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 the much smaller glacial and interglacial periods, which occur as alternating phases within an icehouse period (known as an ice age) and tend to last less than one million years. There are five known icehouse periods in Earth's climate history, namely the Huronian, Cryogenian, Andean-Saharan (also known as Early Paleozoic), Late Paleozoic and Late Cenozoic glaciations.
The main factors involved in changes of the paleoclimate are believed to be the concentration of atmospheric greenhouse gases such as carbon dioxide (CO2) and less importantly methane (CH4), changes in Earth's orbit, long-term changes in the solar constant, and oceanic and orogenic changes from tectonic plate dynamics. Greenhouse and icehouse periods have played key roles in the evolution of life on Earth by directly and indirectly forcing biotic adaptation and turnover at various spatial scales across time.
Timeline of the five known great icehouse periods, shown in blue. The periods in between depict greenhouse conditions.
Greenhouse Earth
A "greenhouse Earth" is a period during which no continental glaciers exist anywhere on the planet. Additionally, the levels of carbon dioxide and other greenhouse gases (such as water vapor and methane) are high, and sea surface temperatures (SSTs) range from 28 °C (82.4 °F) in the tropics to 0 °C (32 °F) in the polar regions. Earth has been in a greenhouse state for about 85% of its history.
The state should not be confused with a hypothetical runaway greenhouse effect, which is an irreversible tipping point that corresponds to the ongoing runaway greenhouse effect on Venus. The IPCC
states that "a 'runaway greenhouse effect'—analogous to [that of]
Venus—appears to have virtually no chance of being induced by anthropogenic activities."
Causes
There are several theories as to how a greenhouse Earth can come
about. Geologic climate proxies indicate that there is a strong
correlation between a greenhouse state and high CO2 levels. However, it is important to recognize that high CO2
levels have traditionally been interpreted as feedback to Earth's
climate rather than as an independent driver, but geologic drivers of CO2 and climate change have been identified. Other phenomena have instead likely played a key role in influencing
global climate by altering oceanic and atmospheric currents and increasing the net amount of solar radiation absorbed by Earth's atmosphere. Such phenomena may include but are not limited to tectonic shifts that result in the release of greenhouse gases (such as CO2 and CH4) via volcanic activity. Volcanoes emit massive amounts of CO2
and methane into the atmosphere when they are active, which can trap
enough heat to cause a greenhouse effect. In a greenhouse Earth,
atmospheric concentrations of greenhouse gases like carbon dioxide (CO2) and methane (CH4)
are higher, trapping solar energy in the atmosphere via the greenhouse
effect. Methane, the main component of natural gas, is responsible for
more than a third of the current global warming. While it has an 80-fold higher global warming potential than CO2
in the 20 years after it has been introduced into the atmosphere (due
to a stronger greenhouse effect), it has a relatively short half life in the atmosphere of 10–20 years. An increase in the solar constant increases the net amount of solar energy absorbed into Earth's atmosphere, and changes in Earth's obliquity and eccentricity increase the net amount of solar radiation absorbed into Earth's atmosphere.
An illustration of ice age Earth at its glacial maximum
Earth is now in an icehouse state, and ice sheets are present in both poles simultaneously. Climatic proxies indicate that greenhouse gas concentrations tend to lower during an icehouse Earth. Similarly, global temperatures are also lower under icehouse conditions. Earth then fluctuates between glacial and interglacial periods, and the
size and the distribution of continental ice sheets fluctuate
dramatically. The fluctuation of the ice sheets results in changes in regional
climatic conditions that affect the range and the distribution of many
terrestrial and oceanic species.
On scales ranging from thousands to hundreds of millions of
years, the Earth's climate has transitioned from warm to chilly
intervals within life-sustaining ranges. There have been three periods
of glaciation in the Phanerozoic Eon (Ordovician, Carboniferous, and Cenozoic),
each lasting tens of millions of years and bringing ice down to sea
level at mid-latitudes. During these frigid "icehouse" intervals, sea
levels were generally lower, CO2 levels in the atmosphere
were lower, net photosynthesis and carbon burial were lower, and oceanic
volcanism was lower than during the alternate "greenhouse" intervals.
Transitions from Phanerozoic icehouse to greenhouse intervals coincided
with biotic crises or catastrophic extinction events, indicating
complicated biosphere-hydrosphere feedbacks.
The glacial and interglacial periods tend to alternate in
accordance with solar and climatic oscillation until Earth eventually
returns to a greenhouse state.
Earth's current icehouse state is known as the Quaternary Ice Age and began approximately 2.58 million years ago. However, an ice sheet has existed in Antarctica for approximately 34 million years. Earth is now in a clement interglacial period that started approximately 11,800 years ago. Earth will likely phase into another interglacial period such as the Eemian,
which occurred between 130,000 and 115,000 years ago, during which
evidence of forest in North Cape, Norway, and hippopotamus in the Rhine
and Thames Rivers can be observed. Earth is expected to continue to transition between glacial and
interglacial periods until the cessation of the Quaternary Ice Age and
will then enter another greenhouse state.
Causes
It is well established that there is strong correlation between low CO2 levels and an icehouse state, with evidence that the balance between geologic CO2 sources, weathering and carbon burial is responsible for secular transitions from Greenhouse and Icehouse climates and a clear role of ocean carbon storage changes in driving most of Quaternary CO2 and climate change. However, other factors such as solar, orbital and plate-tectonic forcing also influence climate and the global carbon cycle.
Potential drivers of previous icehouse states include the
movement of the tectonic plates and the opening and the closing of
oceanic gateways. They seem to play a crucial part in driving Earth into an icehouse
state, as tectonic shifts result in the transportation of cool, deep
water, which circulates to the ocean surface and assists in ice sheet
development at the poles. Examples of oceanic current shifts as a result of tectonic plate dynamics include the opening of the Tasmanian Gateway 36.5 million years ago, which separated Australia and Antarctica, and the opening of the Drake Passage 32.8 million years ago by the separation of South America and Antarctica, both of which are believed to have allowed for the development of the Antarctic ice sheet. The closing of the Isthmus of Panama and of the Indonesian seaway approximately 3 to 4 million years ago may also be a contributor to Earth's current icehouse state.
One proposed driver of the Ordovician Ice Age was the evolution of land plants. Under that paradigm, the rapid increase in photosynthetic biomass gradually removed CO2 from the atmosphere and replaced it with increasing levels of O2, which induced global cooling.
A proposed driver of the Quaternary Ice age is the collision of the Indian subcontinent with Eurasia to form the Himalayas and the Tibetan Plateau. Under that paradigm, the resulting continental uplift revealed massive quantities of unweathered silicate rock CaSiO 3, which reacted with CO2 to produce CaCO 3 (lime) and SiO 2 (silica). The CaCO 3
was eventually transported to the ocean and taken up by plankton, which
then died and sank to the bottom of the ocean, which effectively
removed CO2 from the atmosphere.
Within icehouse states are "glacial" and "interglacial"
periods, where ice sheets build up or retreat. During an icehouse
period, only about 20% of the time is spent in interglacial, or warmer
times. Model simulations suggest that the current interglacial climate state
will continue for at least another 100,000 years because of CO2 emissions, including the complete deglaciation of the Northern Hemisphere.
The main causes for glacial and interglacial periods are variations in the movement of Earth around the Sun. The astronomical components, discovered by the Serbian geophysicist Milutin Milanković and now known as Milankovitch cycles, include the axial tilt of Earth, the orbital eccentricity (or shape of the orbit), and the precession (or wobble) of Earth's rotation.
The tilt of the axis tends to fluctuate from 21.5° to 24.5° and back
every 41,000 years on the vertical axis. The change actually affects the
seasonality on Earth since a change in solar radiation
hits certain areas of the planet more often on a higher tilt, and a
lower tilt creates a more even set of seasons worldwide. The changes can
be seen in ice cores, which also contain evidence that during glacial
times (at the maximum extension of the ice sheets), the atmosphere had
lower levels of carbon dioxide (CO2). That may be caused by change in physical ocean circulation, biological productivity and acid-base chemistry.
A "Snowball Earth" is the complete opposite of greenhouse Earth in
which Earth's surface is completely frozen over. However, a snowball
Earth technically does not have continental ice sheets like during the
icehouse state. "The Great Infra-Cambrian Ice Age" has been claimed to be the host of such a world, and in 1964, the scientist W. Brian Harland brought forth his discovery of indications of glaciers in the low latitudes (Harland and Rudwick). That became a problem for Harland because of the thought of the "Runaway Snowball Paradox" (a kind of Snowball effect) that once Earth enters the route of becoming a snowball Earth, it would never be able to leave that state.
In 1992, geologist Joseph Kirschvink
brought up a solution to the paradox and how Earth could enter and exit
the snowball period. Since the continents were then huddled at the low
and the middle latitudes, there was less ocean water available to absorb
the higher amount solar energy hitting the tropics, and there was also
an increase in rainfall because more land exposed to higher solar energy
might have caused chemical weathering, which would contribute to
removal of CO2 from the atmosphere. Both conditions might have caused a substantial drop in CO2
atmospheric levels which resulted in cooling temperatures and
increasing ice albedo (ice reflectivity of incoming solar radiation),
which would further increase global cooling (a positive feedback). That
might have been the mechanism of entering Snowball Earth state.
Kirschvink explained that the way to get out of Snowball Earth
state could be connected again to carbon dioxide. A possible explanation
is that during Snowball Earth, volcanic activity would not halt but
accumulate atmospheric CO2. At the same time, global ice cover would prevent chemical weathering (particularly hydrolysis), responsible for removal of CO2 from the atmosphere. CO2 therefore accumulated in the atmosphere. Once the atmosphere accumulation of CO2
reached a threshold, temperature would rise enough for ice sheets to
start melting. That would in turn reduce the ice albedo effect, which
would in turn further reduce the ice cover and allow an exit from
Snowball Earth. At the end of Snowball Earth, before the equilibrium
"thermostat" between volcanic activity and the by then slowly resuming
chemical weathering was reinstated, CO2 in the atmosphere had
accumulated enough to cause temperatures to peak to as much as 60 °C,
thrusting the Earth into a brief moist greenhouse state.
Around the same geologic period of Snowball Earth (it is debated if it was the cause or the result of Snowball Earth), the Great Oxygenation Event (GOE) was occurring. The event known as the Cambrian Explosion
followed and produced the beginnings of populous bilateral organisms,
as well as a greater diversity and mobility in multicellular life.
Some biologists claim that a complete snowball Earth could not have happened since photosynthetic life would not have survived under many meters of ice without sunlight. However, sunlight has been observed to penetrate meters of ice in Antarctica. Most scientists now believe that a "hard" Snowball Earth, one completely covered by
ice, is probably impossible. However, a "slushball Earth", with points
of opening near the equator, is considered to be possible.
Recent studies may have again complicated the idea of a snowball
Earth. In October 2011, a team of French researchers announced that the
carbon dioxide during the last speculated "snowball Earth" may have been
lower than originally stated, which provides a challenge in finding out
how Earth got out of its state and whether a snowball or a slushball
Earth occurred.
Transitions
Causes
The Eocene, which occurred between 56.0 and 33.9 million years ago, was Earth's warmest temperature period for 100 million years. However, the "super-greenhouse" period had eventually become an
icehouse period by the late Eocene. It is believed that the decline of
CO2 caused the change, but mechanisms of positive feedback may have contributed to the cooling.
The best available record for a transition from an icehouse to greenhouse period in which plant life existed is for the Permianperiod,
which occurred around 300 million years ago. A major transition took
place over the subsequent 40 million years and caused Earth to change
from a moist, icy planet in which rainforests covered the tropics to a hot, dry, and windy location in which little could survive. Professor Isabel P. Montañez of University of California, Davis,
who has researched the time period, found the climate to be "highly
unstable" and to be "marked by dips and rises in carbon dioxide."
Impacts
The Eocene-Oligocene transition was the latest and occurred
approximately 34 million years ago. It resulted in a rapid global
cooling, the glaciation of Antarctica, and a series of biotic extinction
events. The most dramatic species turnover event associated with the
time period is the Grande Coupure, a period that saw the replacement of European tree-dwelling and leaf-eating mammal species by migratory species from Asia.
Research
Paleoclimatology
is a branch of science that attempts to understand the history of
greenhouse and icehouse conditions over geological time. The study of ice cores, dendrochronology, ocean and lakesediments (varve), palynology, (paleobotany), isotope analysis (such as radiometric dating and stable isotope analysis), and other climate proxies allows scientists to create models of Earth's past energy budgets and the resulting climate. One study has shown that atmospheric carbon dioxide levels during the Permian age rocked back and forth between 250 parts per million, which is close to today's levels, up to 2,000 parts per million. Studies on lake sediments suggest that the "hothouse" or "super-greenhouse" Eocene was in a "permanent El Niño state" after the 10 °C warming of the deep ocean and high latitude surface temperatures shut down the Pacific Ocean's El Niño-Southern Oscillation. A theory was suggested for the Paleocene–Eocene Thermal Maximum on the sudden decrease of the carbon isotopic composition of the global inorganic carbon pool by 2.5 parts per million. A hypothesis posed for this drop of isotopes was the increase of methane hydrates, the trigger for which remains a mystery. The increase of atmospheric methane,
which happens to be a potent but short-lived greenhouse gas, increased
the global temperatures by 6 °C with the assistance of the less potent
carbon dioxide.
List of icehouse and greenhouse periods
A greenhouse period ran from 4.6 to 2.4 billion years ago.
Huronian glaciation – an icehouse period that ran from 2.4 billion to 2.1 billion years ago
A greenhouse period ran from 2.1 billion to 720 million years ago.
Cryogenian – an icehouse period that ran from 720 to 635 million years ago during which the entire Earth was at times frozen over
A greenhouse period ran from 635 million years ago to 450 million years ago.
A greenhouse period ran from 420 million years ago to 360 million years ago.
Late Paleozoic Ice Age – an icehouse period that ran from 360 million to 260 million years ago
A greenhouse period ran from 260 million years ago to 33.9 million years ago.
Late Cenozoic Ice Age – the current icehouse period, which began 33.9 million years ago
Modern conditions
Currently, Earth is in an icehouse climate state. About 34 million years ago, ice sheets began to form in Antarctica; the ice sheets in the Arctic did not start forming until 2 million years ago. Some processes that may have led to the current icehouse may be connected to the development of the Himalayan Mountains and the opening of the Drake Passage between South America
and Antarctica, but climate model simulations suggest that the early
opening of the Drake Passage played only a minor role, and the later
constriction of the Tethys and Central American Seaways is more important in explaining the observed Cenozoic cooling. Scientists have tried to compare the past transitions between icehouse
and greenhouse, and vice versa, to understand what type of climate state
Earth will have next.
Without the human influence on the greenhouse gas concentration, a glacial period would be the next climate state. Predicted changes in orbital forcing suggest that in absence of human-made global warming, the next glacial period would begin at least 50,000 years from now (see Milankovitch cycles), but the ongoing anthropogenic greenhouse gas emissions mean the next climate state will be a greenhouse Earth period. Permanent ice is actually a rare phenomenon in the history of Earth and
occurs only in coincidence with the icehouse effect, which has affected
about 20% of Earth's history.
Map showing global and regional tipping elements: if the global temperature increases past a certain point (color-coded for temperature thresholds), this particular element would be tipped.[1] The result would be a transition to a different state.
In climate science, a tipping point is a critical threshold that, when crossed, leads to large, accelerating and often irreversible changes in the climate system. If tipping points are crossed, they are likely to have severe impacts on human society and may accelerate global warming. Tipping behavior is found across the climate system, for example in ice sheets, mountain glaciers, circulation patterns in the ocean, in ecosystems, and the atmosphere. Examples of tipping points include thawing permafrost, which will release methane, a powerful greenhouse gas, or melting ice sheets and glaciers reducing Earth's albedo,
which would warm the planet faster. Thawing permafrost is a threat
multiplier because it holds roughly twice as much carbon as the amount
currently circulating in the atmosphere.
Tipping points are often, but not necessarily, abrupt. For example, with average global warming somewhere between 0.8 °C (1.4 °F) and 3 °C (5.4 °F), the Greenland ice sheet passes a tipping point and is doomed, but its melt would take place over millennia. Tipping points are possible at today's global warming of just over 1 °C (1.8 °F) above preindustrial times, and highly probable above 2 °C (3.6 °F) of global warming. 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 sheets, the Amazon rainforest and warm-water coral reefs. A 2022 study published in Science
found that exceeding 1.5 °C of global warming could trigger multiple
tipping points, including the collapse of major ice sheets, abrupt
thawing of permafrost, and coral reef die-off, with potential for
cascading system effects.
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, potentially catastrophic, impacts. Crossing a threshold in one part of the climate system may trigger another tipping element to tip into a new state. For example, 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.
Scientists have identified many elements in the climate system which may have tipping points. As of September 2022, nine global core tipping elements and seven regional impact tipping elements are known. Out of those, one regional and three global climate elements will
likely pass a tipping point if global warming reaches 1.5 °C (2.7 °F).
They are the Greenland ice sheet collapse, West Antarctic ice sheet
collapse, tropical coral reef die off, and boreal permafrost abrupt
thaw.
Tipping points exist in a range of systems, for example in the cryosphere,
within ocean currents, and in terrestrial systems. The tipping points
in the cryosphere include: Greenland ice sheet disintegration, West
Antarctic ice sheet disintegration, East Antarctic ice sheet disintegration, arctic sea ice decline, retreat of mountain glaciers, permafrost thaw. The tipping points for ocean current changes include the Atlantic Meridional Overturning Circulation (AMOC), the North Subpolar Gyre and the Southern Ocean overturning circulation.
Lastly, the tipping points in terrestrial systems include Amazon
rainforest dieback, boreal forest biome shift, Sahel greening, and
vulnerable stores of tropical peat carbon.
Definition
A
system going past a tipping point. The system starts (blue) in one of
two alternative stable states, represented by the ball in the left hand
valley. Under external forcing over time (left to right) this state
loses stability (purple), represented by the valley getting shallower,
lowering the hilltop. Past a tipping point the initial stable state
disappears and the system undergoes an abrupt, self-propelling change
into the alternative, remaining stable state (red).Positive tipping point in society
The IPCC Sixth Assessment Report defines a tipping point as a "critical threshold beyond which a system reorganizes, often abruptly and/or irreversibly". It can be brought about by a small disturbance causing a
disproportionately large change in the system. It can also be associated
with self-reinforcing feedbacks, which could lead to changes in the climate system irreversible on a human timescale. For any particular climate component, the shift from one state to a new stable state may take many decades or centuries.
The 2019 IPCC Special Report on the Ocean and Cryosphere in a Changing Climate
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."
In ecosystems and in social systems, a tipping point can trigger a regime shift, a major systems reorganisation into a new stable state. Such regime shifts need not be harmful. In the context of the climate
crisis, the tipping point metaphor is sometimes used in a positive sense,
such as to refer to shifts in public opinion in favor of action to
mitigate climate change, or the potential for minor policy changes to
rapidly accelerate the transition to a green economy.
Comparison of tipping points
Scientists have identified many elements in the climate system which may have tipping points. In the early 2000s the IPCC began considering the possibility of tipping points, originally referred to as large-scale discontinuities.
At that time the IPCC concluded they would only be likely in the event
of global warming of 4 °C (7.2 °F) or more above preindustrial times,
and another early assessment placed most tipping point thresholds at
3–5 °C (5.4–9.0 °F) above 1980–1999 average warming. Since then estimates for global warming thresholds have generally
fallen, with some thought to be possible in the Paris Agreement range
(1.5–2 °C (2.7–3.6 °F)) by 2016. As of 2021 tipping points are considered to have significant
probability at today's warming level of just over 1 °C (1.8 °F), with
high probability above 2 °C (3.6 °F) of global warming. Some tipping points may be 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 global core tipping elements and seven regional impact tipping elements have been identified. Out of those, one regional and three global climate elements are
estimated to likely pass a tipping point if global warming reaches
1.5 °C (2.7 °F), namely Greenland ice sheet collapse, West Antarctic ice
sheet collapse, tropical coral reef die off, and boreal permafrost
abrupt thaw. Two further tipping points are forecast as likely if
warming continues to approach 2 °C (3.6 °F): Barents sea ice abrupt
loss, and the Labrador Seasubpolar gyre collapse.
Global core tipping elements
Proposed climate tipping element (and tipping point)
The paper also
provides the same estimate in terms of equivalent emissions: partial
dieback would be equivalent to the emissions of 30 billion tonnes of
carbon, while total dieback would be equivalent to 75 billion tonnes of
carbon.
The paper also
provides the same estimate in terms of emissions: between 125 and 250
billion tonnes of carbon and between 175 and 350 billion tonnes of
carbon equivalent.
Regional impact tipping elements
Proposed climate tipping element (and tipping point)
The paper clarifies
that this represents a 50% increase of gradual permafrost thaw: it also
provides the same estimate in terms of emissions per each degree of
warming: 10 billion tonnes of carbon and 14 billion tonnes of carbon
equivalent by 2100, and 25/35 billion tonnes of carbon/carbon equivalent
by 2300.
The loss of these
forests would be equivalent to the emissions of 52 billion tons of
carbon, but this would be more than offset by the area's albedo effect increasing and reflecting more sunlight.
Extra forest growth
here would absorb around 6 billion tons of carbon, but because this area
receives a lot of sunlight, this is very minor when compared to reduced
albedo, as this vegetation absorbs more heat than the snow-covered
ground it moves into.
Tipping points in the cryosphere
Greenland ice sheet disintegration
Changes
in extent (colored lines) and thickness (black lines) of the Greenland
ice sheet over time, showing its rapid, sustained melting since 2000
The Greenland ice sheet
is the second largest ice sheet in the world, and completely melting
the water which it holds would raise sea levels globally by 7.2 metres
(24 ft). Due to global warming, the ice sheet is currently melting at an
accelerating rate, adding almost 1 mm to global sea levels every year. Around half of the ice loss occurs via surface melting, and the
remainder occurs at the base of the ice sheet where it touches the sea,
by calving (breaking off) icebergs from its margins.
The Greenland ice sheet has a tipping point because of the melt-elevation feedback.
Surface melting reduces the height of the ice sheet, and air at a lower
altitude is warmer. The ice sheet is then exposed to warmer
temperatures, accelerating its melt. A 2021 analysis of sub-glacial sediment at the bottom of a 1.4
kilometres (0.87 mi) 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
(4.5 °F) maximum temperature increase over the preindustrial conditions
observed over that period. There is some evidence that the Greenland ice sheet is losing stability, and getting close to a tipping point.
West Antarctic ice sheet disintegration
A topographic and bathymetric map of Antarctica without its ice sheets, assuming constant sea levels and no post-glacial rebound
The West Antarctic Ice Sheet (WAIS) is a large ice sheet in Antarctica; in places more than 4 kilometres (2.5 mi) thick. It sits on bedrock mostly below sea level, having formed a deep subglacial basin due to the weight of the ice sheet over millions of years. As such, it is in contact with the heat from the ocean which makes it
vulnerable to fast and irreversible ice loss. A tipping point could be
reached once the WAIS's grounding lines (the point at which ice no
longer sits on rock and becomes floating ice shelves)
retreat behind the edge of the subglacial basin, resulting in
self-sustaining retreat in to the deeper basin - a process known as the Marine Ice Sheet Instability (MISI). Thinning and collapse of the WAIS's ice shelves
is helping to accelerate this grounding line retreat. If completely
melted, the WAIS would contribute around 3.3 metres (11 ft) of sea level
rise over thousands of years.
Ice loss from the WAIS is accelerating, and some outlet glaciers
are estimated to be close to or possibly already beyond the point of
self-sustaining retreat. The paleo record
suggests that during the past few hundred thousand years, the WAIS
largely disappeared in response to similar levels of warming and CO2 emission scenarios projected for the next few centuries.
Like with the other ice sheets, there is a counteracting negative feedback - greater warming also intensifies the effects of climate change on the water cycle, which result in an increased precipitation over the ice sheet in the form of snow during the winter, which would freeze on the surface, and this increase in the surface mass balance (SMB) counteracts some fraction of the ice loss. In the IPCC Fifth Assessment Report,
it was suggested that this effect could potentially overpower increased
ice loss under the higher levels of warming and result in small net ice
gain, but by the time of the IPCC Sixth Assessment Report, improved modelling had proven that the glacier breakup would consistently accelerate at a faster rate.
East Antarctic ice sheet disintegration
The East Antarctic ice sheet
is the largest and thickest ice sheet on Earth, with the maximum
thickness of 4,800 metres (3.0 mi). A complete disintegration would
raise the global sea levels by 53.3 metres (175 ft), but this may not
occur until global warming of 10 °C (18 °F), while the loss of
two-thirds of its volume may require at least 6 °C (11 °F) of warming to
trigger. Its melt would also occur over a longer timescale than the loss of any
other ice on the planet, taking no less than 10,000 years to finish.
However, the subglacial basin portions of the East Antarctic ice sheet may be vulnerable to tipping at lower levels of warming. The Wilkes Basin is of particular concern, as it holds enough ice to raise sea levels by about 3–4 metres (10–13 ft).
Average decadal extent and area of the Arctic Ocean sea ice since the start of satellite observations
Annual trend in the Arctic sea ice extent and area for the 2011–2022 time period
Arctic sea ice 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 hypothesised 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 does not overcome the cooling and the formation of new ice during the Arctic winter.
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 may not reform during the winter even below 2 °C (3.6 °F) 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 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.
Projected loss of mountain glaciers over the 21st century, for different amounts of global warming
Mountain glaciers
are the largest repository of land-bound ice after the Greenland and
the Antarctica ice sheets, and they are also undergoing melting as the
result of climate change. A glacier tipping point is when it enters a
disequilibrium state with the climate and will melt away unless the
temperatures go down. Examples include glaciers of the North Cascade Range,
where even in 2005 67% of the glaciers observed were in disequilibrium
and will not survive the continuation of the present climate, or the French Alps,
where The Argentière and Mer de Glace glaciers are expected to
disappear completely by end of the 21st century if current climate
trends persist. Altogether, it was estimated in 2023 that 49% of the world's glaciers
would be lost by 2100 at 1.5 °C (2.7 °F) of global warming, and 83% of
glaciers would be lost at 4 °C (7.2 °F). This would amount to one
quarter and nearly half of mountain glacier *mass* loss, respectively,
as only the largest, most resilient glaciers would survive the century.
This ice loss would also contribute ~9 cm (3+1⁄2 in)
and ~15 cm (6 in) to sea level rise, while the current likely
trajectory of 2.7 °C (4.9 °F) would result in the SLR contribution of ~11 cm (4+1⁄2 in) by 2100.
The absolute largest amount of glacier ice is located in the Hindu KushHimalaya 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 (2.7 °F), while the intermediate and severeclimate change scenarios (Representative Concentration Pathways
(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 is low: however, irrigation and hydropower
generation would still have to adjust to greater interannual
variability and lower pre-monsoon flows in all of the region's rivers.
Permafrost thaw
Ground collapse caused by abrupt permafrost thaw in Herschel Island, Canada, 2013
Feedback processes related to land and subsea permafrost
Perennially frozen ground, or permafrost, covers large fractions of land – mainly in Siberia, Alaska, northern Canada and the Tibetan plateau – and can be up to a kilometre thick.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 have died and decomposed over thousands of years. Scientists
believe there is nearly twice as much carbon in permafrost than is
present in Earth's atmosphere.
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, some of which
is irreversibly lost. While most thaw is gradual and will take centuries, abrupt thaw can
occur in some places where permafrost is rich in large ice masses, which
once melted cause the ground to slump or form 'thermokarst' lakes over
years to decades. These processes can become self-sustaining, leading to localised
tipping dynamics, and could increase greenhouse gas emissions by around
40%. Because CO2 and methane
are both greenhouse gases, they act as a self-reinforcing feedback on
permafrost thaw, but are unlikely to lead to a global tipping point or
runaway warming process.
The Northern part of the Atlantic meridional overturning circulation
The Atlantic meridional overturning circulation (AMOC), also known as the Gulf Stream System, is a large system of ocean currents. It is driven by differences in the density of water; colder and more salty water is heavier than warmer fresh water. 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 glaciers
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 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. In October 2024, 44 climate scientists published an open letter,
claiming that according to scientific studies in the past few years, the
risk of AMOC collapse has been greatly underestimated, it can occur in
the next few decades, with devastating impacts especially for Nordic
countries. An August 2025 study concluded that the collapse of AMOC could start as early as the 2060s.
North subpolar gyre
Modelled 21st century warming under the "intermediate" climate change scenario (top). The potential collapse of the subpolar gyre in this scenario (middle). The collapse of the entire AMOC (bottom).
Some climate models indicate that the deep convection in Labrador-Irminger Seas could collapse under certain global warming scenarios, which would then collapse the entire circulation in the North subpolar gyre.
It is considered unlikely to recover even if the temperature is
returned to a lower level, making it an example of a climate tipping
point. This would result in rapid cooling, with implications for
economic sectors, agriculture industry, water resources and energy
management in Western Europe and the East Coast of the United States. Frajka-Williams et al. 2017 pointed out that recent changes in cooling
of the subpolar gyre, warm temperatures in the subtropics and cool
anomalies over the tropics, increased the spatial distribution of
meridional gradient in sea surface temperatures, which is not captured by the AMO Index.
A 2021 study found that this collapse occurs in only four CMIP6
models out of 35 analyzed. However, only 11 models out of 35 can
simulate North Atlantic Current with a high degree of accuracy, and this
includes all four models which simulate collapse of the subpolar gyre.
As the result, the study estimated the risk of an abrupt cooling event
over Europe caused by the collapse of the current at 36.4%, which is
lower than the 45.5% chance estimated by the previous generation of
models. In 2022, a paper suggested that previous disruption of subpolar gyre was connected to the Little Ice Age.
Southern Ocean overturning circulation
Since the 1970s, the upper cell of the circulation has strengthened, while the lower cell weakened.
Southern ocean overturning circulation itself consists of two parts,
the upper and the lower cell. The smaller upper cell is most strongly
affected by winds due to its proximity to the surface, while the behaviour of the larger lower cell is defined by the temperature and salinity of Antarctic bottom water. The strength of both halves had undergone substantial changes in the
recent decades: the flow of the upper cell has increased by 50–60% since
1970s, while the lower cell has weakened by 10–20%. This has been partly due to the natural cycle of Interdecadal Pacific Oscillation, and climate change has played a substantial role in both trends, as it had altered the Southern Annular Mode weather pattern, while the massive growth of ocean heat content in the Southern Ocean has increased the melting of the Antarctic ice sheets, and this fresh meltwater dilutes salty Antarctic bottom water.
Paleoclimate
evidence shows that the entire circulation had strongly weakened or
outright collapsed before: some preliminary research suggests that such a
collapse may become likely once global warming reaches levels between
1.7 °C (3.1 °F) and 3 °C (5.4 °F). However, there is far less certainty
than with the estimates for most other tipping points in the climate
system. Even if the circulation's collapse starts in the near future, it is unlikely to be complete until close to 2300, Similarly, impacts such as the reduction in precipitation in the Southern Hemisphere, with a corresponding increase in the North, or a decline of fisheries in the Southern Ocean with a potential collapse of certain marine ecosystems, are also expected to unfold over multiple centuries.
Tipping points in terrestrial systems
As of 2022, 20% of the Amazon rainforest has been "transformed" (deforested) and another 6% has been "highly degraded", causing Amazon Watch to warn that the Amazonia is in the midst of a tipping point crisis.
The Amazon rainforest is the largest tropical rainforest
in the world. It is twice as big as India and spans nine countries in
South America. It produces around half of its own rainfall by recycling
moisture through evaporation and transpiration as air moves across the forest. This moisture recycling expands the area in which there is enough
rainfall for rainforest to be maintained, and without it one model
indicates around 40% of the current forest area would be too dry to
sustain rainforest. However, when forest is lost via climate change (from droughts and wildfires) or deforestation,
there will be less rain in downwind regions, increasing tree stress and
mortality there. Eventually, if enough forest is lost a threshold can
be reached beyond which large parts of the remaining rainforest may die
off and transform into drier degraded forest or savanna landscapes, particularly in the drier south and east. In 2022, a study reported that the rainforest has been losing resilience since the early 2000s. Resilience is measured by recovery-time from short-term perturbations, with delayed return to equilibrium of the rainforest termed as critical slowing down. The observed loss of resilience reinforces the theory that the rainforest could be approaching a critical transition, although it cannot determine exactly when or if a tipping point will be reached.
Boreal forest biome shift
During the last quarter of the twentieth century, the zone of latitude occupied by taiga
experienced some of the greatest temperature increases on Earth. Winter
temperatures have increased more than summer temperatures. In summer,
the daily low temperature has increased more than the daily high
temperature. It has been hypothesised that the boreal environments have only a few states which are stable in the long term - a treeless tundra/steppe, a forest with >75% tree cover and an open woodland
with ≈20% and ≈45% tree cover. Thus, continued climate change would be
able to force at least some of the presently existing taiga forests into
one of the two woodland states or even into a treeless steppe - but it
could also shift tundra areas into woodland or forest states as they
warm and become more suitable for tree growth.
The
response of six tree species common in Quebec's forests to 2 °C
(3.6 °F) and 4 °C (7.2 °F) warming under different precipitation levels
These trends were first detected in the Canadian boreal forests in the early 2010s, and summer warming had also been shown to increase water stress and
reduce tree growth in dry areas of the southern boreal forest in central
Alaska and portions of far eastern Russia. In Siberia, the taiga is converting from predominantly needle-shedding larch trees to evergreen conifers in response to a warming climate.
Subsequent research in Canada found that even in the forests
where biomass trends did not change, there was a substantial shift
towards the deciduous broad-leaved trees with higher drought tolerance
over the past 65 years. A Landsat
analysis of 100,000 undisturbed sites found that the areas with low
tree cover became greener in response to warming, but tree mortality
(browning) became the dominant response as the proportion of existing
tree cover increased. A 2018 study of the seven tree species dominant in the Eastern Canadian
forests found that while 2 °C (3.6 °F) warming alone increases their
growth by around 13% on average, water availability is much more
important than temperature. Also, further warming of up to 4 °C (7.2 °F)
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 moderateSSP2-4.5
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 (2.7 °F) or 3.1 °C (5.6 °F) 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.
The Special Report on Global Warming of 1.5 °C and the IPCC Fifth Assessment Report
indicate that global warming will likely result in increased
precipitation across most of East Africa, parts of Central Africa and
the principal wet season of West Africa. However, there is significant uncertainty related to these projections especially for West Africa.Currently, the Sahel is becoming greener but precipitation has not fully recovered to levels reached in the mid-20th century.
A study from 2022 concluded: "Clearly the existence of a future tipping threshold for the WAM (West African Monsoon)
and Sahel remains uncertain as does its sign but given multiple past
abrupt shifts, known weaknesses in current models, and huge regional
impacts but modest global climate feedback, we retain the Sahel/WAM as a
potential regional impact tipping element (low confidence)."
Some simulations of global warming and increased carbon dioxide concentrations have shown a substantial increase in precipitation in the Sahel/Sahara. This and the increased plant growth directly induced by carbon dioxide
could lead to an expansion of vegetation into present-day desert,
although it might be accompanied by a northward shift of the desert,
i.e. a drying of northernmost Africa.
Vulnerable stores of tropical peat carbon: Cuvette Centrale peatland
Map of Cuvette Centrale location in the Congo Basin.
Three graphs portray the evolution of its peatland carbon content over
the past 20,000 years, as reconstructed from three peat cores.
In 2017, it was discovered that 40% of the Cuvette Centrale wetlands are underlain with a dense layer of peat, which contains around 30 petagrams (billions of tons) of carbon.
This amounts to 28% of all tropical peat carbon, equivalent to the
carbon contained in all the forests of the Congo Basin. In other words,
while this peatland only covers 4% of the Congo Basin area, its carbon
content is equal to that of all trees in the other 96%. It was then estimated that if all of that peat burned, the atmosphere would absorb the equivalent of 20 years of current United Statescarbon dioxide emissions, or three years of all anthropogenic CO2 emissions.
This threat prompted the signing of Brazzaville Declaration in March 2018: an agreement between Democratic Republic of Congo, the Republic of Congo and Indonesia
(a country with longer experience of managing its own tropical
peatlands) aiming to promote better management and conservation of this
region. However, 2022 research by the same team which had originally discovered
this peatland not only revised its area (from the original estimate of
145,500 square kilometres (56,200 sq mi) to 167,600 square kilometres
(64,700 sq mi)) and depth (from 2 m (6.6 ft) to (1.7 m (5.6 ft)) but
also noted that only 8% of this peat carbon is currently covered by the
existing protected areas. For comparison, 26% of its peat is located in areas open to logging, mining or palm oil plantations, and nearly all of this area is open for fossil fuel exploration.
Even in the absence of local disturbance from these activities,
this area is the most vulnerable store of tropical peat carbon in the
world, as its climate is already much drier than that of the other
tropical peatlands in the Southeast Asia and the Amazon rainforest.
A 2022 study suggests that the geologically recent conditions between
7,500 years ago and 2,000 years ago were already dry enough to cause
substantial peat release from this area, and that these conditions are
likely to recur in the near future under continued climate change. In this case, Cuvette Centrale would act as one of the tipping points in the climate system at some yet unknown time.
Bleached coral with normal coral in the background
Around 500 million people around the world depend on coral reefs for food, income, tourism and coastal protection. Since the 1980s, this is being threatened by the increase in sea surface temperatures which is triggering mass bleaching of coral, especially in sub-tropical regions. A sustained ocean temperature spike of 1 °C (1.8 °F) above average is enough to cause bleaching. Under heat stress, corals expel the small colourful algae which live in their tissues, which causes them to turn white. The algae, known as zooxanthellae, have a symbiotic relationship with coral such that without them, the corals slowly die. After these zooxanthellae have disappeared, the corals are vulnerable to a transition towards a seaweed-dominated ecosystem, making it very difficult to shift back to a coral-dominated ecosystem. The IPCC
estimates that by the time temperatures have risen to 1.5 °C (2.7 °F)
above pre-industrial times, "Coral reefs... are projected to decline by a
further 70–90%"; and that if the world warms by 2 °C (3.6 °F), they
will become extremely rare.
Break-up of equatorial stratocumulus clouds
In 2019, a study employed a large eddy simulation model to estimate that equatorial stratocumulus clouds could break up and scatter when CO2 levels rise above 1,200 ppm
(almost three times higher than the current levels, and over 4 times
greater than the preindustrial levels). The study estimated that this
would cause a surface warming of about 8 °C (14 °F) globally and 10 °C
(18 °F) in the subtropics, which would be in addition to at least 4 °C
(7.2 °F) already caused by such CO2 concentrations. In addition, stratocumulus clouds would not reform until the CO2 concentrations drop to a much lower level. It was suggested that this finding could help explain past episodes of unusually rapid warming such as Paleocene-Eocene Thermal Maximum. In 2020, further work from the same authors revealed that in their
large eddy simulation, this tipping point cannot be stopped with solar radiation modification: in a hypothetical scenario where very high CO2
emissions continue for a long time but are offset with extensive solar
radiation modification, the break-up of stratocumulus clouds is simply
delayed until CO2 concentrations hit 1,700 ppm, at which point it would still cause around 5 °C (9.0 °F) of unavoidable warming.
However, because large eddy simulation models are simpler and smaller-scale than the general circulation models used for climate projections, with limited representation of atmospheric processes like subsidence, this finding is currently considered speculative. Other scientists say that the model used in that study unrealistically
extrapolates the behavior of small cloud areas onto all cloud decks, and
that it is incapable of simulating anything other than a rapid
transition, with some comparing it to "a knob with two settings". Additionally, CO2 concentrations would only reach 1,200 ppm if the world follows Representative Concentration Pathway 8.5, which represents the highest possible greenhouse gas emission scenario and involves a massive expansion of coal infrastructure. In that case, 1,200 ppm would be passed shortly after 2100.
Cascading tipping points
A proposed tipping cascade with four tipping elements
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
(3.6 °F) – the upper limit set by the Paris Agreement 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". A network model analysis suggested that temporary overshoots of climate change – increasing global temperature beyond Paris Agreement
goals temporarily as often projected – can substantially increase risks
of climate tipping cascades ("by up to 72% compared with non-overshoot
scenarios").
Formerly considered tipping elements
Earlier (2008) list of tipping elements in the climate system. When compared to later lists, the major differences are that in 2008 ENSO, Indian summer monsoon, Arctic ozone hole and all of Arctic sea ice
were all listed as tipping points. Labrador-Irminger circulation,
mountain glaciers and East Antarctic ice however were not included. This
2008 list also includes Antarctic bottom water (part of the Southern Ocean overturning circulation), which was left out of the 2022 list, but included in some subsequent ones.
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 is 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 °C (6.3 °F) and 7 °C (13 °F) 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, but the layout of the ocean was significantly different from now. 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
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
Illustration
of three types of tipping point; (a), (b) noise-, (c), (d) bifurcation-
and (e), (f) rate-induced. (a), (c), (e) example time-series (coloured
lines) through the tipping point with black solid lines indicating
stable climate states (e.g. low or high rainfall) and dashed lines
represent the boundary between stable states. (b), (d), (f) stability
landscapes provide an understanding for the different types of tipping
point. The valleys represent different climate states the system can
occupy with hill tops separating the stable states.
Tipping point behaviour in the climate can be described in
mathematical terms. Three types of tipping points have been identified—bifurcation, noise-induced and rate-dependent.
Bifurcation-induced tipping
Bifurcation-induced tipping happens when a particular parameter in
the climate (for instance a change in environmental conditions or forcing),
passes a critical level – at which point a bifurcation takes place –
and what was a stable state loses its stability or simply disappears. The Atlantic Meridional Overturning Circulation (AMOC)
is an example of a tipping element that can show bifurcation-induced
tipping. Slow changes to the bifurcation parameters in this system – the
salinity and temperature of the water – may push the circulation
towards collapse.
Many types of bifurcations show hysteresis, which is the dependence of the state of a system on its history. For
instance, depending on how warm it was in the past, there can be
differing amounts of ice on the poles at the same concentration of
greenhouse gases or temperature.
Early warning signals
For tipping points that occur because of a bifurcation, it may be
possible to detect whether a system is getting closer to a tipping
point, as it becomes less resilient to perturbations on approach of the
tipping threshold. These systems display critical slowing down, with an increased memory (rising autocorrelation) and variance. Depending on the nature of the tipping system, there may be other types of early warning signals. Abrupt change is not an early warning signal (EWS) for tipping points,
as abrupt change can also occur if the changes are reversible to the
control parameter.
These EWSs are often developed and tested using time series from
the paleo record, like sediments, ice caps, and tree rings, where past
examples of tipping can be observed. It is not always possible to say whether increased variance and
autocorrelation is a precursor to tipping, or caused by internal
variability, for instance in the case of the collapse of the AMOC. 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 does not change. Because they are unpredictable, such occurrences are often described as a "one-in-x-year" event. An example is the Dansgaard–Oeschger events during the last ice age, with 25 occurrences of sudden climate fluctuations over a 500-year period.
Rate-induced tipping
Rate-induced tipping occurs when a change in the environment is
faster than the force that restores the system to its stable state. 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.
Potential impacts
Schematic of some possible interactions and cascading effects between the Earth's climate system and humanity's social system
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 for hundreds of millions. Other impacts would likely take place over longer timescales, such as the melting of the ice caps.
The circa 10 metres (33 ft) of sea level rise from the combined melt of
Greenland and West Antarctica would require moving many cities inland
over the course of centuries, but would also accelerate sea level rise
this century, with Antarctic ice sheet instability projected to expose
120 million more people to annual floods in a mid-emissions scenario. A collapse of the Atlantic Overturning Circulation would cause over
10 °C of cooling in parts of Europe, cause drying in Europe, Central
America, West Africa, and southern Asia, and lead to about 1 metre (3+1⁄2 ft) of sea level rise in the North Atlantic. The impacts of AMOC collapse would have serious implications for food
security, with one projection showing reduced yields of key crops across
most world regions, with for example arable agriculture becoming
economically infeasible in Britain. These impacts could happen simultaneously in the case of cascading tipping points. 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 and regime shifts led to the retreat of pastoral societies in North Africa and a change of dynasty in Egypt.
Some scholars have proposed a threshold which, if crossed, could
trigger multiple tipping points and self-reinforcing feedback loops that
would prevent stabilisation of the climate, causing much greater
warming and sea-level rises and leading to severe disruption to
ecosystems, society, and economies. This scenario is sometimes called the Hothouse Earth
scenario. The researchers proposed that this scenario could unfold
beyond a threshold of around 2 °C above pre-industrial levels. However,
while this scenario is possible, the existence and value of this
threshold remains speculative, and doubts have been raised if tipping
points would lock in much extra warming in the shorter term.Decisions taken over the next decade could influence the climate of the
planet for tens to hundreds of thousands of years and potentially even
lead to conditions which are inhospitable to current human societies.
The report also states that there is a possibility of a cascade of
tipping points being triggered even if the goal outlined in the Paris Agreement to limit warming to 1.5–2.0 °C (2.7–3.6 °F) is achieved.
Geological timescales
Meltwater pulse 1A was a period of abrupt sea level rise around 14,000 years ago. It may be an example of a tipping point.
A runaway greenhouse effect is a tipping point so extreme that oceans evaporate and the water vapour escapes to space, an irreversible climate state that happened on Venus. A runaway greenhouse effect has virtually no chance of being caused by people. Venus-like conditions on the Earth require a large long-term forcing
that is unlikely to occur until the sun brightens by a ten of percents,
which will take 600–700 million years.
