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climate science Climatology (from Greek , ''klima'', "place, zone"; and , '' -logia'') or climate science is the scientific study of Earth's climate, typically defined as weather conditions averaged over a period of at least 30 years. This modern field of stu ...
, a tipping point is a critical threshold that, when crossed, leads to large and often irreversible changes in the
climate system Earth's climate system is a complex system having five interacting components: the atmosphere (air), the hydrosphere (water), the cryosphere (ice and permafrost), the lithosphere (earth's upper rocky layer) and the biosphere (living things). '' ...
. If tipping points are crossed, they are likely to have severe impacts on human society. Tipping behaviour is found across the climate system, in ecosystems, ice sheets, and the circulation of the ocean and atmosphere. Tipping points are often, but not necessarily, abrupt. For example, with average global warming somewhere between 0.8 and 3 °C, the
Greenland ice sheet The Greenland ice sheet ( da, Grønlands indlandsis, kl, Sermersuaq) is a vast body of ice covering , roughly near 80% of the surface of Greenland. It is sometimes referred to as an ice cap, or under the term ''inland ice'', or its Danish equi ...
passes a tipping point and is doomed, but the melt would take place over millennia. Tipping points are possible at today's global warming of just over 1 °C above preindustrial times, and highly probable above 2 °C of global warming. The
geological record The geologic record in stratigraphy, paleontology and other natural sciences refers to the entirety of the layers of rock stratum, strata. That is, deposits laid down by volcanism or by Deposition (sediment), deposition of sediment derived from w ...
shows many abrupt changes that suggest tipping points may have been crossed in ancient times. It is possible that some tipping points are close to being crossed or have already been crossed, like those of the West Antarctic and
Greenland ice sheet The Greenland ice sheet ( da, Grønlands indlandsis, kl, Sermersuaq) is a vast body of ice covering , roughly near 80% of the surface of Greenland. It is sometimes referred to as an ice cap, or under the term ''inland ice'', or its Danish equi ...
s, the
Amazon rainforest The Amazon rainforest, Amazon jungle or ; es, Selva amazónica, , or usually ; french: Forêt amazonienne; nl, Amazoneregenwoud. In English, the names are sometimes capitalized further, as Amazon Rainforest, Amazon Forest, or Amazon Jungle. ...
and warm-water coral reefs. A danger is that if the tipping point in one system is crossed, this could cause a cascade of other tipping points, leading to severe impacts.


Definition

The sixth report from the United Nations Intergovernmental Panel on Climate Change (IPCC), released in 2021, defines a tipping point as a "critical threshold beyond which a system reorganizes, often abruptly and/or irreversibly". It can be brought about by a small disturbance causing a disproportionately large change in the system. One set of definitions of "tipping points" also requires self-reinforcing
feedback Feedback occurs when outputs of a system are routed back as inputs as part of a chain of cause-and-effect that forms a circuit or loop. The system can then be said to ''feed back'' into itself. The notion of cause-and-effect has to be handled ...
s, which could lead to changes in the climate system irreversible on a human timescale. For any particular climate component, the shift from one state to a new stable state may take many decades or centuries. In
ecosystem An ecosystem (or ecological system) consists of all the organisms and the physical environment with which they interact. These biotic and abiotic components are linked together through nutrient cycles and energy flows. Energy enters the syst ...
s and in social systems, a tipping point can trigger a ''regime shift'', a major systems reorganisation into a new stable state. The 2019 IPCC
Special Report on the Ocean and Cryosphere in a Changing Climate The United Nations' Intergovernmental Panel on Climate Change's (IPCC) Special Report on the Ocean and Cryosphere in a Changing Climate (SROCC) is a report about the effects of climate change on the world's seas, sea ice, icecaps and glaciers. ...
defines a tipping point as: "A level of change in system properties beyond which a system reorganises, often in a non-linear manner, and does not return to the initial state even if the drivers of the change are abated. For the climate system, the term refers to a critical threshold at which global or regional climate changes from one stable state to another stable state.".


Geological record

The geological record shows that there have been abrupt changes in the
climate system Earth's climate system is a complex system having five interacting components: the atmosphere (air), the hydrosphere (water), the cryosphere (ice and permafrost), the lithosphere (earth's upper rocky layer) and the biosphere (living things). '' ...
that indicate ancient tipping points. For instance, the Dansgaard–Oeschger events during the last ice age were periods of abrupt warming (within decades) in Greenland and Europe, that may have involved the abrupt changes in major ocean currents. During the
deglaciation Deglaciation is the transition from full glacial conditions during ice ages, to warm interglacials, characterized by global warming and sea level rise due to change in continental ice volume. Thus, it refers to the retreat of a glacier, an ice shee ...
in the early
Holocene The Holocene ( ) is the current geological epoch. It began approximately 11,650 cal years Before Present (), after the Last Glacial Period, which concluded with the Holocene glacial retreat. The Holocene and the preceding Pleistocene togeth ...
,
sea level rise Globally, sea levels are rising due to human-caused climate change. Between 1901 and 2018, the globally averaged sea level rose by , or 1–2 mm per year on average.IPCC, 2019Summary for Policymakers InIPCC Special Report on the Ocean and Cr ...
was not smooth, but rose abruptly during meltwater pulses. The monsoon in North Africa saw abrupt changes on decadal timescales during the African humid period. This period, spanning from 15,000 to 5,000 years ago, also ended suddenly in a drier state.


Tipping elements

Scientists have identified many elements in the climate system which may have tipping points. in the early 2000s the IPCC began considering the possibility of tipping points. At that time the IPCC concluded they would only be likely in the event of global warming of 4 °C or more above preindustrial times. As of 2021 tipping points are considered to have significant probability at today's warming level of just over 1 °C, with high probability above 2 °C of global warming, and it is possible that some tipping points are close to being crossed or have already been crossed, like those of the ice sheets in West Antarctic and Greenland, warm-water coral reefs, and the Amazon rainforest. As of September 2022, nine core tipping elements and seven regional impact tipping elements were identified. Out of those, one regional and three core climate elements are estimated to likely pass a tipping point if global warming reaches 1.5 °C, namely the Greenland ice sheet collapse, the West Antarctic ice sheet collapse, tropical coral reef die off, and the boreal permafrost abrupt thaw. Two further tipping points are forecast as likely if warming continues to approach 2 °C : Barents sea ice abrupt loss, and the Labrador sea subpolar gyre collapse.


Greenland ice sheet disintegration

The
Greenland ice sheet The Greenland ice sheet ( da, Grønlands indlandsis, kl, Sermersuaq) is a vast body of ice covering , roughly near 80% of the surface of Greenland. It is sometimes referred to as an ice cap, or under the term ''inland ice'', or its Danish equi ...
is the second largest ice sheet in the world, and is three times the size of the American state of
Texas Texas (, ; Spanish: ''Texas'', ''Tejas'') is a state in the South Central region of the United States. At 268,596 square miles (695,662 km2), and with more than 29.1 million residents in 2020, it is the second-largest U.S. state by ...
. It holds enough water, that if completely melted, could raise sea levels globally by 7.2 metres. Due to global warming, the ice sheet is melting at an accelerating rate, adding almost 1 mm to global sea levels every year. Around half of the ice loss occurs via surface melting, and the remainder occurs at the base of the ice sheet where the ice sheet touches the sea, by calving (breaking off) icebergs from its margins. The Greenland ice sheet has a tipping point because of the melt-elevation
feedback Feedback occurs when outputs of a system are routed back as inputs as part of a chain of cause-and-effect that forms a circuit or loop. The system can then be said to ''feed back'' into itself. The notion of cause-and-effect has to be handled ...
. Surface melting reduces the height of the ice sheet. As air at a lower altitude is warmer, the ice sheet is then exposed to warmer temperatures, accelerating the melt. A 2021 analysis of sub-glacial sediment at the bottom of a 1.4 km Greenland ice core finds that the Greenland ice sheet melted away at least once during the last million years, and therefore strongly suggests that its tipping point is below the 2.5 °C maximum positive temperature excursion over that period. There is some evidence that the Greenland ice sheet is losing stability, and getting close to a tipping point.


West Antarctic ice sheet disintegration

The West Antarctic Ice Sheet (WAIS) is a large ice sheet in Antarctica; in places more than 4 kilometres thick. It sits on
bedrock In geology, bedrock is solid rock that lies under loose material ( regolith) within the crust of Earth or another terrestrial planet. Definition Bedrock is the solid rock that underlies looser surface material. An exposed portion of be ...
mostly below sea level. As such, it is in contact with the heat from the ocean which makes it vulnerable to fast and irreversible ice loss. A tipping point could be reached if thinning or collapse of the WAIS's ice shelves triggers a feedback loop that leads to rapid and irreversible loss of its ice into the ocean. If completely melted, the ice sheet would contribute around 3.3 metres of sea level rise. Ice loss from the WAIS is accelerating. The paleo record suggests that during the past few hundred thousand years, the WAIS largely disappeared in response to similar levels of warming and emission scenarios projected for the next few centuries. Like with the other ice sheets, there is a counteracting negative feedback - greater warming also intensifies the
effects of climate change on the water cycle The effects of climate change on the water cycle are profound and have been described as an "intensification" or an overall "strengthening" of the water cycle (also called hydrologic cycle).Douville, H., K. Raghavan, J. Renwick, R.P. Allan, P.A. ...
, which result in an increased
precipitation In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail. ...
over the ice sheet in the form of
snow Snow comprises individual ice crystals that grow while suspended in the atmosphere—usually within clouds—and then fall, accumulating on the ground where they undergo further changes. It consists of frozen crystalline water throughou ...
during the winter, which would freeze on the surface, and this increase in the surface mass balance (SMB) counteracts some fraction of the ice loss. In the
IPCC Fifth Assessment Report The Fifth Assessment Report (AR5) of the United Nations Intergovernmental Panel on Climate Change (IPCC) is the fifth in a series of such reports and was completed in 2014.IPCC (2014The IPCC’s Fifth Assessment Report (AR5) leaflet/ref> As ha ...
, it was suggested that this effect could potentially overpower increased ice loss under the higher levels of warming and result in small net ice gain, but by the time of the IPCC Sixth Assessment Report, improved modelling had proven that the glacier breakup would consistently accelerate at a faster rate.


North Subpolar Gyre


East Antarctic ice sheet disintegration

East Antarctic ice sheet is the largest and thickest ice sheet on Earth, with the maximum thickness of 4,800 m. A complete disintegration would raise the global sea levels by 53.3 m, but this may not occur until global warming of 10 degrees Celsius, while the loss of two-thirds of its volume may require at least 6 degrees of warming to trigger. Its melt would also occur over a longer timescale than the loss of any other ice on the planet, taking no less than 10,000 years to finish. However, the
subglacial basin A subglacial lake is a lake that is found under a glacier, typically beneath an ice cap or ice sheet. Subglacial lakes form at the boundary between ice and the underlying bedrock, where gravitational pressure decreases the pressure melting point o ...
portions of the East Antarctic ice sheet may be vulnerable to tipping at lower levels of warming. The Wilkes Basin is of particular concern, as it holds enough ice to raise sea levels by about 3 to 4 metres.


Amazon rainforest dieback

The
Amazon rainforest The Amazon rainforest, Amazon jungle or ; es, Selva amazónica, , or usually ; french: Forêt amazonienne; nl, Amazoneregenwoud. In English, the names are sometimes capitalized further, as Amazon Rainforest, Amazon Forest, or Amazon Jungle. ...
is the largest
tropical rainforest Tropical rainforests are rainforests that occur in areas of tropical rainforest climate in which there is no dry season – all months have an average precipitation of at least 60 mm – and may also be referred to as ''lowland equator ...
in the world. It is twice as big as India and spans nine countries in South America. It produces around half of its own rainfall by recycling moisture through evaporation and
transpiration Transpiration is the process of water movement through a plant and its evaporation from aerial parts, such as leaves, stems and flowers. Water is necessary for plants but only a small amount of water taken up by the roots is used for growth ...
as air moves across the forest. When forest is lost via climate change (droughts and fires) or
deforestation Deforestation or forest clearance is the removal of a forest or stand of trees from land that is then converted to non-forest use. Deforestation can involve conversion of forest land to farms, ranches, or urban use. The most concentrated ...
, there will be less rain and more trees will die. Eventually, large parts of the rainforest may die off and transform into a dry
savanna A savanna or savannah is a mixed woodland- grassland (i.e. grassy woodland) ecosystem characterised by the trees being sufficiently widely spaced so that the canopy does not close. The open canopy allows sufficient light to reach the ground ...
landscape. In 2022, a study reported that the rainforest has been losing resilience since the early 2000s. Resiliency is measured by recovery-time from short-term perturbations. This delayed return to equilibrium of the rainforest is termed ''critical slowing down''. The observed loss of resilience reinforces the theory that the rainforest is approaching a
critical transition Critical transitions are abrupt shifts in the state of ecosystems, the climate, financial systems or other complex dynamical systems that may occur when changing conditions pass a critical or bifurcation point. As such, they are a particular type ...
.


Permafrost thaw

Perennially frozen ground, or
permafrost Permafrost is ground that continuously remains below 0 °C (32 °F) for two or more years, located on land or under the ocean. Most common in the Northern Hemisphere, around 15% of the Northern Hemisphere or 11% of the global surface ...
, covers large fractions of land – mainly in
Siberia Siberia ( ; rus, Сибирь, r=Sibir', p=sʲɪˈbʲirʲ, a=Ru-Сибирь.ogg) is an extensive geographical region, constituting all of North Asia, from the Ural Mountains in the west to the Pacific Ocean in the east. It has been a part ...
,
Alaska Alaska ( ; russian: Аляска, Alyaska; ale, Alax̂sxax̂; ; ems, Alas'kaaq; Yup'ik: ''Alaskaq''; tli, Anáaski) is a state located in the Western United States on the northwest extremity of North America. A semi-exclave of the U ...
,
northern Canada Northern Canada, colloquially the North or the Territories, is the vast northernmost region of Canada variously defined by geography and politics. Politically, the term refers to the three territories of Canada: Yukon, Northwest Territories an ...
and the
Tibetan plateau The Tibetan Plateau (, also known as the Qinghai–Tibet Plateau or the Qing–Zang Plateau () or as the Himalayan Plateau in India, is a vast elevated plateau located at the intersection of Central, South and East Asia covering most of the Ti ...
– and can be up to a kilometre thick. Subsea permafrost up to 100 metres thick also occurs on the sea floor under part of the Arctic Ocean. This frozen ground holds vast amounts of carbon from plants and animals that died and decomposed over thousands of years. Scientists believe there is nearly twice as much carbon in permafrost than is present in Earth's atmosphere. As the climate warms and the permafrost begins to thaw, carbon dioxide and methane are released into the atmosphere. With higher temperatures, microbes become active and decompose the biological material in the permafrost. This could happen rapidly, or over longer timespans, and the loss would be irreversible. Because and
methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane ...
are both greenhouse gases, they act as a self-reinforcing feedback on permafrost melt.


Atlantic Meridional Overturning Circulation

The Atlantic Meridional Overturning Circulation (AMOC), also known as the Gulf Stream System, is a large system of
ocean current An ocean current is a continuous, directed movement of sea water generated by a number of forces acting upon the water, including wind, the Coriolis effect, breaking waves, cabbeling, and temperature and salinity differences. Depth conto ...
s. It is driven by differences in the density of water; colder and more salty water is heavier than warmer fresh water. The AMOC acts as a conveyor belt, sending warm surface water from the tropics north, and carrying cold fresh water back south. As warm water flows northwards, some evaporates which increases salinity. It also cools when it is exposed to cooler air. Cold, salty water is more dense and slowly begins to sink. Several kilometres below the surface, cold, dense water begins to move south. Increased rainfall and the melting of ice due to global warming dilutes the salty surface water, and warming further decreases its density. The lighter water is less able to sink, slowing down the circulation. Theory, simplified models, and reconstructions of abrupt changes in the past suggest the AMOC has a tipping point. If freshwater input from melting
glacier A glacier (; ) is a persistent body of dense ice that is constantly moving under its own weight. A glacier forms where the accumulation of snow exceeds its ablation over many years, often centuries. It acquires distinguishing features, such a ...
s reaches a certain threshold, it could collapse into a state of reduced flow. Even after melting stops, the AMOC may not return to its current state. It is unlikely that the AMOC will tip in the 21st century, but it may do so before 2300 if
greenhouse gas emissions Greenhouse gas emissions from human activities strengthen the greenhouse effect, contributing to climate change. Most is carbon dioxide from burning fossil fuels: coal, oil, and natural gas. The largest emitters include coal in China and ...
are very high. A weakening of 24% to 39% is expected depending on greenhouse emissions, even without tipping behaviour. If the AMOC does shut down, a new stable state could emerge that lasts for thousands of years, possibly triggering other tipping points. In 2021, a study which used a "primitive" finite-difference ocean model estimated that AMOC collapse could be invoked by a sufficiently fast increase in ice melt even if it never reached the common thresholds for tipping obtained from slower change. Thus, it implied that the AMOC collapse is more likely than what is usually estimated by the complex and large-scale climate models. Another 2021 study found early-warning signals in a set of AMOC indices, suggesting that the AMOC may be close to tipping. However, it was contradicted by another study published in the same journal the following year, which found a "largely stable" AMOC which had so far not been affected by climate change beyond its own natural variability. Two more studies published in 2022 have also suggested that the modelling approaches commonly used to evaluate AMOC appear to overestimate the risk of its collapse.


Arctic sea ice

Arctic sea ice The Arctic ice pack is the sea ice cover of the Arctic Ocean and its vicinity. The Arctic ice pack undergoes a regular seasonal cycle in which ice melts in spring and summer, reaches a minimum around mid-September, then increases during fall a ...
was once identified as a potential tipping element. The loss of sunlight-reflecting sea ice during summer exposes the (dark) ocean, which would warm. Arctic sea ice cover is likely to melt entirely under even relatively low levels of warming, and it was hypothesized that this could eventually transfer enough heat to the ocean to prevent sea ice recovery even if the global warming is reversed. Modelling now shows that this heat transfer during the
Arctic summer The midnight sun is a natural phenomenon that occurs in the summer months in places north of the Arctic Circle or south of the Antarctic Circle, when the Sun remains visible at the local midnight. When the midnight sun is seen in the Arctic, th ...
does not overcome the cooling and the formation of new ice during the
Arctic winter The polar night is a phenomenon where the nighttime lasts for more than 24 hours that occurs in the northernmost and southernmost regions of Earth. This occurs only inside the polar circles. The opposite phenomenon, the polar day, or midnig ...
. As such, the loss of Arctic ice during the summer is not a tipping point for as long as the Arctic winter remains cool enough to enable the formation of new Arctic sea ice. However, if the higher levels of warming prevent the formation of new Arctic ice even during winter, then this change may become irreversible. Consequently, Arctic Winter Sea Ice is included as a potential tipping point in a 2022 assessment. Additionally, the same assessment argued that while the rest of the ice in the Arctic Ocean may recover from a total summertime loss during the winter, ice cover in the
Barents Sea The Barents Sea ( , also ; no, Barentshavet, ; russian: Баренцево море, Barentsevo More) is a marginal sea of the Arctic Ocean, located off the northern coasts of Norway and Russia and divided between Norwegian and Russian terr ...
may not reform during the winter even below 2 degrees of warming. This is because the Barents Sea is already the fastest-warming part of the Arctic: in 2021-2022 it was found that while the warming within the
Arctic Circle The Arctic Circle is one of the two polar circles, and the most northerly of the five major circles of latitude as shown on maps of Earth. Its southern equivalent is the Antarctic Circle. The Arctic Circle marks the southernmost latitude at ...
has already been nearly four times faster than the global average since 1979, Barents Sea warmed up to seven times faster than the global average. This tipping point matters because of the decade-long history of research into the connections between the state of Barents- Kara Sea ice and the weather patterns elsewhere in Eurasia.


Coral reef die-off

Around 500 million people around the world depend on
coral reef A coral reef is an underwater ecosystem characterized by reef-building corals. Reefs are formed of Colony (biology), colonies of coral polyp (zoology), polyps held together by calcium carbonate. Most coral reefs are built from stony corals, wh ...
s for food, income, tourism and coastal protection. Since the 1980s, this is being threatened by the increase in
sea surface temperature Sea surface temperature (SST), or ocean surface temperature, is the ocean temperature close to the surface. The exact meaning of ''surface'' varies according to the measurement method used, but it is between and below the sea surface. Air mas ...
s which is triggering mass bleaching of coral, especially in sub-tropical regions. A sustained ocean temperature spike of 1 °C above average is enough to cause bleaching. Under heat stress, corals expel the small colourful
algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from unicellular micr ...
which live in their tissues, which causes them to turn white. The algae, known as
zooxanthellae Zooxanthellae is a colloquial term for single-celled dinoflagellates that are able to live in symbiosis with diverse marine invertebrates including demosponges, corals, jellyfish, and nudibranchs. Most known zooxanthellae are in the genus ''S ...
, have a
symbiotic Symbiosis (from Greek , , "living together", from , , "together", and , bíōsis, "living") is any type of a close and long-term biological interaction between two different biological organisms, be it mutualistic, commensalistic, or para ...
relationship with coral such that without them, the corals slowly die. After these zooxanthellae have disappeared, the corals are vulnerable to a transition towards a
seaweed Seaweed, or macroalgae, refers to thousands of species of macroscopic, multicellular, marine algae. The term includes some types of '' Rhodophyta'' (red), ''Phaeophyta'' (brown) and '' Chlorophyta'' (green) macroalgae. Seaweed species such as ...
-dominated
ecosystem An ecosystem (or ecological system) consists of all the organisms and the physical environment with which they interact. These biotic and abiotic components are linked together through nutrient cycles and energy flows. Energy enters the syst ...
, making it very difficult to shift back to a coral-dominated ecosystem. The
IPCC The Intergovernmental Panel on Climate Change (IPCC) is an intergovernmental body of the United Nations. Its job is to advance scientific knowledge about climate change caused by human activities. The World Meteorological Organization (WMO) ...
estimates that by the time temperatures have risen to 1.5 °C above pre-industrial times, between 70% and 90% of coral reefs that exist today will disappear; and that if the world warms by 2 °C, they will become extremely rare.


Mountain glaciers

Mountain
glacier A glacier (; ) is a persistent body of dense ice that is constantly moving under its own weight. A glacier forms where the accumulation of snow exceeds its ablation over many years, often centuries. It acquires distinguishing features, such a ...
s are the largest repository of land-bound ice after the Greenland and the Antarctica ice sheets, and they are also undergoing melting as the result of climate change. A glacier tipping point is when it enters a disequilibrium state with the climate and will melt away unless the temperatures go down. Examples include glaciers of the North
Cascade Range The Cascade Range or Cascades is a major mountain range of western North America, extending from southern British Columbia through Washington and Oregon to Northern California. It includes both non-volcanic mountains, such as the North Cascades, ...
, where even in 2005 67% of the glaciers observed were in disequilibrium and will not survive the continuation of the present climate, or the French
Alps The Alps () ; german: Alpen ; it, Alpi ; rm, Alps ; sl, Alpe . are the highest and most extensive mountain range system that lies entirely in Europe, stretching approximately across seven Alpine countries (from west to east): France, Sw ...
, where The Argentière and Mer de Glace glaciers are expected to disappear completely by end of the 21st century if current climate trends persist. The absolute largest amount of glacier ice is located in the
Hindu Kush The Hindu Kush is an mountain range in Central and South Asia to the west of the Himalayas. It stretches from central and western Afghanistan, Quote: "The Hindu Kush mountains run along the Afghan border with the North-West Frontier Province ...
Himalaya The Himalayas, or Himalaya (; ; ), is a mountain range in Asia, separating the plains of the Indian subcontinent from the Tibetan Plateau. The range has some of the planet's highest peaks, including the very highest, Mount Everest. Over 10 ...
region, which is colloquially known as the Earth's "Third Pole" as the result. It is believed that one third of that ice will be lost by 2100 even if the warming is limited to 1.5 °C, while the "intermediate" and "severe" climate change scenarios ( RCP 4.5 and 8.5) are likely to lead to the losses of 50% and >67% of the region's glaciers over the same timeframe. Glacier melt is projected to accelerate regional river flows until the amount of meltwater peaks around 2060, going into an irreversible decline afterwards. Since regional precipitation will continue to increase even as the glacier meltwater contribution declines, annual river flows are only expected to diminish in the western basins where contribution from the
monsoon A monsoon () is traditionally a seasonal reversing wind accompanied by corresponding changes in precipitation but is now used to describe seasonal changes in atmospheric circulation and precipitation associated with annual latitudinal oscil ...
is low: however,
irrigation Irrigation (also referred to as watering) is the practice of applying controlled amounts of water to land to help grow crops, landscape plants, and lawns. Irrigation has been a key aspect of agriculture for over 5,000 years and has been devel ...
and
hydropower Hydropower (from el, ὕδωρ, "water"), also known as water power, is the use of falling or fast-running water to produce electricity or to power machines. This is achieved by converting the gravitational potential or kinetic energy of ...
generation would still have to adjust to greater interannual variability and lower pre-monsoon flows in all of the region's rivers.


Sahel greening


Boreal forest biome shift

During the last quarter of the twentieth century, the zone of latitude occupied by
taiga Taiga (; rus, тайга́, p=tɐjˈɡa; relates to Mongolic and Turkic languages), generally referred to in North America as a boreal forest or snow forest, is a biome characterized by coniferous forests consisting mostly of pines, sp ...
experienced some of the greatest temperature increases on Earth. Winter temperatures have increased more than summer temperatures. In summer, the daily low temperature has increased more than the daily high temperature. It has been hypothesized that the boreal environments have only a few states which are stable in the long term - a treeless
tundra In physical geography, tundra () is a type of biome where tree growth is hindered by frigid temperatures and short growing seasons. The term ''tundra'' comes through Russian (') from the Kildin Sámi word (') meaning "uplands", "treeless mou ...
/
steppe In physical geography, a steppe () is an ecoregion characterized by grassland plains without trees apart from those near rivers and lakes. Steppe biomes may include: * the montane grasslands and shrublands biome * the temperate gras ...
, a forest with >75% tree cover and an open
woodland A woodland () is, in the broad sense, land covered with trees, or in a narrow sense, synonymous with wood (or in the U.S., the '' plurale tantum'' woods), a low-density forest forming open habitats with plenty of sunlight and limited shade (see ...
with ~20% and ~45% tree cover. Thus, continued climate change would be able to force at least some of the presently existing taiga forests into one of the two woodland states or even into a treeless steppe - but it could also shift tundra areas into woodland or forest states as they warm and become more suitable for tree growth. These trends were first detected in the Canadian boreal forests in the early 2010s, and summer warming had also been shown to increase water stress and reduce tree growth in dry areas of the southern boreal forest in central Alaska and portions of far eastern Russia. In Siberia, the taiga is converting from predominantly needle-shedding larch trees to evergreen conifers in response to a warming climate. Subsequent research in Canada found that even in the forests where biomass trends did not change, there was a substantial shift towards the deciduous broad-leaved trees with higher drought tolerance over the past 65 years, and a
Landsat The Landsat program is the longest-running enterprise for acquisition of satellite imagery of Earth. It is a joint NASA / USGS program. On 23 July 1972, the Earth Resources Technology Satellite was launched. This was eventually renamed to La ...
analysis of 100,000 undisturbed sites found that the areas with low tree cover became greener in response to warming, but tree mortality (browning) became the dominant response as the proportion of existing tree cover increased. A 2018 study of the seven tree species dominant in the Eastern Canadian forests found that while 2 °C warming alone increases their growth by around 13% on average, water availability is much more important than temperature and further warming of up to 4 °C would result in substantial declines unless matched by increases in precipitation. A 2021 paper had confirmed that the boreal forests are much more strongly affected by climate change than the other forest types in Canada and projected that most of the eastern Canadian boreal forests would reach a tipping point around 2080 under the RCP 8.5 scenario, which represents the largest potential increase in anthropogenic emissions. Another 2021 study projected that under the "moderate" SSP2-4.5 scenario, boreal forests would experience a 15% worldwide increase in biomass by the end of the century, but this would be more than offset by the 41% biomass decline in the tropics. In 2022, the results of a 5-year warming experiment in North America had shown that the juveniles of tree species which currently dominate the southern margins of the boreal forests fare the worst in response to even 1.5°C or +3.1 °C of warming and the associated reductions in precipitation. While the temperate species which would benefit from such conditions are also present in the southern boreal forests, they are both rare and have slower growth rates.


Equatorial stratocumulus clouds


Formerly considered tipping elements

The possibility that the El Niño–Southern Oscillation (ENSO) is a tipping element had attracted attention in the past. Normally strong winds blow west across the South Pacific Ocean from South America to Australia. Every two to seven years, the winds weaken due to pressure changes and the air and water in the middle of the Pacific warms up, causing changes in wind movement patterns around the globe. This known as El Niño and typically leads to droughts in India, Indonesia and Brazil, and increased flooding in Peru. In 2015/2016, this caused food shortages affecting over 60 million people. El Niño-induced droughts may increase the likelihood of forest fires in the Amazon. The threshold for tipping was estimated to be between 3.5 and 7 °C of global warming in 2016. After tipping, the system would be in a more permanent El Niño state, rather than oscillating between different states. This has happened in Earth's past, in the
Pliocene The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58 So far, there is no definitive evidence indicating changes in ENSO behaviour, and the IPCC Sixth Assessment Report concluded that it is "virtually certain that the ENSO will remain the dominant mode of interannual variability in a warmer world." Consequently, the 2022 assessment no longer includes it in the list of likely tipping elements. The Indian summer monsoon is another part of the climate system which was considered suspectible to irreversible collapse in the earlier research. However, more recent research has demonstrated that warming tends to strengthen the Indian monsoon, and it is projected to strengthen in the future.
Methane hydrate Methane clathrate (CH4·5.75H2O) or (8CH4·46H2O), also called methane hydrate, hydromethane, methane ice, fire ice, natural gas hydrate, or gas hydrate, is a solid clathrate compound (more specifically, a clathrate hydrate) in which a large amou ...
deposits in the Arctic were once thought to be vulnerable to a rapid dissociation which would have a large impact on global temperatures, in a dramatic scenario known as a clathrate gun hypothesis. Later research found that it takes millennia for methane hydrates to respond to warming, while methane emissions from the seafloor rarely transfer from the water column into the atmosphere. IPCC Sixth Assessment Report states "It is very unlikely that gas clathrates (mostly methane) in deeper terrestrial permafrost and subsea clathrates will lead to a detectable departure from the emissions trajectory during this century".


Mathematical theory

Tipping point behaviour in the climate can be described in mathematical terms. Three types of tipping points have been identified— bifurcation,
noise Noise is unwanted sound considered unpleasant, loud or disruptive to hearing. From a physics standpoint, there is no distinction between noise and desired sound, as both are vibrations through a medium, such as air or water. The difference aris ...
-induced and rate-dependent.


Bifurcation-induced tipping

Bifurcation-induced tipping happens when a particular parameter in the climate (for instance a change in environmental conditions or forcing), passes a critical level – at which point a bifurcation takes place – and what was a stable state loses its stability or simply disappears. The Atlantic Meridional Overturning Circulation (AMOC) is an example of a tipping element that can show bifurcation-induced tipping. Slow changes to the bifurcation parameters in this system – the salinity and temperature of the water – may push the circulation towards collapse. Many types of bifurcations show
hysteresis Hysteresis is the dependence of the state of a system on its history. For example, a magnet may have more than one possible magnetic moment in a given magnetic field, depending on how the field changed in the past. Plots of a single component of ...
, which is the dependence of the state of a system on its history. For instance, depending on how warm it was in the past, there can be differing amounts of ice on the poles at the same concentration of greenhouse gases or temperature.


Early warning signals

For tipping points that occur because of a bifurcation, it may be possible to detect whether a system is getting closer to a tipping point, as it becomes less resilient to perturbations on approach of the tipping threshold. These systems display
critical slowing down Critical or Critically may refer to: *Critical, or critical but stable, medical states **Critical, or intensive care medicine * Critical juncture, a discontinuous change studied in the social sciences. *Critical Software, a company specializing i ...
, with an increased memory (rising
autocorrelation Autocorrelation, sometimes known as serial correlation in the discrete time case, is the correlation of a signal with a delayed copy of itself as a function of delay. Informally, it is the similarity between observations of a random variable ...
) and
variance In probability theory and statistics, variance is the expectation of the squared deviation of a random variable from its population mean or sample mean. Variance is a measure of dispersion, meaning it is a measure of how far a set of numbe ...
. Depending on the nature of the tipping system, there may be other types of early warning signals. Abrupt change is not an early warning signal (EWS) for tipping points, as abrupt change can also occur if the changes are reversible to the control parameter. These EWSs are often developed and tested using time series from the paleo record, like sediments, ice caps, and tree rings, where past examples of tipping can be observed. It is not always possible to say whether increased variance and autocorrelation is a precursor to tipping, or caused by internal variability, for instance in the case of the collapse of the AMOC. Quality limitations of paleodata further complicate the development of EWSs. They have been developed for detecting tipping due to drought in forests in California, and melting of the Pine Island Glacier in West Antarctica, among other systems. Using early warning signals (increased autocorrelation and variance of the melt rate time series), it has been suggested that the Greenland ice sheet is currently losing resilience, consistent with modelled early warning signals of the ice sheet. Human-induced changes in the climate system may be too fast for early warning signals to become evident, especially in systems with inertia.


Noise-induced tipping

Noise-induced tipping is the transition from one state to another due to random fluctuations or internal variability of the system. Noise-induced transitions do not show any of the early warning signals which occur with bifurcations. This means they are unpredictable because the underlying
potential Potential generally refers to a currently unrealized ability. The term is used in a wide variety of fields, from physics to the social sciences to indicate things that are in a state where they are able to change in ways ranging from the simple r ...
does not change. Because they are unpredictable, such occurrences are often described as a "one-in-x-year" event. An example is the Dansgaard–Oeschger events during the last
ice age An ice age is a long period of reduction in the temperature of Earth's surface and atmosphere, resulting in the presence or expansion of continental and polar ice sheets and alpine glaciers. Earth's climate alternates between ice ages and gre ...
, with 25 occurrences of sudden climate fluctuations over a 500 year period.


Rate-induced tipping

Rate-induced tipping occurs when a change in the environment is faster than the force that restores the system to its stable state. In peatlands, for instance, after years of relative stability, rate-induced tipping can lead to an " explosive release of soil carbon from peatlands into the atmosphere" – sometimes known as "compost bomb instability". The AMOC may also show rate-induced tipping: if the rate of ice melt increases too fast, it may collapse, even before the ice melt reaches the critical value where the system would undergo a bifurcation.


Cascading tipping points

Crossing a threshold in one part of the climate system may trigger another tipping element to tip into a new state. Such sequences of thresholds are called cascading tipping points, an example of a domino effect. Ice loss in West Antarctica and Greenland will significantly alter ocean circulation. Sustained warming of the northern high latitudes as a result of this process could activate tipping elements in that region, such as permafrost degradation, and boreal forest dieback. Thawing permafrost is a threat multiplier because it holds roughly twice as much carbon as the amount currently circulating in the atmosphere. Loss of ice in Greenland likely destabilises the West Antarctic ice sheet via sea level rise, and vice-versa, especially if Greenland were to melt first as West Antarctica is particularly vulnerable to contact with warm sea water. A 2021 study with three million computer simulations of a climate model showed that nearly one-third of those simulations resulted in domino effects, even when temperature increases were limited to 2 °C – the upper limit set by the
Paris Agreement The Paris Agreement (french: Accord de Paris), often referred to as the Paris Accords or the Paris Climate Accords, is an international treaty on climate change. Adopted in 2015, the agreement covers climate change mitigation, adaptation, and ...
in 2015. The authors of the study said that the science of tipping points is so complex that there is great uncertainty as to how they might unfold, but nevertheless, argued that the possibility of cascading tipping points represents "an existential threat to civilisation".


Impacts

Tipping points can have very severe impacts. They can exacerbate current dangerous impacts of climate change, or give rise to new impacts. Some potential tipping points would take place abruptly, such as disruptions to the Indian monsoon, with severe impacts on
food security Food security speaks to the availability of food in a country (or geography) and the ability of individuals within that country (geography) to access, afford, and source adequate foodstuffs. According to the United Nations' Committee on World ...
for hundred of millions. Other impacts would likely take place over longer timescales, such as the melt of the ice caps. The 10 m of sea level rise from the combined melt of Greenland and West Antarctica would require moving many cities inland. A collapse of the Atlantic Overturning Circulation would alter Europe radically, and lead to a metre of sea level rise in the North Atlantic. These impacts could happen simultaneously in the case of cascading tipping points. A review of abrupt changes over the last 30,000 years showed that tipping points can lead to a large set of cascading impacts in climate, ecological and social systems. For instance, the abrupt termination of the African humid period cascaded, and
desertification Desertification is a type of land degradation in drylands in which biological productivity is lost due to natural processes or induced by human activities whereby fertile areas become increasingly arid. It is the spread of arid areas caused ...
and regime shifts led to the retreat of pastoral societies in North Africa and a change of dynasty in Egypt. A 2021 meta analysis on the potential economic impact of tipping points found that they raise global risk; the medium estimate was that they increase the social cost of carbon by about 25%, with a 10% chance of tipping points more than doubling it. The social cost of carbon reflects the economic damage from carbon emissions. It included a detailed table of the estimated additional impact from each tipping point per every tonne of carbon under the "intermediate" Representative Concentration Pathway 4.5 Since it was written before the publication of the IPCC Sixth Assessment Report and the 2022 assessment of tipping points, it includes some tipping points they no longer consider plausible, like methane hydrates and the Indian summer monsoon, and excludes the more recently identified ones like the Labrador-Irminger circulation. In 2022, this assessment was severely criticized by a group of scientists including Steve Keen and Timothy Lenton, who considered those values, as well as the paper's suggestion that tipping points activated at 6 °C of global warming would reduce global per capita consumption by around 1.4%, to be an enormous underestimate. The authors of the assessment responded to this criticism by noting that their assessment should be treated as the starting point in economic assessment of tipping points rather than the final word, and since most of the literature included in their meta-analysis lacks the ability to estimate nonmarket climate damages, their numbers are likely to be underestimates. They have also noted that if climate change is assumed to impact the
economic growth Economic growth can be defined as the increase or improvement in the inflation-adjusted market value of the goods and services produced by an economy in a financial year. Statisticians conventionally measure such growth as the percent rate o ...
rate rather than the base level of economic activity (which they consider unlikely, but still a possibility) then the social cost of carbon becomes nearly 100 times larger, while the impact of tipping points becomes around 3 times larger.


Runaway greenhouse effect

A runaway greenhouse effect is a tipping point so extreme that oceans evaporate and the water vapour escapes to space, an irreversible
climate state Throughout Earth's climate history (Paleoclimate) its climate has fluctuated between two primary states: greenhouse and icehouse Earth. Both climate states last for millions of years and should not be confused with glacial and interglacial period ...
that happened on
Venus Venus is the second planet from the Sun. It is sometimes called Earth's "sister" or "twin" planet as it is almost as large and has a similar composition. As an interior planet to Earth, Venus (like Mercury) appears in Earth's sky never f ...
. A runaway greenhouse effect has virtually no chance of being caused by people. Venus-like conditions on the Earth require a large long-term forcing that is unlikely to occur until the sun brightens by a few tens of percents, which will take a few billion years.


See also

* Greenhouse and icehouse Earth *
Climate sensitivity Climate sensitivity is a measure of how much Earth's surface will cool or warm after a specified factor causes a change in its climate system, such as how much it will warm for a doubling in the atmospheric carbon dioxide () concentration. In te ...
*
Planetary boundaries Planetary boundaries is a concept highlighting human-caused perturbations of Earth systems making them relevant in a way not accommodated by the environmental boundaries separating the three ages within the Holocene epoch. Crossing a planetary ...
* World Scientists' Warning to Humanity


References


External links


Global Systems Institute
Tipping points research at Exeter University
The climate tipping points
2022 BBC radio documentary {{DEFAULTSORT:Tipping Point (Climatology) Climate change feedbacks Climatology Politics of climate change