A deep geological repository is a way of storing hazardous or radioactive waste within a stable geologic environment, typically 200–1,000 m below the surface of the earth. It entails a combination of waste form, waste package, engineered seals
and geology that is suited to provide a high level of long-term
isolation and containment without future maintenance. This is intended
to prevent radioactive dangers. A number of mercury, cyanide and arsenic waste repositories are operating worldwide including Canada (Giant Mine) and Germany (potash mines in Herfa-Neurode and Zielitz). Radioactive waste storage sites are under construction with the Onkalo in Finland being the most advanced.
Principles and background
Highly
toxic waste that cannot be further recycled must be stored in
isolation, to avoid contamination of air, ground and underground water.
Deep geological repository is a type of long-term storage that isolates
waste in geological structures that are expected to be stable for
millions of years, with a number of natural and engineered barriers.
Natural barriers include water-impermeable (e.g. clay) and
gas-impermeable (e.g. salt) layers of rock above and surrounding the
underground storage. Engineered barriers include bentonite clay and cement.
It is widely accepted that spent nuclear fuel and high-level
reprocessing and plutonium wastes require well-designed storage for
periods ranging from tens of thousands to a million years, to minimize
releases of the contained radioactivity into the environment. Safeguards
are also required to ensure that neither plutonium nor highly enriched
uranium is diverted to weapon use. There is general agreement that
placing spent nuclear fuel in repositories hundreds of meters below the
surface would be safer than indefinite storage of spent fuel on the
surface [of the earth].
Common elements of repositories include the radioactive waste, the
containers enclosing the waste, other engineered barriers or seals
around the containers, the tunnels housing the containers, and the
geologic makeup of the surrounding area.
A storage space hundreds of metres below the ground needs to withstand the effects of one or more future glaciations, with thick ice sheets resting on top of the rock. The presence of ice sheets affects the hydrostatic pressure at
repository depth, groundwater flow and chemistry, and the potential for
earthquakes. This is being taken into consideration by organizations
preparing for long-term waste repositories in Sweden, Finland, Canada
and some other countries that have to assess the effects of future
glaciations.
Despite a long-standing agreement among many experts that
geological disposal can be safe, technologically feasible and
environmentally sound, a large part of the general public in many
countries remains skeptical as a result of anti-nuclear campaigns. One of the challenges facing the supporters of these efforts is to
demonstrate confidently that a repository will contain wastes for so
long that any releases that might take place in the future will pose no
significant health or environmental risk.
Nuclear reprocessing
does not eliminate the need for a repository, but reduces the volume,
the long-term radiation hazard, and long-term heat dissipation capacity
needed. Reprocessing does not eliminate the political and community
challenges to repository siting.
Natural uranium ore deposits serve as proof of concept for stability of radioactive elements in geological formations—Cigar Lake Mine for example is a natural deposit of highly concentrated uranium ore located under sandstone and a quartz layer at a depth of 450 m, that is 1 billion years old with no radioactive leaks to the surface.
The ability of natural geologic barriers to isolate radioactive waste is demonstrated by the natural nuclear fission reactors at Oklo, Gabon. During their long reaction period about 5.4 tonnes of fission products as well as 1.5 tonnes of plutonium together with other transuranic elements
were generated in the uranium ore body. This plutonium and the other
transuranics remained immobile until the present day, a span of almost 2
billion years. This is remarkable as ground water had ready access to the deposits and they were not in a chemically inert form, such as glass.
Research
Deep geologic disposal has been studied for several decades, including laboratory tests, exploratory boreholes, and the construction and operation of underground research laboratories where large-scale in-situ tests are being conducted. Major underground test facilities are listed below.
A schematic of a geologic repository under construction at Olkiluoto Nuclear Power Plant site, FinlandA demonstration tunnel in Olkiluoto.In February 2014, radioactive materials leaked from a damaged storage drum at the U. S. Waste Isolation Pilot Plant. Analysis of several accidents, by the U.S. Department Of Energy, have shown lack of a "safety culture".
The process of selecting appropriate deep final repositories is under
way in several countries, with the first expected to be commissioned
some time after 2010.
Australia
There was a proposal in the early 2000s for an international high level waste repository in Australia and Russia. Since the proposal for a global repository in Australia, which has
never produced nuclear power, and has one research reactor, was raised,
domestic political objections have been loud and sustained, making such a
facility in Australia unlikely.
Giant Mine has been used as a deep repository for storage of highly toxic arsenic
waste in the form of powder. As of 2020 there is ongoing research to
reprocess the waste into a frozen block form which is more chemically
stable and prevents water contamination.
On Nov 28, 2024, the NWMO
selected the Wabigoon Lake Ojibway Nation-Ignace area as the site for
Canada's deep geological repository for used nuclear fuel.
Finland
The Onkalo site in Finland based on the KBS-3
technology, is the furthest along the road to becoming operational
among repositories worldwide. Posiva started construction of the site in
2004. The Finnish government issued the company a licence for
constructing the final disposal facility in November 2015. As of June
2019, continuous delays mean that Posiva expects operations to begin in 2023.
Germany
A number of repositories including potash mines in Herfa-Neurode and Zielitz have been used for years for the storage of highly toxic mercury, cyanide and arsenic waste. There is little debate in Germany regarding toxic waste, in spite of
the fact that unlike nuclear waste, it does not lose toxicity with time.
There is a debate about the search for a final repository for radioactive waste, accompanied by protests, especially in the Gorleben village in the Wendland
area, which was seen as ideal for the final repository until 1990
because of its location in a remote, economically depressed corner of
West Germany, next to the closed border to the former East Germany.
After reunification, the village is now close to the center of Germany,
and is now used for temporary storage of nuclear waste.
Approval
was granted in January 2022 for the construction of a direct disposal
facility using KBS-3 technology, on the site of the Forsmark nuclear power plant.
United Kingdom
The
UK Government, in common with many other countries and supported by
scientific advice, has identified permanent deep underground disposal as
the most appropriate means of disposing of higher activity radioactive
waste.
Radioactive Waste Management (RWM) was established in 2014 to deliver a Geological Disposal Facility (GDF)
and is a subsidiary of the Nuclear Decommissioning Authority (NDA) which is responsible for clean-up of the UK's historical nuclear sites.
In 2022, Nuclear Waste Services (NWS) formed from the merger of RWM
with the Low Level Waste Repository in Cumbria.
A GDF will be delivered through a community consent-based process,
working in close partnership with communities, building trust for the
long term and ensuring a GDF supports local interests and priorities.
The first Working Groups were established in Copeland and Allerdale in Cumbria during late 2020 and early 2021. These Working Groups have
started the process of obtaining consent for hosting a GDF in their
areas. These Working Groups are believed to be a critical step in the
process to find a willing community and a suitable, feasible and
acceptable site for a GDF. Allerdale withdrew from the process to select
a deep waste repository site in 2023. NWS explained this decision in
terms of there being insufficient extent of potentially suitable geology
in which to undertake a site selection process.
RWM continues to have discussions in a range of places across
England with people and organisations who are interested in exploring
the benefits of hosting a GDF. More Working Groups are anticipated to
form across the country in the next year or two.
In 2025, the HM Treasury's National Infrastructure and Service Transformation Authority
annual review graded the project, estimated to cost up to £54 billion,
as "unachievable", stating it had "major issues with project definition,
schedule, budget, quality and/or benefits delivery, which at this stage
do not appear to be manageable or resolvable."
In 1978, the U.S. Department of Energy (DOE) began studying Yucca Mountain, within the secure boundaries of the Nevada Test Site in Nye County, Nevada,
to determine whether it would be suitable for a long-term geologic
repository for spent nuclear fuel and high-level radioactive waste. This
project faced significant opposition and suffered delays due to
litigation by the Agency for Nuclear Projects for the State of Nevada (Nuclear Waste Project Office) and others. The Obama administration rejected use of the site in the 2009 United States Federal Budget
proposal, which eliminated all funding except that needed to answer
inquiries from the U.S. Nuclear Regulatory Commission (NRC), "while the
Administration devises a new strategy toward nuclear waste disposal."
In March 2009, Energy SecretarySteven Chu told a Senate hearing the Yucca Mountain site is no longer viewed as an option for storing reactor waste.
In June 2018, the Trump administration and some members of
Congress again began proposing using Yucca Mountain, with senators from
Nevada raising opposition.
In February 2020, U.S. President Donald Trump tweeted about a
potential change of policy on plans to use Yucca Mountain in Nevada as a
repository for nuclear waste. Trump's previous budgets have included funding for Yucca Mountain but,
according to Nuclear Engineering International, two senior
administration officials said that the latest spending blueprint will
not include any money for licensing the project. On February 7, Energy Secretary Dan Brouillette
echoed Trump's sentiment and stated that the U.S. administration may
investigate other types of [nuclear] storage, such as interim or
temporary sites in other parts of the country.
Though no formal plan had solidified from the federal government,
the private sector moved forward with their own plans. Holtec
International submitted a license application to the NRC for an
autonomous consolidated interim storage facility (CISF) in southeastern
New Mexico in March 2017. Similarly, Interim Storage Partners is also
planning to build and operate a CISF in Andrews County, Texas. Meanwhile, other companies have indicated that they are prepared to bid
on an anticipated procurement from the DOE to design a facility for
interim storage of nuclear waste. The NRC issued a licence for the Andrews County CISF in September 2021. A group including the State of Texas petitioned for a court review of the licence. In August 2023, the United States Court of Appeals for the Fifth Circuit
ruled that the NRC does not have the authority from Congress to license
such a temporary storage facility that is not at a nuclear power
station or federal site, nullifying the purported license. The other New
Mexico CISF is similarly being challenged in the United States Court of Appeals for the Tenth Circuit.
Deep Isolation, a corporation based in Berkeley, California, proposed a solution involving horizontal storage of radioactive waste
canisters in directional boreholes, using technology developed for oil
and gas mining. An 18" borehole can be directed vertically to the depth
of several thousand feet in geologically stable formations, and then a
horizontal waste disposal section of similar length can be created where
waste canisters are stored before the borehole is sealed.
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. 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 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)
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
degrees Celsius 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.
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 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.
