In climate science, a tipping point is a key moment when a small change causes big, fast, and often permanent changes in Earth's climate. If tipping points are reached, they can cause serious problems for people and may speed up global warming. These tipping points happen in many parts of Earth's climate, such as ice sheets, glaciers, ocean currents, ecosystems, and the atmosphere. Examples include permafrost thawing, which releases methane, a strong greenhouse gas, or melting ice sheets and glaciers, which lower Earth's ability to reflect sunlight and make the planet warm faster. Permafrost thawing is a big risk because it holds about twice as much carbon as is currently in the air.
Tipping points may happen suddenly or take a long time. For example, if Earth warms between 0.8°C (1.4°F) and 3°C (5.4°F), the Greenland ice sheet might reach a tipping point and begin to melt, but the melting would take thousands of years. Scientists say tipping points could already be near or have already been reached, such as in the West Antarctic and Greenland ice sheets, the Amazon rainforest, and warm-water coral reefs. A 2022 study in Science found that warming above 1.5°C (2.7°F) could cause many tipping points, like ice sheet collapse, permafrost thaw, and coral reef loss, which could lead to major problems in Earth's systems.
A danger is that crossing one tipping point might cause others to happen, leading to serious or even catastrophic effects. For example, ice loss in Antarctica and Greenland could change ocean currents. Long-term warming in northern areas might then cause more tipping points, like permafrost loss or forest dieback.
Scientists have found many parts of Earth's climate that might have tipping points. As of September 2022, nine global and seven regional tipping points are known. If Earth warms to 1.5°C (2.7°F), four of these are likely to reach a tipping point: the collapse of the Greenland ice sheet, the collapse of the West Antarctic ice sheet, the loss of tropical coral reefs, and the sudden thaw of boreal permafrost.
Tipping points occur in many systems, such as ice-covered areas, ocean currents, and land-based systems. In ice-covered areas, tipping points include the breakdown of ice sheets in Greenland, Antarctica, and the Arctic, as well as the loss of mountain glaciers and permafrost. In ocean currents, tipping points include changes in the Atlantic Meridional Overturning Circulation, the North Subpolar Gyre, and the Southern Ocean circulation. On land, tipping points include the loss of the Amazon rainforest, changes in boreal forests, the greening of 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 completely, often suddenly and/or permanently." A small change can cause a much larger effect in the system. It may also involve feedback loops that make changes happen faster, leading to climate changes that cannot be reversed within a human lifetime. 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 completely, often in an unpredictable way, and does not return to its original state even if the causes of the change are reduced. In the climate system, this term refers to a point where global or regional 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 term "tipping point" is sometimes used positively, such as when public opinion changes to support climate action or when small policy changes lead to faster progress toward a green economy.
Comparison of tipping points
Scientists have found many parts of the climate system that may reach critical thresholds. In the early 2000s, the Intergovernmental Panel on Climate Change (IPCC) started studying these thresholds, which were first called large-scale discontinuities. At that time, the IPCC said these thresholds would likely occur only if global temperatures rose 4°C (7.2°F) or more above preindustrial levels. An earlier study suggested most thresholds would occur at 3–5°C (5.4–9.0°F) above the average warming from 1980–1999. Over time, estimates for when these thresholds might occur have decreased. Some scientists now think certain thresholds could be reached within the Paris Agreement range (1.5–2°C (2.7–3.6°F)) by 2016. As of 2021, scientists believe tipping points are likely at today’s warming level of just over 1°C (1.8°F), and very likely if warming reaches 2°C (3.6°F). Some tipping points may already have been reached or are very close, such as those involving ice sheets in West Antarctica and Greenland, warm-water coral reefs, and the Amazon rainforest.
As of September 2022, scientists have identified nine global and seven regional tipping elements. Of these, one regional and three global elements are likely to cross a tipping point if global warming reaches 1.5°C (2.7°F). These include the collapse of the Greenland and West Antarctic ice sheets, the death 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 in the world. If all the ice melted, it would raise global sea levels by 7.2 meters (24 feet). Because of global warming, the ice sheet is melting faster each year, adding nearly 1 millimeter to sea levels every year. About half of the ice loss happens when ice on the surface melts. The other half happens where the ice meets the sea, as icebergs break off from the edges.
The Greenland ice sheet has a tipping point because of a feedback loop. When ice melts, the ice sheet becomes shorter, and air at lower altitudes is warmer. This exposes the ice to even warmer temperatures, making it melt faster. A 2021 study of sediment from a 1.4-kilometer (0.87-mile) Greenland ice core showed that the ice sheet melted completely at least once in the past million years. This suggests its tipping point may be below a 2.5°C (4.5°F) increase in temperature since preindustrial times. Some evidence shows the ice sheet is becoming less stable and may be near a tipping point.
The West Antarctic Ice Sheet (WAIS) is a large ice sheet in Antarctica, up to 4 kilometers (2.5 miles) thick in some places. It sits mostly on bedrock below sea level, forming a deep basin over millions of years. This makes it vulnerable to ocean heat, which can cause fast and irreversible ice loss. A tipping point may occur if the grounding lines (where ice meets rock and becomes floating ice shelves) retreat into the basin. This process, called Marine Ice Sheet Instability (MISI), causes the ice to retreat on its own. Thinning and collapse of ice shelves are speeding up this retreat. If the WAIS melted completely, it would raise sea levels by about 3.3 meters (11 feet) over thousands of years.
Ice loss from the WAIS is increasing. Some glaciers may already be beyond the point of self-sustaining retreat. Historical records show that the WAIS largely disappeared during past warming events similar to those expected in the next few centuries.
Like other ice sheets, there is a counteracting effect: warmer temperatures increase snowfall in winter, which freezes on the surface and adds to the ice. This could slow some ice loss. However, newer models show that glacier breakup will likely speed up as warming continues.
The East Antarctic ice sheet is the largest and thickest on Earth, with a maximum thickness of 4,800 meters (3.0 miles). If it completely melted, it would raise sea levels by 53.3 meters (175 feet), 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 meters (10–13 feet).
Arctic sea ice was once thought to be a tipping point. When sea ice melts in summer, the dark ocean absorbs more heat, warming the area. Scientists once thought this could prevent ice from returning even if warming stopped. However, newer models show that winter cooling can still form new ice. If warming prevents winter ice formation, Arctic sea ice could become irreversible. A 2022 report noted that ice in the Barents Sea may not recover even with 2°C (3.6°F) of warming, as it is warming much faster than other Arctic areas. 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 ice sheets. They are melting due to climate change. A tipping point occurs when glaciers can no longer balance with the climate and will melt unless temperatures drop. For example, 67% of glaciers in the North Cascade Range were already in disequilibrium by 2005 and may not survive current warming. In the French Alps, glaciers like Argentière and Mer de Glace may vanish by 2100 if trends continue. By 2100, 49% of glaciers could 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 inches) and 15 cm (6 inches) to sea level rise, respectively.
The largest glacier ice is in the Hindu Kush Himalaya region, called the "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 than 67% of glaciers could be lost. Meltwater from these glaciers will increase river flows until around 2060, then decline. While river flows may stay stable in some areas, irrigation and hydropower will need to adapt to more unpredictable water patterns.
Tipping points related to ocean current collapse
The Atlantic meridional overturning circulation (AMOC), also called the Gulf Stream System, is a large system of ocean currents. It is caused by differences in water density; colder and saltier water is heavier than warmer fresh water. The AMOC acts like a conveyor belt, moving warm surface water from the tropics toward the north and carrying cold fresh water back south. As warm water flows north, some of it evaporates, increasing salinity. It also cools when it meets cooler air. Cold, salty water is denser and begins to sink slowly. Several kilometers below the surface, cold, dense water moves south. Increased rainfall and melting ice from global warming dilute salty surface water, reducing its density. Lighter water is less likely to sink, slowing the circulation.
Studies and models suggest the AMOC has a tipping point. If enough freshwater from melting glaciers enters the ocean, the AMOC might collapse into a state with much slower flow. Even if melting stops, the AMOC may not return to its current state. It is unlikely to tip in the 21st century, but could happen before 2300 if greenhouse gas emissions are very high. A weakening of 24% to 39% is expected depending on emissions, even without tipping behavior. If the AMOC collapses, a new stable state could form, lasting thousands of years and possibly triggering other tipping points.
A 2021 study used a simple ocean model to show that AMOC collapse could occur even without reaching usual tipping thresholds, suggesting it might be more likely than complex models estimate. Another 2021 study found signs that the AMOC might be near a tipping point, but a 2022 study disagreed, saying the AMOC has remained stable within natural variations. More 2022 studies suggested that models might overestimate the risk of collapse. In 2024, 44 scientists said the risk of collapse has been underestimated, possibly happening in the next few decades. A 2025 study suggested collapse could start as early as the 2060s.
Some models show that deep convection in the Labrador-Irminger Seas could collapse under certain warming scenarios, leading to the entire North subpolar gyre circulation collapsing. This might not recover even if temperatures drop, making it a climate tipping point. This could cause rapid cooling, affecting agriculture, water, and energy in Europe and the U.S. A 2017 study noted changes in temperature gradients not captured by the AMO Index.
A 2021 study found that only four out of 35 models predicted subpolar gyre collapse. Eleven models accurately simulate the North Atlantic Current, including those four. The study estimated a 36.4% chance of abrupt cooling in Europe, lower than previous estimates. A 2022 study linked past subpolar gyre disruptions to the Little Ice Age.
The Southern Ocean’s circulation has two parts: an upper cell affected by wind and a lower cell influenced by Antarctic bottom water temperature and salinity. The upper cell’s flow has increased by 50–60% since the 1970s, while the lower cell has weakened by 10–20%. This is partly due to natural cycles and climate change, which altered weather patterns and increased Antarctic ice melt, diluting the bottom water.
Paleoclimate evidence shows the AMOC has weakened or collapsed before. Some research suggests collapse might occur when global warming reaches 1.7°C to 3°C. However, there is less certainty than with other tipping points. Even if collapse starts soon, it is unlikely to be complete until near 2300. Impacts like reduced Southern Hemisphere rainfall and Southern Ocean fisheries decline are expected over centuries.
Tipping points in terrestrial systems
The Amazon rainforest is the largest tropical rainforest in the world. 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 that about 40% of the current forest area might become too dry to support rainforests. However, if the forest is lost due to climate change (like droughts and wildfires) or deforestation, less rain will fall in nearby regions, increasing stress and death of trees there. If enough 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 after short-term changes, and delayed recovery is called "critical slowing down." The loss of resilience supports the idea that the rainforest might be nearing a critical change, though it cannot predict exactly when or if a tipping point will occur.
During the last quarter of the twentieth 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 summer daily lows increased more than daily highs. Scientists think that boreal environments (like taiga) have only a few long-term stable states: a treeless tundra or steppe, a forest with more than 75% tree cover, or an open woodland with about 20% to 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 and become more suitable for trees.
These trends were first noticed in Canada’s boreal forests in the early 2010s. Summer warming has increased water stress and reduced tree growth in dry parts of the southern boreal forest in central Alaska and far eastern Russia. In Siberia, the taiga is changing from mostly larch trees (which shed needles) to evergreen conifers due to warming.
Later research in Canada found that even in forests where total tree growth did not change, there was a shift toward more deciduous broad-leaved trees (which are more drought-tolerant) over the past 65 years. A study using satellite data found that areas with low tree cover became greener as temperatures rose, but tree death (browning) became more common in areas with higher tree cover. A 2018 study of seven tree species in eastern Canada found that a 2°C warming increased their growth by about 13% on average, but water availability was more important than temperature. Further warming up to 4°C could cause large declines in growth unless rainfall also increases.
A 2021 study confirmed that boreal forests are more strongly affected by climate change than other forest types in Canada. It projected that most of the eastern Canadian boreal forests might reach a tipping point around 2080 under the RCP 8.5 scenario, which represents the highest potential increase in human-caused emissions. Another 2021 study projected that under a moderate climate scenario, boreal forests worldwide might gain 15% more biomass 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 the most with even 1.5°C or 3.1°C of warming and reduced rainfall. While temperate tree species that could benefit from these conditions exist in the southern boreal forests, they are rare and grow slowly.
The Special Report on Global Warming of 1.5°C and the IPCC Fifth Assessment Report suggest that global warming may increase rainfall in most of East Africa, parts of Central Africa, and the main wet season in West Africa. However, predictions for West Africa are uncertain. Currently, the Sahel region is becoming greener, but rainfall has not returned to levels from the mid-20th century.
A 2022 study concluded that it is unclear whether the West African Monsoon (WAM) and Sahel region have a future tipping point, but past abrupt changes and model limitations suggest they could be a potential tipping element with low confidence.
Some simulations show that global warming and higher carbon dioxide levels could increase rainfall in the Sahel and Sahara, leading to more plant growth and vegetation expanding into deserts. However, this might also cause the desert to shift northward, drying northern Africa.
In 2017, scientists discovered that 40% of the Cuvette Centrale wetlands have a thick layer of peat (a type of soil) 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. Although the peatland covers only 4% of the Congo Basin, its carbon content matches that of the rest of the basin’s forests. If all this peat burned, it would release carbon equal to 20 years of current U.S. carbon dioxide emissions or three years of all human-caused emissions.
This discovery led to the Brazzaville Declaration in 2018, where the Democratic Republic of Congo, the Republic of Congo, and Indonesia (a country with experience managing peatlands) agreed to better protect this region. However, a 2022 study revised the peatland’s size (from 145,500 to 167,600 square kilometers) and depth (from 2 meters to 1.7 meters). It also found that only 8% of the peat is protected, while 26% is in areas open to logging, mining, or palm oil plantations. Nearly all of this area is also open to fossil fuel exploration.
Even without human activity, this peatland is highly vulnerable because its climate is drier than other tropical peatlands in Southeast Asia and the Amazon. A 2022 study suggests that conditions between 7,500 and 2,000 years ago were already dry enough to release peat carbon, and similar conditions may return with continued climate change. In this case, the Cuvette Centrale could become a tipping point in the climate system at an unknown future time.
Other tipping points
About 500 million people worldwide rely on coral reefs for food, jobs, tourism, and protection from coastal storms. Since the 1980s, rising sea surface temperatures have harmed coral reefs, causing widespread coral bleaching, especially in sub-tropical areas. If ocean temperatures stay 1 °C (1.8 °F) above normal for a long time, coral bleaching occurs. When corals are stressed by heat, they push out tiny colorful algae called zooxanthellae, which live inside their tissues. This causes the corals to turn white. These algae have a special relationship with corals, and without them, corals slowly die. After zooxanthellae disappear, coral reefs may shift to ecosystems dominated by seaweed, making it hard to return to a coral-dominated state. The IPCC predicts that if global temperatures rise by 1.5 °C (2.7 °F) above pre-industrial levels, coral reefs could decline by 70–90%. If temperatures rise by 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 today’s levels and over four times pre-industrial levels), equatorial stratocumulus clouds might break apart and scatter. This could lead to about 8 °C (14 °F) of global warming and 10 °C (18 °F) in sub-tropical regions, adding to at least 4 °C (7.2 °F) already caused by such CO₂ levels. The study suggested that these clouds would not reform until CO₂ levels dropped significantly. This finding may explain past periods of rapid warming, like 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 stratocumulus 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 and smaller than the models typically used for climate predictions. These models do not fully represent complex atmospheric processes, such as subsidence, and their findings are considered speculative. Some scientists argue that the study’s model overestimates the impact of small cloud areas and cannot simulate gradual changes, comparing it to a switch with only two settings. CO₂ levels reaching 1,200 ppm would only happen if the world follows the most extreme greenhouse gas emission scenario (Representative Concentration Pathway 8.5), which involves a large expansion of coal use. In that case, CO₂ levels would reach 1,200 ppm shortly after the year 2100.
Crossing a threshold in one part of Earth’s climate system can trigger other tipping points, leading to sudden and irreversible changes. These linked tipping points are called cascading tipping points, similar to a chain reaction. For example, ice loss in West Antarctica and Greenland could disrupt ocean currents. Continued warming in northern regions might trigger other tipping points, 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 destabilize the West Antarctic ice sheet due to rising sea levels, and vice versa, especially if Greenland melts first, as West Antarctica is more vulnerable to warm ocean water.
A 2021 study using three million computer simulations of a climate model found that nearly one-third of the simulations showed cascading tipping points, even if global temperatures rose no more than 2 °C (3.6 °F)—the limit set by the Paris Agreement in 2015. The researchers noted that while the science of tipping points is complex and uncertain, the possibility of cascading tipping points poses a major risk to human societies. A network model analysis also found 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) is a climate pattern that has been studied as a possible tipping element. Usually, strong winds blow west across the South Pacific Ocean from South America to Australia. 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, these droughts led to food shortages affecting over 60 million people. El Niño-related droughts may make forest fires in the Amazon more likely. Scientists estimated that ENSO could become a tipping element if global temperatures rise between 3.5 °C (6.3 °F) and 7 °C (13 °F) by 2016. If this happened, ENSO would stay in a permanent El Niño state instead of switching between states. This situation occurred in Earth's past during the Pliocene era, but ocean conditions were very different then. Currently, there is no clear evidence that ENSO behavior has changed. The IPCC Sixth Assessment Report states that ENSO is expected to remain the main cause of yearly climate changes in a warmer world. Because of this, the 2022 report no longer lists ENSO as a likely tipping element.
The Indian summer monsoon is another climate system that was once thought to be at risk of not recovering from changes. However, newer research shows that warming tends to make the monsoon stronger, and it is expected to become stronger in the future.
Methane hydrate deposits in the Arctic were once considered a risk because they could break apart quickly, releasing large amounts of methane and raising 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 released from the seafloor rarely reaches the atmosphere. The IPCC Sixth Assessment Report states that it is very unlikely that methane from deep permafrost or underwater hydrates 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 happens when a specific factor in the climate, such as changes in temperature or salinity, crosses a critical level. At this point, a sudden shift occurs, and a stable state may become unstable or disappear. The Atlantic Meridional Overturning Circulation (AMOC) is an example of a system that could experience this type of tipping. Slow changes in the AMOC’s salinity and temperature might lead to its collapse.
Some bifurcations show hysteresis, meaning the system’s current state depends on its past. For example, the amount of ice on Earth’s poles at a certain temperature might vary based on previous climate conditions.
For systems near a bifurcation tipping point, signs may appear before the tipping occurs. These include reduced resilience to disturbances, increased memory (autocorrelation), and higher variability. Other early warning signals might exist depending on the system. However, sudden changes are not reliable signs of tipping, as they can also happen if conditions return to normal.
Early warning signals are often studied using ancient records, such as sediment layers, ice cores, and tree rings, which show past climate changes. It is not always clear whether increased variability or memory is a sign of tipping or caused by natural changes. These signals have been tested in systems like California forests during droughts and the Pine Island Glacier in Antarctica. For example, signs of reduced resilience have been found in the Greenland ice sheet, matching computer model predictions.
Human-caused climate changes may happen too quickly for early warning signals to be detected, especially in systems that take time to adjust.
Noise-induced tipping occurs when random changes or internal system variations cause a shift between states. These shifts do not show early warning signals because the system’s underlying conditions remain unchanged. These events are often described as rare, such as the Dansgaard–Oeschger events during the last ice age, which caused sudden climate changes 25 times over 500 years.
Rate-induced tipping happens when environmental changes occur faster than the system can recover. For example, in peatlands, rapid changes could cause a sudden release of stored carbon into the atmosphere, known as "compost bomb instability." The AMOC might also experience this: if ice melt happens too quickly, the circulation could collapse before reaching a critical level that would trigger a 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 develop more slowly, such as ice caps melting. If Greenland and West Antarctica completely melt, sea levels could rise about 10 meters (33 feet) over centuries, which would require moving many cities inland. However, this melting could also speed up sea level rise this century, increasing the risk of flooding for 120 million more people in a mid-emissions scenario. If the Atlantic Overturning Circulation collapses, parts of Europe could cool by more than 10°C, cause droughts 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 in Britain too expensive to support. These effects could happen at the same time if multiple tipping points are triggered together. A study of climate changes over the last 30,000 years found that tipping points can lead to many linked problems in climate, ecosystems, and human societies. For example, the end of a wet period in Africa led to desertification, changes in ecosystems, and the decline of pastoral communities in North Africa and a shift in leadership in Egypt.
Some scientists suggest that crossing a certain temperature threshold could cause many tipping points and feedback loops that make climate change worse, leading to extreme warming, rising seas, and serious harm to ecosystems, societies, and economies. This possibility is sometimes called the "Hothouse Earth" scenario. Researchers think this could happen if global temperatures rise more than 2°C above pre-industrial levels. However, whether this threshold exists and what it is remains uncertain. Some experts question if tipping points would cause long-term warming quickly. Choices made in the next decade could affect Earth'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°C (2.7–3.6°F) is met, a chain of tipping points could still occur.
Geological timescales
The geological record shows sudden changes over long periods of time that may indicate tipping points were reached in ancient times. For example, during the last ice age, the Dansgaard–Oeschger events were times of sudden warming in Greenland and Europe that may have been linked to changes in ocean currents. During the early Holocene, when ice was melting, sea levels did not rise gradually. Instead, they rose suddenly during periods of rapid melting. In North Africa, the monsoon changed suddenly over decades during the African humid period, which lasted from 15,000 to 5,000 years ago. This period ended quickly, leading to a much drier climate.
A runaway greenhouse effect is a tipping point so extreme that oceans evaporate, and water vapor escapes into space, creating a climate state that cannot be reversed. This happened on the planet Venus. A runaway greenhouse effect is extremely unlikely to be caused by human activities. On Earth, conditions similar to Venus would require a very large and long-lasting change in climate, which is not expected until the sun becomes much brighter in about 600–700 million years.