In climate science, a tipping point is a key level that, when reached, causes big, fast, and often permanent changes in Earth's climate system. If these points are crossed, they can lead to serious problems for people and may speed up global warming. Tipping points are found in many parts of the climate system, 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, causing the planet to warm faster. Permafrost thawing is a major threat because it holds about twice as much carbon as is currently in the atmosphere.
Tipping points may happen suddenly or gradually. For example, if global warming reaches between 0.8°C (1.4°F) and 3°C (5.4°F), the Greenland ice sheet may cross a tipping point and begin to melt, but this process would take thousands of years. Today, with global warming just over 1°C (1.8°F) above preindustrial levels, some tipping points may already be close to being crossed or have already been reached. These include the West Antarctic and Greenland ice sheets, the Amazon rainforest, and warm-water coral reefs. A 2022 study in Science found that reaching 1.5°C of global warming could cause several tipping points, such as the collapse of major ice sheets, sudden permafrost thaw, and coral reef death, which might lead to more widespread problems.
A danger is that crossing one tipping point could cause other tipping points to be reached, leading to serious, possibly catastrophic effects. For example, ice loss in West Antarctica and Greenland could greatly change ocean currents. Continued warming in northern regions could 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 and West Antarctic ice sheets, the death of tropical coral reefs, and sudden permafrost thaw in boreal regions.
Tipping points occur in various systems, such as the cryosphere (ice-related areas), ocean currents, and land-based systems. Cryosphere tipping points include the breakdown of the Greenland, West Antarctic, and East Antarctic ice sheets, the loss of Arctic sea ice, shrinking mountain glaciers, and permafrost thaw. Ocean current tipping points include changes in the Atlantic Meridional Overturning Circulation (AMOC), the North Subpolar Gyre, and Southern Ocean circulation. Terrestrial 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 describes a tipping point as 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. This can also involve feedback loops that make changes happen faster, 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 defines a tipping point as: "A level of change in a system beyond which it reorganizes, often in a way that is not straightforward, 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 critical threshold 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 regime shift, which is a major change 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 Earth's climate system that may reach a point where changes happen quickly and cannot be reversed. In the early 2000s, the IPCC started studying these points, which were first called large-scale discontinuities. At that time, the IPCC believed these changes would only likely happen if global temperatures rose 4 °C (7.2 °F) or more above preindustrial levels. An earlier study suggested most tipping points would occur at warming levels of 3–5 °C (5.4–9.0 °F) compared to the average from 1980–1999. Over time, scientists have lowered their estimates, with some tipping points possibly happening within the Paris Agreement range (1.5–2 °C (2.7–3.6 °F)) by 2016. As of 2021, scientists believe some tipping points may already be close to being reached or have already been crossed at today’s warming level of just over 1 °C (1.8 °F). A high chance of crossing these points exists if warming reaches 2 °C (3.6 °F) or more. Examples of tipping points that may already be near or crossed include the 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 in the climate system. 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 abrupt thaw of boreal permafrost. Two additional tipping points are expected to occur if warming continues toward 2 °C (3.6 °F): the abrupt 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 about 1 millimeter to sea levels every year. About half of the ice loss happens from melting on the surface, and the other half happens where the ice meets the ocean, as icebergs break off from the edges.
The Greenland ice sheet has a tipping point because of a process called melt-elevation feedback. When ice melts, the ice sheet becomes shorter, and air at lower altitudes is warmer. This exposes the ice to warmer temperatures, making it melt even faster. A 2021 study of sediment from a 1.4-kilometer (0.87-mile) deep ice core in Greenland found that the ice sheet melted completely at least once in the last million years. This suggests that its tipping point may occur if global temperatures rise by more than 2.5°C (4.5°F) compared to preindustrial times. Evidence shows the ice sheet is becoming less stable and may be nearing this tipping point.
The West Antarctic Ice Sheet (WAIS) is a large ice sheet in Antarctica, with some parts over 4 kilometers (2.5 miles) thick. It sits mostly on bedrock below sea level, which makes it vulnerable to melting because ocean heat can reach it. A tipping point may occur if the grounding lines (the edge where ice meets rock) retreat into a deep basin under the ice. This process, called Marine Ice Sheet Instability (MISI), could cause the ice sheet to collapse quickly. Thinning of ice shelves is 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, and some glaciers are close to or may already have passed the point of self-sustaining retreat. Historical records show that the WAIS disappeared in the past during similar levels of warming and carbon dioxide emissions expected in the future.
Like other ice sheets, there is a counteracting effect: warmer temperatures increase snowfall in winter, which can add ice to the sheet. This might slow some ice loss. However, newer research shows that ice loss will likely continue to speed up even with more snowfall.
The East Antarctic ice sheet is the largest and thickest ice sheet on Earth, with some parts up to 4,800 meters (3.0 miles) thick. If it melted completely, it would raise sea levels by 53.3 meters (175 feet), but this would likely happen only if global temperatures rise by 10°C (18°F). Losing two-thirds of its volume might require a temperature rise of at least 6°C (11°F). This melt would take tens of thousands of years. However, parts of the East Antarctic ice sheet, such as 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, dark ocean surfaces absorb more heat, which warms the area. However, new models show that heat absorbed in summer is not enough to stop ice from forming again in winter, as long as winter temperatures stay cold enough. If warming prevents ice formation even in winter, this change could become permanent. A 2022 study included Arctic winter sea ice as a potential tipping point.
The same study noted that ice in the Barents Sea may not recover even if warming is limited to 2°C (3.6°F). The Barents Sea is warming faster than other Arctic regions, with temperatures rising up to seven times faster than the global average. This matters because changes in Barents Sea ice affect weather patterns across Eurasia.
Mountain glaciers hold the most land ice after the Greenland and Antarctic ice sheets. They are melting due to climate change. A glacier tipping point occurs when it becomes unstable and will melt unless temperatures drop. For example, in the North Cascade Range, 67% of glaciers were already unstable by 2005 and may not survive current climate trends. 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 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 Hindu Kush Himalaya region, known as the Earth’s Third Pole, holds the most glacier ice outside the polar regions. Even with limited warming to 1.5°C (2.7°F), one-third of its glaciers could be lost by 2100. Under higher warming scenarios, up to 67% may be lost. Glacier melt will increase river flows until about 2060, after which flows will decline. While river flows may remain stable in some areas due to increased rainfall, regions with less monsoon rain will face reduced water availability, affecting irrigation and hydropower.
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 moves water based on differences in water density: colder and saltier water is heavier than warmer, fresher water. The AMOC acts like a conveyor belt, carrying warm surface water from the tropics toward the north and returning cold, fresh water southward. As warm water moves north, some of it evaporates, increasing its saltiness. It also cools when exposed to colder air. Cold, salty water becomes denser and slowly sinks. Below the surface, this dense water flows south. Increased rainfall and melting ice from global warming dilute salty surface water, making it less dense. Lighter water is less likely to sink, which can slow the circulation.
Studies, simplified models, and past records suggest the AMOC has a tipping point. If freshwater from melting glaciers reaches a certain level, the AMOC could collapse into a state with much weaker flow. Even if melting stops, the AMOC may not return to its current state. Scientists believe the AMOC is unlikely to collapse 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 collapse. If the AMOC does shut down, a new stable state might form, lasting thousands of years and possibly triggering other tipping points.
In 2021, a study using a simple ocean model suggested that AMOC collapse could happen even if ice melt does not reach common tipping thresholds, as long as the melting happens quickly. This implies the collapse might be more likely than some large climate models suggest. Another 2021 study found early signs that the AMOC may be near a tipping point. However, a later study in the same journal found the AMOC has remained stable so far, with changes likely due to natural variability. Two studies in 2022 suggested that models often overestimate the risk of AMOC collapse. In 2024, 44 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 serious effects on Nordic countries. A 2025 study suggested collapse could begin as early as the 2060s.
Some models show that deep water movement in the Labrador-Irminger Seas could collapse under certain global warming scenarios, leading to the collapse of the entire North subpolar gyre circulation. This collapse might not be reversible, even if temperatures drop, making it a climate tipping point. This could cause rapid cooling, affecting agriculture, water resources, and energy systems in Western Europe and the U.S. East Coast. A 2017 study noted 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, and these include the four models that predict collapse. The study estimated a 36.4% chance of abrupt cooling in Europe due to AMOC collapse, lower than previous estimates of 45.5%. A 2022 study linked past disruptions of the subpolar gyre to the Little Ice Age.
The Southern Ocean overturning circulation has two parts: the upper and lower cells. The upper cell is more affected by wind, while the lower cell depends on the temperature and salinity of Antarctic bottom water. In recent decades, 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 climate patterns like the Interdecadal Pacific Oscillation and climate change, which altered the Southern Annular Mode weather pattern. Increased ocean heat content has also caused Antarctic ice to melt, adding fresh water that dilutes salty Antarctic bottom water.
Paleoclimate evidence shows the AMOC has weakened or collapsed before. Some research suggests collapse may become likely if global warming reaches 1.7–3.0°C. However, there is less certainty about this than for other climate tipping points. Even if collapse begins soon, it may not fully occur until near 2300. Effects 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 develop over centuries.
Tipping points in terrestrial systems
The Amazon rainforest is the largest tropical rainforest on Earth. It covers an area twice as large as India and spans nine countries in South America. The rainforest creates about half of its own rainfall by releasing moisture into the air 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 rainforest area might become too dry to support rainforests. However, if the rainforest is lost due to climate change (like droughts and wildfires) or deforestation, less rain will reach areas downwind, causing more stress and death for trees there. If enough forest is lost, a threshold could be reached where large parts of the remaining rainforest might die and turn into drier forests or savanna-like landscapes, especially in the drier southern and eastern regions. A 2022 study found that the rainforest has been losing resilience since the early 2000s. Resilience refers to how quickly the rainforest can recover after short-term disturbances, and delayed recovery is called critical slowing down. The loss of resilience supports the idea that the rainforest may be nearing a critical change, though it cannot predict exactly when or if a tipping point will occur.
In the last quarter of the twentieth century, the taiga (boreal forest) region experienced some of the fastest temperature increases on Earth. Winter temperatures rose more than summer temperatures, and in summer, daily low temperatures increased more than daily high temperatures. Scientists think that boreal environments have only a few stable long-term 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, but it could also cause tundra areas to become forests 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 also increased water stress and reduced tree growth in dry areas of the southern boreal forest in central Alaska and parts of far eastern Russia. In Siberia, the taiga is changing from mainly needle-shedding larch trees to evergreen conifers because of a warming climate.
Later research in Canada found that even in forests where total tree growth did not change, there was a shift toward more drought-tolerant deciduous broad-leaved trees over the past 65 years. A study using satellite images of 100,000 undisturbed sites 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 main tree species in eastern Canadian forests found that while a 2°C (3.6°F) temperature increase alone boosts their growth by about 13% on average, water availability is more important than temperature. Further warming up to 4°C (7.2°F) could cause significant declines unless rainfall also increases.
A 2021 paper confirmed that boreal forests in Canada are more strongly affected by climate change than other forest types and predicted that most of the eastern Canadian boreal forests could reach a tipping point around 2080 under the RCP 8.5 scenario, which represents the highest possible 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 of biomass in tropical forests. A 2022 experiment in North America found that young trees in the southern edges of boreal forests struggle the most with even 1.5°C (2.7°F) or 3.1°C (5.6°F) of warming and reduced rainfall. While some temperate tree species that would benefit from these conditions exist in the southern boreal forests, they are rare and grow more slowly.
Reports from the IPCC and the Special Report on Global Warming of 1.5°C suggest that most of East Africa, parts of Central Africa, and the main wet season in West Africa may see increased rainfall due to global warming. However, predictions for West Africa are uncertain. The Sahel region is becoming greener, but rainfall has not returned to levels seen in the mid-20th century.
A 2022 study concluded that while the future tipping point for the West African Monsoon (WAM) and Sahel is uncertain, the region remains a potential tipping element due to past abrupt changes and the impact of climate on local conditions. Some models show that global warming and higher carbon dioxide levels could increase rainfall in the Sahel/Sahara, leading to more vegetation growth in desert areas. However, this might also shift the desert northward, causing drier conditions in northern Africa.
In 2017, scientists discovered that 40% of the Cuvette Centrale wetlands are covered by a thick layer of peat containing about 30 billion tons of carbon. This peat holds 28% of all tropical peat carbon, equal to the carbon stored in all the forests of the Congo Basin. Although this peatland covers only 4% of the Congo Basin, its carbon content matches that of the other 96% of the basin’s forests. If all this peat burned, the atmosphere would absorb carbon equal to 20 years of current U.S. carbon dioxide emissions, or three years of all human-caused emissions worldwide.
This discovery led to the Brazzaville Declaration in 2018, an agreement between the Democratic Republic of Congo, the Republic of Congo, and Indonesia (a country with experience managing tropical peatlands) to improve conservation efforts. However, a 2022 study revised earlier estimates, finding the peatland covers 167,600 square kilometers (64,700 square miles) and has an average depth of 1.7 meters (5.6 feet). Only 8% of this peat is protected, while 26% is in areas open to logging, mining, or palm oil plantations, and nearly all of it is accessible for fossil fuel exploration.
Even without human activity, this area is the most vulnerable tropical peatland due to its drier climate compared to Southeast Asia and the Amazon. A 2022 study suggests that conditions similar to those 7,500 to 2,000 years ago—when the region was already dry enough to release significant peat carbon—could return in the near future due to climate change. If this happens, the Cuvette Centrale peatland could act as a tipping point in the climate system at an 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 large-scale bleaching, especially in sub-tropical regions. A long period of ocean temperatures that are 1 degree Celsius (1.8 degrees Fahrenheit) higher than normal can lead to bleaching. When corals experience heat stress, they expel the small, colorful algae that live inside their tissues, causing them to turn white. These algae, called zooxanthellae, live in a mutually beneficial relationship with corals. Without them, corals slowly die. After zooxanthellae disappear, corals become vulnerable to ecosystems dominated by seaweed, making it very difficult to return to coral-dominated ecosystems. The IPCC estimates that if global temperatures rise to 1.5 degrees Celsius (2.7 degrees Fahrenheit) above pre-industrial levels, coral reefs could decline by 70–90%. If temperatures rise to 2 degrees Celsius (3.6 degrees Fahrenheit), coral reefs would become extremely rare.
In 2019, a study used a type of computer model to estimate that if carbon dioxide levels rise above 1,200 parts per million (almost three times current levels and over four times preindustrial levels), equatorial stratocumulus clouds could break apart and scatter. This would cause global surface warming of about 8 degrees Celsius (14 degrees Fahrenheit) and 10 degrees Celsius (18 degrees Fahrenheit) in sub-tropical regions, in addition to at least 4 degrees Celsius (7.2 degrees Fahrenheit) already caused by these CO2 levels. The study suggested that these clouds would not reform until CO2 levels drop significantly. This finding may 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 were used to offset high CO2 emissions, the breakup of stratocumulus clouds would only be delayed until CO2 levels reached 1,700 ppm, still causing about 5 degrees Celsius (9.0 degrees Fahrenheit) of warming.
However, because the models used in this study are simpler and smaller-scale than the models used for climate projections, and they do not fully represent atmospheric processes like subsidence, this finding is considered speculative. Some scientists argue that the study’s model oversimplifies cloud behavior, making it unable to simulate gradual changes. CO2 levels would only reach 1,200 ppm if the world follows the highest greenhouse gas emission scenario, which involves expanding coal infrastructure. In that case, 1,200 ppm would be reached shortly after 2100.
Crossing a threshold in one part of the climate system may trigger other tipping points, leading to cascading changes, similar to a domino effect. For example, ice loss in West Antarctica and Greenland could disrupt ocean circulation. Sustained warming in northern high latitudes might activate tipping elements like permafrost degradation and boreal forest dieback. Thawing permafrost is a major risk because it holds about twice as much carbon as currently in the atmosphere. Loss of Greenland ice could destabilize the West Antarctic ice sheet through rising sea levels, and vice versa, especially if Greenland melts first, as West Antarctica is vulnerable to warm seawater.
A 2021 study using three million computer simulations of a climate model found that nearly one-third of the simulations showed domino effects, even with temperature increases limited to 2 degrees Celsius (3.6 degrees Fahrenheit)—the upper limit set by the Paris Agreement. The study’s authors noted that while the science of tipping points is complex and uncertain, cascading tipping points could pose an existential threat to 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. Normally, 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. This causes the middle of the Pacific Ocean to warm, which changes wind patterns worldwide. This event is called El Niño. It often causes droughts in India, Indonesia, and Brazil, and more flooding in Peru. In 2015/2016, El Niño led to food shortages affecting over 60 million people. El Niño-related droughts may also increase the risk of forest fires in the Amazon. Scientists estimated that ENSO could reach a tipping point if global warming reaches between 3.5 °C (6.3 °F) and 7 °C (13 °F) in 2016. If this tipping point is reached, the system might stay in a permanent El Niño state instead of changing between states. This happened in Earth’s past during the Pliocene, but ocean conditions were very different then. So far, 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 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 part of the climate system that earlier research suggested might collapse permanently. However, more recent studies show 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 thought to be at risk of rapid release, which could greatly affect global temperatures. This idea is called the clathrate gun hypothesis. Later research 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 hydrates in deeper permafrost or under the ocean 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 a change in environmental conditions, crosses a critical level. At this point, a bifurcation happens, causing a stable state to become unstable or disappear. The Atlantic Meridional Overturning Circulation (AMOC) is an example of a system that can experience bifurcation-induced tipping. Slow changes in factors like water salinity and temperature in this system may 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 level of greenhouse gases or temperature can vary based on past warmth.
For systems near a bifurcation-induced tipping point, signs like reduced resilience to disturbances may appear. These systems show critical slowing down, with increased memory (autocorrelation) and variability. Other early warning signals may exist depending on the system. Sudden changes are not early warning signals, as they can also happen if changes are reversible.
Early warning signals are often studied using data from the past, such as sediments, ice cores, and tree rings, where past tipping events are recorded. It is not always clear if increased variability and autocorrelation signal an approaching tipping point or result from natural system changes. For example, the AMOC’s collapse may be influenced by data quality issues. These signals have been used to detect tipping in systems like California’s forests during droughts and the Pine Island Glacier in Antarctica. Studies suggest the Greenland ice sheet is currently losing resilience, matching modeled early warning signals.
Human-caused changes may happen too quickly for early warning signals to be detected, especially in systems that take time to adjust.
Noise-induced tipping happens when random fluctuations or internal changes in a system cause a shift between states. These transitions lack early warning signals because the system’s underlying structure does not change. This unpredictability makes such events hard to forecast, often described as "one-in-x-year" events. An example is the Dansgaard–Oeschger events during the last ice age, which caused 25 sudden climate shifts over 500 years.
Rate-induced tipping occurs when environmental changes happen faster than the system can recover. In peatlands, for instance, rapid changes can cause a sudden release of stored carbon into the atmosphere, sometimes called "compost bomb instability." The AMOC may also experience rate-induced tipping: if ice melt accelerates too quickly, the system could collapse before reaching the critical level that would trigger a bifurcation.
Potential impacts
Tipping points can cause very serious problems. They can make current climate change dangers worse or create new dangers. Some tipping points might happen suddenly, like changes in the Indian monsoon, which could harm food supplies for hundreds of millions of people. Other changes might happen slowly, such as ice caps melting. If Greenland and West Antarctica melt completely, sea levels could rise about 10 meters (33 feet) over many centuries. This would require moving many cities inland, but it would also speed up sea level rise this century, possibly putting 120 million more people at risk of yearly floods in some areas. A collapse of the Atlantic Overturning Circulation could cause temperatures in parts of Europe to drop 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 could seriously affect food supplies, with one study showing that major crop yields might drop in many parts of the world, including making farming in Britain too expensive to support. These effects might 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 cause many connected problems in climate, ecosystems, and human societies. For example, the sudden end of a wet period in Africa led to desertification and changes in ecosystems, which caused pastoral communities 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 start a chain of tipping points and feedback loops that make climate change harder to control, leading to much higher temperatures, rising seas, and serious harm to ecosystems, societies, and economies. This possible future is called the Hothouse Earth scenario. However, whether this threshold exists and what it exactly means is still 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 of thousands of years and might create conditions that are hard for human societies to survive. The report also says that even if the goal in the Paris Agreement—to limit warming to 1.5–2.0 degrees Celsius—is met, a series of tipping points could still be triggered.
Geological timescales
The geological record shows many sudden changes over long periods of time that suggest tipping points may have been reached in ancient times. For example, the Dansgaard–Oeschger events during the last ice age were times of sudden warming in Greenland and Europe that happened quickly, within decades. These changes might have been caused by sudden shifts in major ocean currents. During the early Holocene, when ice was melting, sea levels did not rise smoothly. Instead, they rose suddenly during periods of rapid melting. In North Africa, the monsoon changed suddenly over about ten years during the African humid period. This period, which lasted from 15,000 to 5,000 years ago, ended quickly with a much drier climate.
A runaway greenhouse effect is a tipping point so extreme that oceans turn into vapor, and the water vapor leaves Earth's atmosphere. This irreversible climate state occurred 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, which is not expected to happen until the sun becomes 10% brighter. This will take 600–700 million years.