland ice sheet is the second largest ice sheet on Earth. If all the ice melted, it would raise global sea levels by 7.2 metres (24 ft). Because of global warming, the ice sheet is melting faster each year, adding nearly 1 mm to sea levels yearly. About half of the ice loss happens from melting on the surface, while the other half occurs at the bottom of the ice sheet, where icebergs break off from the edges.
The Greenland ice sheet has a tipping point due to a feedback process. When the surface melts, the ice sheet becomes shorter, and air at lower altitudes is warmer. This exposes the ice to higher temperatures, speeding up melting. A 2021 study of sediment found at the bottom of a 1.4 km (0.87 mi) Greenland ice core shows that the ice sheet melted completely at least once in the last million years. This suggests its tipping point may be reached if global temperatures rise by more than 2.5 °C (4.5 °F) above preindustrial levels. Some evidence shows the ice sheet is becoming less stable and nearing this tipping point.
The West Antarctic Ice Sheet (WAIS) is a large ice sheet in Antarctica, with parts over 4 km (2.5 mi) thick. It sits mostly on bedrock below sea level, forming a deep basin over millions of years. This makes it vulnerable to ocean heat, leading to rapid and irreversible ice loss. A tipping point could occur if the grounding lines (where ice meets rock) retreat into the basin, causing a process called Marine Ice Sheet Instability (MISI). Thinning and collapse of ice shelves are speeding up this retreat. If all the WAIS melted, it would raise sea levels by about 3.3 metres (11 ft) over thousands of years.
Ice loss from the WAIS is increasing, and some glaciers may already be beyond the point of self-sustaining retreat. Historical records show the WAIS likely disappeared during past warming events similar to those projected for the future.
Like other ice sheets, there is a counteracting effect: warmer temperatures increase snowfall in winter, which can slow ice loss. Earlier studies suggested this might reduce ice loss under high warming, but newer models show ice loss will continue to accelerate.
The East Antarctic ice sheet is the largest and thickest on Earth, reaching 4,800 metres (3.0 mi) in some places. If it fully melted, it would raise sea levels by 53.3 metres (175 ft), but this may not happen until global warming reaches 10 °C (18 °F). Losing two-thirds of its volume would require at least 6 °C (11 °F) of warming. This melt would take at least 10,000 years. However, parts of the East Antarctic ice sheet, like the Wilkes Basin, may be vulnerable to tipping at lower warming levels. The Wilkes Basin alone could raise sea levels by 3–4 metres (10–13 ft).
Arctic sea ice was once considered a tipping element. Losing reflective sea ice in summer exposes darker ocean, which absorbs more heat. Earlier models suggested this could prevent ice recovery even if warming stopped, but new research shows winter cooling helps form new ice, so summer ice loss is not a tipping point unless warming prevents winter ice formation. However, the Barents Sea may not recover even with 2 °C (3.6 °F) of warming, as it is warming much faster than other Arctic regions. This matters because changes in Barents-Kara Sea ice affect weather patterns in Eurasia.
Mountain glaciers hold the most land ice after the Greenland and Antarctic sheets and are melting due to climate change. A glacier tipping point occurs when it becomes out of balance with the climate and will melt unless temperatures drop. For example, in the North Cascade Range, 67% of glaciers were already out of balance by 2005 and may not survive current conditions. In the French Alps, glaciers like Argentière and Mer de Glace are expected to disappear by the end of the 21st century if warming continues. By 2100, 49% of glaciers may be lost at 1.5 °C (2.7 °F) of warming, and 83% at 4 °C (7.2 °F). This would contribute about 9 cm (3.5 in) and 15 cm (6 in) to sea level rise, respectively.
The largest glacier ice is in the Hindu Kush Himalaya region, called the Earth’s Third Pole. Even with 1.5 °C (2.7 °F) of warming, one-third of its ice may be lost by 2100. Under higher warming scenarios, 50% or more of the region’s glaciers could vanish. Glacier melt will increase river flows until around 2060, after which flows will decline. While annual river flows may decrease in some areas, irrigation and hydropower will need to adapt to more variable water supplies.
Tipping points related to ocean current collapse
The Atlantic meridional overturning circulation (AMOC), also called the Gulf Stream System, is a major system of ocean currents. It is caused by differences in water density; colder and saltier water is heavier than warmer fresh water. The AMOC moves warm surface water from the tropics toward the north and carries cold fresh water back south. As warm water flows north, some of it evaporates, increasing salinity. It also cools when exposed to colder air. Cold, salty water becomes denser and slowly sinks. Below the surface, this cold, dense water moves south. Increased rainfall and melting ice from global warming dilute salty surface water, reducing its density. Lighter water is less able to sink, which slows the circulation.
Studies, simplified models, and past climate changes suggest the AMOC has a tipping point. If enough freshwater from melting glaciers enters the ocean, the AMOC could collapse into a state of reduced flow. Even if melting stops, the AMOC may not return to its current state. Scientists believe the AMOC is unlikely to tip in the 21st century but could do so before 2300 if greenhouse gas emissions remain very high. A weakening of 24% to 39% is expected, even without a complete collapse. If the AMOC shuts down, a new stable state could form, lasting for thousands of years and possibly triggering other tipping points.
In 2021, a study using a basic ocean model suggested that AMOC collapse could happen even if ice melt does not reach common tipping thresholds. This implies the risk of collapse might be higher than estimated by complex climate models. Another 2021 study found early signs that the AMOC may be near a tipping point. However, a study published the next year found the AMOC has remained stable, unaffected by climate change beyond natural changes. Two 2022 studies suggested common methods for evaluating the AMOC may overestimate the risk of collapse. In 2024, 44 climate scientists published a letter stating recent studies suggest the risk of AMOC collapse has been underestimated and could occur in the next few decades, with major impacts on Nordic countries. A 2025 study suggested AMOC collapse could begin as early as the 2060s.
Some climate models show that deep water mixing in the Labrador-Irminger Seas could collapse under certain global warming scenarios, which would disrupt the entire circulation in the North subpolar gyre. This collapse might not be reversible, even if temperatures decrease, making it a climate tipping point. This could cause rapid cooling, affecting agriculture, water resources, and energy in Western Europe and the U.S. East Coast. A 2017 study noted recent changes in ocean temperatures and currents that are not fully captured by the AMO Index.
A 2021 study found that only four of 35 climate models predicted AMOC collapse. Of the 35 models, only 11 accurately simulate the North Atlantic Current, including the four that predict collapse. The study estimated a 36.4% chance of abrupt cooling in Europe from AMOC collapse, lower than previous estimates. A 2022 study linked past disruptions of the subpolar gyre to the Little Ice Age.
The Southern Ocean overturning circulation has two parts: an upper cell and a lower cell. The upper cell is affected by wind, while the lower cell is influenced by the temperature and salinity of Antarctic bottom water. Recent decades show the upper cell’s flow has increased by 50–60% since the 1970s, while the lower cell has weakened by 10–20%. These changes are partly due to natural cycles like the Interdecadal Pacific Oscillation and climate change, which altered weather patterns and increased Antarctic ice melt. Freshwater from melting ice dilutes salty Antarctic bottom water.
Paleoclimate evidence shows the AMOC has weakened or collapsed before. Some research suggests collapse may become likely when global warming reaches 1.7°C to 3°C. However, predictions about the AMOC’s tipping point are less certain than for other climate tipping points. Even if collapse begins soon, it is unlikely to be complete until near 2300. Impacts like reduced rainfall in the Southern Hemisphere and increased rainfall in the North, or declines in Southern Ocean fisheries and marine ecosystems, are expected to take centuries to fully develop.
Tipping points in terrestrial systems
The Amazon rainforest is the largest tropical rainforest on Earth. It is about twice the size of India and covers nine countries in South America. The rainforest creates about half of its own rainfall by recycling water through evaporation and transpiration as air moves over the forest. This process helps maintain rainfall in a larger area, and without it, one model suggests about 40% of the rainforest might become too dry to support rainforest life. However, if the rainforest is lost due to climate change (like droughts and wildfires) or deforestation, areas downwind will receive less rain, causing more stress and tree deaths. If too much forest is lost, a threshold could be reached where large parts of the rainforest might die and turn into drier landscapes, especially in the south and east. A 2022 study found that the rainforest has been losing resilience since the early 2000s. Resilience is measured by how quickly the rainforest recovers from short-term changes, and slower recovery is called critical slowing down. This loss of resilience supports the idea that the rainforest might be nearing a major change, though the exact timing or occurrence of a tipping point is unclear.
During the last quarter of the 20th century, the taiga (a type of forest in cold regions) experienced some of the largest temperature increases on Earth. Winter temperatures rose more than summer temperatures, and in summer, daily low temperatures increased more than daily high temperatures. Scientists believe that boreal environments (like the taiga) have only a few stable long-term states: treeless tundra or steppe, a dense forest with more than 75% tree cover, or open woodlands with about 20% or 45% tree cover. Continued climate change could push some taiga forests into one of these woodland states or even into a treeless steppe. It could also cause tundra areas to become forested as they warm.
These changes were first noticed in Canada’s boreal forests in the early 2010s. Summer warming has increased water stress and reduced tree growth in dry areas of Alaska and parts of Russia. In Siberia, the taiga is changing from larch trees (which shed needles) to evergreen conifers due to warming.
Research in Canada found that even in forests where total plant growth hasn’t changed, there has been a shift toward deciduous broad-leaved trees, which are more drought-tolerant, over the past 65 years. A study using satellite images found that areas with low tree cover became greener as temperatures rose, but tree death (browning) became more common in areas with more existing tree cover. A 2018 study of seven tree species in eastern Canada showed that 2°C warming increases their growth by about 13% on average, but water availability is more important than temperature. If warming reaches 4°C without more rain, growth would decline significantly.
A 2021 study confirmed that boreal forests in Canada are more strongly affected by climate change than other forest types. It projected that most eastern Canadian boreal forests could reach a tipping point around 2080 under a high-emission scenario. Another 2021 study suggested that under a moderate emissions scenario, boreal forests might gain 15% in biomass globally by the end of the century, but this would be offset by a 41% loss in tropical forests. A 2022 experiment in North America showed that young trees in the southern edges of boreal forests struggle with even 1.5°C or 3.1°C warming and reduced rainfall. While some temperate tree species that could thrive in these conditions exist in the boreal region, they are rare and grow slowly.
Reports from the IPCC and a 2017 study suggest that global warming may increase rainfall in parts of East Africa, Central Africa, and West Africa’s wet season. However, predictions for West Africa are uncertain. The Sahel region is becoming greener, but rainfall has not returned to levels from the mid-20th century. A 2022 study noted that while the West African Monsoon and Sahel may reach a tipping point in the future, the evidence is unclear. Some models predict more rainfall in the Sahel and Sahara, which could allow more vegetation to grow in deserts. However, this might also shift deserts northward, drying parts of northern Africa.
In 2017, scientists discovered that 40% of the Cuvette Centrale wetlands in the Congo Basin are covered by thick peat layers containing about 30 billion tons of carbon. This is 28% of all tropical peat carbon, equal to the carbon stored in all the forests of the Congo Basin. Though the peatland covers only 4% of the basin, it holds the same amount of carbon as the rest of the region combined. If all this peat burned, it would release carbon equal to 20 years of U.S. emissions or three years of global human-caused emissions.
To address this risk, the Brazzaville Declaration was signed in 2018 by the Democratic Republic of Congo, the Republic of Congo, and Indonesia (which has experience managing peatlands). However, a 2022 study revised the size of the peatland to 167,600 square kilometers and its average depth to 1.7 meters. It also found that only 8% of the peat is protected, while 26% is open to logging, mining, or palm oil farming, and nearly all of it is accessible for fossil fuel exploration.
Even without human activity, the Cuvette Centrale is highly vulnerable because its climate is drier than other tropical peatlands. A 2022 study suggested that conditions similar to those 7,500 to 2,000 years ago, which caused significant peat loss, may return under future climate change. If this happens, the Cuvette Centrale could act as a tipping point in the climate system at some unknown time.
Other tipping points
Approximately 500 million people worldwide rely on coral reefs for food, income, tourism, and protection from coastal storms. Since the 1980s, rising sea surface temperatures have threatened coral reefs, causing widespread coral bleaching, especially in sub-tropical areas. A long period of ocean temperatures that are 1 °C (1.8 °F) higher than normal can lead to bleaching. When corals are stressed by heat, they expel the small, colorful algae that live in their tissues. This causes the corals to turn white. These algae, called zooxanthellae, live together with coral in a helpful way. Without them, corals slowly die. After zooxanthellae disappear, corals become more likely to be replaced by seaweed, making it hard to return to a coral-dominated ecosystem. The IPCC estimates that if global temperatures rise to 1.5 °C (2.7 °F) above pre-industrial times, coral reefs could decline by 70–90%. If temperatures rise to 2 °C (3.6 °F), coral reefs may become extremely rare.
In 2019, a study used a computer model to estimate that if carbon dioxide (CO₂) levels rise above 1,200 ppm (almost three times current levels and over four times pre-industrial levels), certain types of clouds called equatorial stratocumulus clouds might break up. This could cause global surface warming of about 8 °C (14 °F) and 10 °C (18 °F) in sub-tropical areas, in addition to warming already caused by high CO₂ levels. The study suggested that these clouds would not reform until CO₂ levels drop significantly. This finding might help explain past periods of rapid warming, such as the Paleocene-Eocene Thermal Maximum. In 2020, the same researchers found that even if solar radiation modification (a method to reduce warming) were used, the breakup of these clouds would only be delayed until CO₂ levels reached 1,700 ppm, still causing about 5 °C (9.0 °F) of warming.
However, the models used in this study are simpler than the large-scale models used for climate projections. These models may not accurately represent processes like air sinking in the atmosphere. Some scientists argue the study’s model oversimplifies cloud behavior, comparing it to a switch with only two settings. CO₂ levels reaching 1,200 ppm would require the world to follow a scenario called Representative Concentration Pathway 8.5, which involves massive coal use. In this case, CO₂ levels would reach 1,200 ppm shortly after 2100.
Crossing a threshold in one part of the climate system can cause other parts to change suddenly. These linked changes are called cascading tipping points, like a chain reaction. For example, ice loss in West Antarctica and Greenland could disrupt ocean currents. Continued warming in northern regions might trigger other changes, such as permafrost melting and boreal forest dieback. Thawing permafrost is dangerous because it holds about twice as much carbon as is currently in the atmosphere. Ice loss in Greenland could also destabilize the West Antarctic ice sheet, and vice versa, especially if Greenland melts first.
A 2021 study used a computer model to run three million simulations. Nearly one-third of these simulations showed cascading tipping points, even if global warming was limited to 2 °C (3.6 °F), the upper limit set by the Paris Agreement. The study’s authors noted that the science of tipping points is complex and uncertain, but they warned that cascading tipping points could pose a serious risk to human civilization. A network model analysis suggested that temporarily exceeding Paris Agreement temperature goals could increase the risk of cascading tipping points by up to 72% compared to scenarios without such overshoots.
Formerly considered tipping elements
The El Niño–Southern Oscillation (ENSO) has been studied as a possible tipping element in the past. Usually, strong winds blow from South America toward Australia across the South Pacific Ocean. Every two to seven years, these winds weaken because of changes in air pressure, causing the middle of the Pacific Ocean to warm. This warming changes wind patterns worldwide and is called El Niño. El Niño often causes droughts in India, Indonesia, and Brazil, and more flooding in Peru. In 2015/2016, this led to food shortages affecting over 60 million people. El Niño-related droughts could increase the risk of forest fires in the Amazon. Scientists estimated that ENSO might tip into a permanent El Niño state if global warming reaches 3.5 °C (6.3 °F) to 7 °C (13 °F). After tipping, the system would stay in a constant El Niño state instead of switching between states. This has occurred in Earth's past, such as during the Pliocene era, but ocean conditions were very different back then. Currently, there is no clear evidence that ENSO behavior has changed. The IPCC Sixth Assessment Report states that ENSO is likely to remain the main cause of year-to-year climate changes in a warmer world. Therefore, the 2022 assessment no longer lists ENSO as a likely tipping element.
The Indian summer monsoon is another climate system once thought to be at risk of not recovering once it changes. However, recent research shows that warming tends to strengthen the monsoon, and it is expected to become stronger in the future.
Methane hydrate deposits in the Arctic were once believed to be vulnerable to rapid breakdown, which could greatly affect global temperatures in a scenario called the clathrate gun hypothesis. Later studies found that methane hydrates take thousands of years to respond to warming, and methane emissions from the seafloor rarely reach the atmosphere. The IPCC Sixth Assessment Report states that it is very unlikely that methane clathrates in deep permafrost or under the sea will cause a noticeable change in emissions during this century.
Mathematical theory
Tipping point behavior in the climate can be explained using math. Scientists have identified three types of tipping points: bifurcation, noise-induced, and rate-dependent.
Bifurcation-induced tipping occurs when a specific factor in the climate, such as changes in environmental conditions, crosses a critical threshold. At this point, the system may shift from a stable state to an unstable one. The Atlantic Meridional Overturning Circulation (AMOC) is an example of a system that could experience bifurcation-induced tipping. Gradual changes in conditions, like the salinity and temperature of ocean water, might lead to the collapse of the AMOC.
Some types of bifurcations show hysteresis, meaning the system’s current state depends on its past. For example, the same level of greenhouse gases or temperature could result in different amounts of ice on Earth’s poles, depending on previous climate conditions.
For systems that tip due to bifurcations, signs may appear before a tipping point is reached. These include reduced resilience to disturbances, increased memory (autocorrelation), and greater variability in the system. Other early warning signals may exist depending on the system. However, sudden changes are not reliable early warnings, as they can also occur when changes are reversible.
Scientists often use data from the past, such as sediments, ice cores, and tree rings, to study tipping points. However, it can be difficult to determine whether increased variability or memory is a sign of an approaching tipping point or caused by natural changes. Early warning signals have been tested for tipping in systems like California forests and the Pine Island Glacier in Antarctica. For example, increased variability in the Greenland ice sheet’s melt rate suggests it may be losing resilience.
Human-caused changes in the climate may happen too quickly for early warning signals to be detected, especially in systems that respond slowly.
Noise-induced tipping happens when random fluctuations or natural changes in a system cause it to shift from one state to another. These changes do not show early warning signals because the system’s underlying structure remains unchanged. This makes noise-induced tipping unpredictable, often described as a "one-in-x-year" event. An example is the Dansgaard–Oeschger events during the last ice age, which involved 25 sudden climate shifts over 500 years.
Rate-induced tipping occurs when environmental changes happen faster than the system can adjust. For example, in peatlands, rapid changes could lead to a sudden release of stored carbon into the atmosphere, known as "compost bomb instability." The AMOC may also experience rate-induced tipping if ice melts too quickly, causing collapse before reaching a critical threshold for bifurcation.
Potential impacts
Tipping points can cause very serious problems. They can make existing climate change effects worse or create new dangers. Some tipping points might happen suddenly, like changes to the Indian monsoon, which could harm food supplies for hundreds of millions of people. Other effects might happen slowly, such as ice caps melting. If Greenland and West Antarctica’s ice melted completely, it would raise sea levels by about 10 meters (33 feet). This would require moving many cities inland over centuries, but it could also speed up sea level rise this century. In a mid-emissions scenario, Antarctic ice sheet instability could put 120 million more people at risk of yearly floods. A collapse of the Atlantic Overturning Circulation (AMOC) would cause temperatures in parts of Europe to drop by more than 10 degrees Celsius, dry out regions in Europe, Central America, West Africa, and southern Asia, and raise sea levels by about 1 meter (3.5 feet) in the North Atlantic. These changes would greatly affect food supplies, with some studies showing major crop losses worldwide, including making farming unprofitable in Britain. These effects could happen at the same time if multiple tipping points are triggered together. A review of climate changes over the past 30,000 years shows that tipping points can cause many linked problems in climate, ecosystems, and human societies. For example, the sudden end of the African humid period led to desertification and changes in ecosystems, which caused pastoral groups in North Africa to move and led to a change in Egypt’s leadership.
Some scientists suggest that crossing a certain temperature threshold—about 2 degrees Celsius above pre-industrial levels—could cause many tipping points and feedback loops that make climate change worse, leading to extreme warming, rising seas, and major harm to ecosystems, societies, and economies. This situation is called the "Hothouse Earth" scenario. However, this threshold is not certain, and some experts question whether tipping points would cause long-term warming quickly. Choices made in the next decade could shape the planet’s climate for tens to hundreds of thousands of years and might create conditions unsuitable for human life today. The report also notes that even if the Paris Agreement goal of limiting warming to 1.5–2.0 degrees Celsius (2.7–3.6 degrees Fahrenheit) is met, a chain of tipping points could still occur.
Geological timescales
The geological record shows sudden changes over long periods of time that suggest tipping points may have been reached in ancient times. For example, during the last ice age, the Dansgaard–Oeschger events were times when Greenland and Europe experienced sudden warming that happened quickly, possibly because major ocean currents changed. During the early Holocene, when ice was melting, sea levels did not rise gradually. Instead, they rose suddenly during times when large amounts of ice melted rapidly. In North Africa, the monsoon changed suddenly over a few years during the African humid period, which lasted from about 15,000 to 5,000 years ago. This period ended quickly, and the region became much drier.
A runaway greenhouse effect is a type of tipping point so extreme that oceans evaporate and water vapor escapes into space, creating a climate state that cannot be reversed. This happened on Venus. A runaway greenhouse effect is extremely unlikely to be caused by humans. Venus-like conditions on Earth would require a very large and long-lasting change in Earth's climate, which is not expected to happen until the sun becomes significantly brighter in about 600 to 700 million years.