Guest Post: The Irreversible Emissions Of A Permafrost ‘Tipping Point’

Carbon Brief - Christina Schädel

Permafrost thaw in the Gates of the Arctic National Park, Alaska. Credit: Natural History Archive / Alamy Stock Photo.

Dr Christina Schädel Dr Christina Schädel is assistant research professor in the Center for Ecosystem Science and Society at Northern Arizona University. She is also lead coordinator of the Permafrost Carbon Network and co-leader of the Permafrost Collaboration Team.

Across vast swaths of the northern hemisphere’s higher reaches, frozen ground holds billions of tonnes of carbon.
As global temperatures rise, this “permafrost” land is at increasing risk of thawing out, potentially releasing its long-held carbon into the atmosphere.
Abrupt permafrost thaw is one of the most frequently discussed “tipping points” that could be crossed in a warming world.
However, research suggests that, while this thawing is already underway, it can be slowed with climate change mitigation.
Yet, what is irreversible is the escape of the carbon that has been – and is being – emitted.
The carbon released from permafrost goes into the atmosphere and stays there, exacerbating global warming.

Tipping points This article is part of a week-long special series on “tipping points”, where a changing climate could push parts of the Earth system into abrupt or irreversible change Explainer: Nine ‘tipping points’ that could be triggered by climate change Guest post: Could the Atlantic Overturning Circulation ‘shut down’? Guest post: The irreversible emissions of a permafrost ‘tipping point’ Guest post: Could climate change and deforestation spark Amazon ‘dieback’? Guest post: How close is the West Antarctic ice sheet to a ‘tipping point’?

In short, what happens in the Arctic does not stay in the Arctic.

Permafrost and the global climate
Permafrost is ground that has been frozen for at least two consecutive years.
Its thickness ranges from less than one metre to more than a kilometre.
Typically, it sits beneath an “active layer” that thaws and refreezes every year.
A warming climate puts this perennially frozen ground at risk. When temperatures rise, permafrost thaws – it does not melt.
There is a simple analogy: compare what happens to an ice cube and a frozen chicken when they are taken out of the freezer.
At room temperature, the former will have melted, leaving a small pool of water, but the chicken will have thawed, leaving a raw chicken.
Eventually, that chicken will start to decompose.
This is exactly what happens to permafrost when temperatures increase.
One quarter of the landmass of the northern hemisphere is underlain by permafrost, which acts like Earth’s gigantic freezer and keeps enormous amounts of organic matter frozen.

Global permafrost map, International Permafrost Association. Credit: Brown, J., O.J. Ferrians, Jr., J.A. Heginbottom, and E.S. Melnikov, eds. 1997. Circum-Arctic map of permafrost and ground-ice conditions. Washington, DC: U.S. Geological Survey in Cooperation with the Circum-Pacific Council for Energy and Mineral Resources. Circum-Pacific Map Series CP-45, scale 1:10,000,000, 1 sheet.

 This organic material includes the remnants of dead plants, animals and microbes that accumulated in the soil and were frozen into permafrost thousands of years ago.

Permafrost including ancient bones (left image) and organic material (right image) in the Permafrost tunnel near Fox, Alaska. Credit: C. Schädel

Arctic temperatures have been increasing more than twice as fast as the global average. This has caused permafrost thaw in many locations and triggered newly awakened microbes to decompose the organic material thereby releasing CO2 or methane into the atmosphere.
Both gases are greenhouse gases, but methane is 28-36 times more potent than CO2 over a century. However, there is more CO2 than methane in the atmosphere and methane is oxidised to CO2 on timescales of about a decade. So, it is the change in atmospheric CO2 concentration that really matters for long-term climate change.

Carbon release from permafrost
So, what role will permafrost play in future carbon emissions? And is there a tipping point that could trigger rapid thaw?
Scientists estimate that there is about twice as much carbon stored in permafrost as circulating in the atmosphere. This is approximately 1460bn-1600bn tonnes of carbon.
Most of it is currently frozen and preserved, but if even a small fraction is released into the atmosphere, the emissions would likely be large – potentially similar in magnitude to carbon release from other environmental fluxes, such as deforestation.
This would still be about one order of magnitude smaller than emissions from fossil-fuel burning by the end of this century. Nevertheless, every additional molecule of CO2 or methane added to the atmosphere accelerates climate change and affects the whole planet and its climate.

Collapsing permafrost with large ice volume. Credit: A. Balser

To our current knowledge, carbon release from permafrost is a gradual and sustained process that continually adds carbon to the atmosphere – thus, further reinforcing warming.
Once the organic matter within permafrost decomposes and releases CO2 and methane, there is no getting it back. In this sense, permafrost thaw is irreversible – meeting one of the conditions of the definition of a tipping point.
However, recent research suggests that if temperature rise were to slow and stop, permafrost thaw, too, would slow – and potentially stop, thus, preventing further emissions.
This suggests that permafrost as a whole will not have shifted to a completely new state – as is the case with some tipping points, such as the melting of the Greenland ice sheet. As a result, it would be possible to prevent further emissions were global warming to be halted.
But, as things stand, permafrost thaw has already been observed in many locations in the Arctic. And as the recent special report on the ocean and cryosphere by the Intergovernmental Panel on Climate Change (IPCC) points out, warming this century will cause substantial emissions from permafrost:

“By 2100, near-surface permafrost area will decrease by 2-66% for RCP2.6 and 30–99% for RCP8.5. This could release 10s to 100s of gigatonnes of carbon as CO2 and methane to the atmosphere for RCP8.5, with the potential to accelerate climate change.”

How to add certainty to permafrost carbon release
The ultimate contribution of permafrost carbon to climate change depends on a variety of factors: how much of the carbon will come out as CO2 or methane, for example, and how much can plants and trees offset some of the additional carbon release.
Permafrost degradation can occur as gradual top down thaw or as abrupt collapse of thawing soil. Both processes release carbon to the atmosphere. Gradual top-down thaw is the result of warmer air temperatures causing the soil to thaw from the top down, whereas abrupt thaw occurs suddenly and unpredictably.
Permafrost can contain up to 80% ice. If the ice melts – remember the ice does melt even though the soil does not – the ground suddenly collapses and deep layers get exposed to air temperature.
Collapsing ground can leave the landscape pockmarked by “thermokarst” lakes, filled with meltwater, rain and snow. These wet conditions can promote the release of the more potent greenhouse gas methane.

Thermokarst landscape. Credit: A. Balser

In uplands, natural drainage creates drier soil conditions after permafrost thaw, thereby accelerating organic matter decomposition and releasing large amounts of CO2. The ultimate impact of carbon release from permafrost will be stronger when a larger percentage of the permafrost zone dries out after thaw.
What fraction of the landscape will become wetter or drier after thaw depends on the distribution of ground ice, but current ice measurements are only sporadic and better spatial coverage and more up to date measurements are urgently needed.
Another important factor in the carbon balance of the permafrost zone is the carbon uptake by plants. The question is how much carbon release from thawing permafrost can be offset by increased plant growth? Plants take up carbon from the atmosphere and use it to grow and maintain their metabolism.
Warmer conditions in the Arctic and all its associated changes stimulate plant growth, which means that some of the carbon added to the atmosphere from thawing permafrost is taken up by the boost to plant growth. But it is unclear how much carbon will be offset by plants and it is unclear how sustained this process is.
Improving model projections of permafrost carbon release is crucial in determining the overall impact of thawing permafrost to the global climate. Recent results from the Canadian Arctic show that permafrost thaw is happening a lot earlier than scientists expected given current model projections.
For the moment, models only account for gradual top-down thaw, but recent estimates show that abrupt thaw and collapsing soil could double carbon release from permafrost. One thing is clear: the less temperatures increase in the Arctic, the more permafrost will stay frozen and the more carbon will stay locked up in permafrost.

Methane hydrates
Often mentioned in the same breath as permafrost thaw is the potential danger associated with the breakdown of methane hydrates, also known as “clathrates”. This is methane “ice” that forms at low temperatures and high pressures in continental margin marine sediments or within and beneath permafrost.
Of particular concern are the methane hydrates stored beneath the East Siberian Arctic Shelf (ESAS), a shallow coastal region to the north of Russia. Studies have suggested that thawing permafrost is releasing this methane, letting it bubble up and out of the seawater. This has led to research warning that the escape of large quantities of methane could have “catastrophic consequences for the climate system” and media reports of an impending “methane timebomb”.

Graphic: Carbon Brief. © Esri

In conversation with Dr Carolyn Ruppel, chief scientist for the US Geological Survey’s Gas Hydrates Project, she tells me that methane hydrates trap about one-sixth of Earth’s methane carbon and that some deposits may, in fact, be degrading now as the climate warms. But, she says:

“If the methane released during gas hydrate degradation reaches the ocean, it would mostly be consumed by bacteria in the water column and not reach the atmosphere.  In permafrost areas, degrading gas hydrate is usually deeply buried, so permafrost thaw is the more important contributor to greenhouse gas emissions.”

While there “may be substantial methane leaking from Arctic continental shelves in areas of thawing subsea permafrost”, says Ruppel, “studies have shown that the flux rates are probably overestimated and the most likely source of the leaking methane is not thawing gas hydrates”. She adds:

“Permafrost-associated hydrates are not that widespread and often occur deeper than the shallower sources of methane that can more readily leak into the atmosphere.”

So, the latest research suggests that a methane bomb from thawing hydrates is not on the horizon. However, for permafrost, the science shows that thaw is already underway and the carbon it is releasing will already be contributing to our warming climate.

Links

• Explainer: Nine ‘Tipping Points’ That Could Be Triggered By Climate Change
• Guest Post: Could The Atlantic Overturning Circulation ‘Shut Down’?
• Arctic sea ice summer minimum in 2019 is ‘joint-second lowest’ on record
• Northern hemisphere’s extreme heatwave in 2018 ‘impossible’ without climate change
• Cold-war spy photos reveal ‘doubling’ of glacier ice loss in Himalayas
• Climate change has influenced global drought risk for ‘more than a century’
• State of the climate: How the world warmed in 2019
• Mapped: How every part of the world has warmed – and could continue to warm

Guest Post: How Close Is The West Antarctic Ice Sheet To A ‘Tipping Point’?

Carbon Brief - Christina Hulbe

Satellite image of Pine Island glacier in West Antarctica. Credit: NG Images / Alamy Stock Photo.

Prof Christina Hulbe Prof Christina Hulbe is a geophysicist in the National School of Surveying at the University of Otago in New Zealand.

Between its east and west ice sheets and its peninsula, Antarctica holds enough ice to raise global sea levels by around 60m.
The West Antarctic ice sheet (WAIS) is a relatively small part, containing an amount of ice equivalent to 3.3m of sea level rise. Yet, most of it sits in a precarious position and is considered “theoretically unstable”.
As a result, how the WAIS will change in response to human-caused warming is generally thought to be the largest source of uncertainty for long-term sea level projections.

Tipping points This article is part of a week-long special series on “tipping points”, where a changing climate could push parts of the Earth system into abrupt or irreversible change Explainer: Nine ‘tipping points’ that could be triggered by climate change Guest post: Could the Atlantic Overturning Circulation ‘shut down’? Guest post: The irreversible emissions of a permafrost ‘tipping point’ Guest post: Could climate change and deforestation spark Amazon ‘dieback’? Guest post: How close is the West Antarctic ice sheet to a ‘tipping point’?

The most pressing aspect of this uncertainty is understanding whether instability thresholds of ice have been crossed, whether the retreat we are now measuring is destined to continue, and whether ice that appears unchanging today will remain that way in the future.
The latest research says that the threshold for irreversible loss of the WAIS likely lies between 1.5C and 2C of global average warming above pre-industrial levels. With warming already at around 1.1C and the Paris Agreement aiming to limit warming to 1.5C or “well-below 2C”, the margins for avoiding this threshold are fine indeed.

Marine ice sheet
According to the recent special report on the ocean and cryosphere (SROCC) by the Intergovernmental Panel on Climate Change (IPCC), there are two main controls on how much global sea levels will rise this century: future human-caused greenhouse gas emissions and how warming affects the Antarctic ice sheet. The IPCC says:

“Beyond 2050, uncertainty in climate change induced SLR [sea level rise] increases substantially due to uncertainties in emission scenarios and the associated climate changes, and the response of the Antarctic ice sheet in a warmer world.”

The concern around the vulnerability of the WAIS principally lies in something called “marine ice sheet instability” (MISI) – “marine” because the base of the ice sheet is below sea level and “instability” for the fact that, once it starts, the retreat is self-sustaining.
Ice sheets can be thought of as huge freshwater reservoirs. Snow accumulates in the cold interior, slowly compacts to become glacier ice and then begins to flow like a very thick fluid back toward the ocean.
In some places, the ice reaches the coast and floats on the ocean surface, forming an ice shelf. The boundary between ice resting on the land surface (or the sea floor in the case of a marine ice sheet) is called the “grounding line”. The grounding line is where water stored in the ice sheet returns to the ocean. And when it moves seaward, we say the ice sheet has a positive “mass balance” – that is to say, it is gaining more ice mass than it is losing back to the sea.
But when the grounding line retreats, the balance is negative. A negative ice sheet balance means a positive contribution to the ocean and, thus, to global sea level.

Instability
This basic picture of ice sheet mass balance is all you need to understand why glaciologists are concerned about MISI.
Changes to the ice shelf on the floating side of the grounding line – such as thinning – can cause ice on the grounded side to lift off from the seafloor. As this ice floats, the grounding line will retreat. Because the ice flows more rapidly when it is floating than it does when grounded, the rate of ice flow near the grounding line will increase. Stretching caused by the faster flow becomes a new source of thinning near the grounding line.
This is illustrated in the figure below. As the newly floating ice flows and thins more quickly, it can cause more ice to lift off and float, driving the grounding line back.
In addition, the areas of the ice sheet at risk of MISI have a reverse, or “retrograde” gradient, which means it gets deeper further inland. As the grounding line retreats further into thicker parts of the ice sheet, the flow speeds up, further increasing ice loss. The reverse gradient makes this process self-sustaining as a positive feedback loop – this is what makes MISI an instability.

Illustration of Marine Ice Sheet Instability, or MISI. Thinning of the buttressing ice shelf leads to acceleration of the ice sheet flow and thinning of the marine-terminated ice margin. Because bedrock under the ice sheet is sloping towards ice sheet interior, thinning of the ice causes retreat of the grounding line followed by an increase of the seaward ice flux, further thinning of the ice margin, and further retreat of the grounding line. Credit: IPCC SROCC (2019) Fig CB8.1a

It is not clear yet if the MISI threshold has been crossed anywhere in Antarctica. We do know that grounding lines are retreating along the Amundsen Sea coastline – most spectacularly on the Thwaites Glacier. And the driver for the retreat appears to be relatively warm ocean water – about 2C warmer than the historical average – flowing toward the grounding line and causing stronger than usual melting.

Graphic: Carbon Brief. Credit: Quantarctica/Norwegian Polar Institute.

If the instability has not started and if the ocean warming stops, then the grounding line should find a new balancing point at a new location. But if it has started, then the retreat will continue no matter what happens next.

Faster flow
Even if the threshold has been crossed – or even if it is crossed in the future – the retreat can proceed at different rates depending on how hard we were “pushing” when it started.
Here’s how that works. The instability depends on a balance of forces within the ice sheet. A force due to gravity causes the ice to flow at a speed that depends in part on its thickness and its surface slope.
A larger melt rate on the floating side and faster flow across the grounding line will draw down the surface of the ice more quickly than smaller rates will. The faster draw-down generates a steeper surface slope and, thus, faster flow and faster retreat.

Pine Island Glacier ice shelf rift. Credit: NASA Image Collection / Alamy Stock Photo.

A modelling study of this feedback, published last year, found that when MISI started with a larger push (a larger melt rate), it proceeded more quickly than when it started with a smaller push, even after the extra melting was removed.
This means that even if MISI is invoked, cutting global emissions and slowing warming will give more time to get ready for its consequences.

Ice cliffs
There appears to be a second source of instability for marine ice sheets – one that comes into play if the ice shelves are lost entirely.
Some of the most spectacular images of glacier change are of iceberg calving – in other words, breaking off – from the heavily crevassed fronts of marine-terminating glaciers.
This calving is caused by melting of the underside of the ice shelf, as well as “hydro-fracturing” – where meltwater forming on the surface of the ice shelf seeps into the ice and causes cracking – or a combination of the two.
How quickly calving happens depends on the height of the ice cliff face above the waterline – the higher the cliff stands above the water, the larger the calving rate.
As is the case for MISI, the declining gradient of seafloor beneath the WAIS means that as the ice cliff retreats into thicker ice it will continue to expose an ever-higher cliff to the ocean and the calving rate must increase.
This process, illustrated below, is called “marine ice cliff instability” (MICI). The theory suggests that where the height of a glacier face exceeds around 100m above the ocean surface, the cliff will be too tall to support its own weight. It will, therefore, inevitably collapse, exposing a similarly tall cliff face behind it, which, too, will collapse. And so on.
The IPCC’s SROCC says that “Thwaites Glacier is particularly important because it extends into the interior of the WAIS, where the bed is >2000m below sea level in places”. (Although, the SROCC also notes that while MISI requires a retrograde bed slope to occur, MICI could even happen on a flat or seaward-inclined bed.)
This recently identified process is not as well studied as MISI, but this is sure to change in the years ahead, as scientists continue to observe fast-changing systems such as the Thwaites Glacier.

Illustration of Marine Ice Cliff Instability. If the cliff is tall enough (at least ~800m of total ice thickness, or about 100m of ice above the water line), the stresses at the cliff face exceed the strength of the ice, and the cliff fails structurally in repeated calving events. Credit: IPCC SROCC (2019) Fig CB8.1b

A Nature study in 2016 on MICI concluded that Antarctica “has the potential to contribute more than a metre of sea level rise by 2100 and more than 15 metres by 2500”. More recent research concluded this is likely to be an overestimate, but noted it is not yet clear what role MICI could play this century. Another study has also suggested that rapid loss of ice through MICI may be mitigated by a slower loss of the ice shelves that hold the glaciers back.

Threshold close
Late last year, a large team of modellers assessed different studies of ice sheet response to the Paris climate target to keep global average warming “well below” 2C.
The models all point in the same direction. Namely, that the threshold for irreversible ice loss in both the Greenland ice sheet and the WAIS is somewhere between 1.5C and 2C global average warming. And we are already at a bit more than 1C warming right now.
This 1.5-2C window is key for the “survival of Antarctic ice shelves”, the review paper explained, and thus their “buttressing” effect on the glaciers they hold back.

Glossary RCP2.6: The RCPs (Representative Concentration Pathways) are scenarios of future concentrations of greenhouse gases and other forcings. RCP2.6 (also sometimes referred to as “RCP3-PD”) is a “peak and decline” scenario where stringent mitigation… Read More

Another threshold may lie between 2C and 2.7C, the authors added. Reaching this level of global temperature rise could trigger the “activation of several larger systems, such as the Ross and Ronne-Filchner drainage basins, and onset of much larger SLR contributions”.
The Ross and Ronne-Filchner are the two largest ice shelves in Antarctica. These could be substantially reduced “within 100–300 years”, another study says, in scenarios where global emissions exceed the RCP2.6 scenario. This emissions pathway is generally considered to be consistent with limiting warming to 2C.
These findings imply that preventing substantial Antarctic ice loss relies on limiting global emissions to – or below – RCP2.6. As the paper concludes: “Crossing these thresholds implies commitment to large ice-sheet changes and SLR that may take thousands of years to be fully realised and be irreversible on longer timescales.”

Links

• Explainer: Nine ‘Tipping Points’ That Could Be Triggered By Climate Change
• Arctic sea ice summer minimum in 2019 is ‘joint-second lowest’ on record
• Northern hemisphere’s extreme heatwave in 2018 ‘impossible’ without climate change
• Cold-war spy photos reveal ‘doubling’ of glacier ice loss in Himalayas
• Climate change has influenced global drought risk for ‘more than a century’
• State of the climate: How the world warmed in 2019
• Mapped: How every part of the world has warmed – and could continue to warm

Guest Post: Could The Atlantic Overturning Circulation ‘Shut Down’?

Carbon Brief - Richard Wood | Laura Jackson

An argo float measures ocean temperatures. Credit: Argo program, Germany/Ifremer.

Dr Richard Wood leads the Climate, Cryosphere and Oceans Group at the UK’s Met Office Hadley Centre. Dr Laura Jackson is a scientist with the Group. Her resarch includes understanding global ocean circulation.

Generally, we think of climate change as a gradual process: the more greenhouse gases that humans emit, the more the climate will change. But are there any “points of no return” that commit us to irreversible change?

The “Atlantic Meridional Overturning Circulation”, known as “AMOC”, is one of the major current systems in the world’s oceans and plays a crucial role in regulating climate.
It is driven by a delicate balance of ocean temperatures and salinity, which is at risk from being upset by a warming climate.
The latest research suggests that AMOC is very likely to weaken this century, but a collapse is very unlikely. However, scientists are some way from being able to define exactly how much warming might push AMOC past a tipping point.

Overturning
The figure below shows an illustration of the AMOC. In the North Atlantic, warm water from the subtropics travels northwards near the surface and cold – and, hence, more dense – water is travelling southwards at depth, typically 2–4km below the surface.
In the north, the warm surface water is cooled by the overlying atmosphere, converted to cold, dense water, and sinks to supply the deep, southward branch. Elsewhere, the cold water upwells and is warmed, re-supplying the upper, warm branch and completing the circuit.

Schematic of the AMOC. The red pathways show warmer water nearer the surface, while the purple pathways show colder, more dense water moving at depth. Credit: Met Office

Could the AMOC collapse?

Tipping points This article is part of a week-long special series on “tipping points”, where a changing climate could push parts of the Earth system into abrupt or irreversible change Explainer: Nine ‘tipping points’ that could be triggered by climate change Guest post: Could the Atlantic Overturning Circulation ‘shut down’? Guest post: The irreversible emissions of a permafrost ‘tipping point’ Guest post: Could climate change and deforestation spark Amazon ‘dieback’? Guest post: How close is the West Antarctic ice sheet to a ‘tipping point’?

The AMOC is vulnerable to climate change. As the atmosphere warms due to increasing greenhouse gases, the ability of the ocean to lose heat from the North Atlantic surface is diminished and one of the driving factors of the AMOC is weakened.
Climate-model projections of global warming this century consistently point to a weakening of the AMOC. The most recent assessments of the Intergovernmental Panel on Climate Change (IPCC) – the fifth assessment report (AR5) and special report on oceans and cryosphere in a changing climate (SROCC) – both conclude that the AMOC is “very likely” to weaken over the 21st century.
Such a weakening would have a cooling effect on climate around the North Atlantic region, as the northward heat supply is slowed down. This effect is included in the climate projections, but the direct warming effect from rising concentrations of greenhouse gases is stronger, so the net result is still warming over land regions.
But more dramatic changes are theoretically possible. A “tipping point” may exist beyond which the current strong AMOC becomes unsustainable.
Evidence for this goes back to a seminal paper published in 1961 by one of the fathers of modern oceanography, Henry Stommel. Stommel realised that the AMOC is a kind of competition between the effects of temperature and salinity, both of which influence the density of seawater.

The figure below illustrates the different possible AMOC states. In today’s climate, temperature dominates and the cold, dense high latitude water drives a strong AMOC (red curve). But in other climate states it is possible for fresh water (from rainfall or ice melt) to freshen – and so lighten – the high-latitude water; in this case, the water is not dense enough to drive the AMOC, which collapses (blue curve).
If the freshwater input to the Atlantic were strong enough – from rapid melting of the Greenland ice sheet, for example – the blue dot would move to the right in the figure. According to Stommel’s model, at some point the strong AMOC state (red) becomes unsustainable and the AMOC collapses to the “off” state (blue). Then, even if the driving climate change were later reversed (the blue dot moving back to the left on the figure), the AMOC would stay on the blue curve and would not switch back on again until the climate had overshot the present day conditions in the opposite direction. This phenomenon is known as “hysteresis”.

Tipping points and hysteresis of the AMOC in Stommel’s simple model. Possible states of the AMOC depend on the amount of freshwater input to the Atlantic Ocean (x-axis). AMOC strength is shown on the y-axis. [Note that both are measured in Sverdrups (Sv), where 1 Sv denotes one million cubic metres of water transported per second.] When there is low freshwater input, temperature dominates the flow and only a strong AMOC is possible (red curve). For high freshwater input, only a collapsed state is possible (blue curve). In between, both states are possible. If the freshwater input were to increase beyond a critical value (the tipping point), the AMOC would collapse. Then, even if the freshwater input were returned to its original state, the AMOC would remain off. Credit: Met Office.

Long-term projections
Stommel’s idea has evolved over the years, but the fundamental insight is still relevant. There is evidence that AMOC changes may have played a role in some major climate shifts of the past – most recently around 8,200 years ago as the world was emerging from the last ice age.
At that time, a huge lake in northwest Canada was being held back by an ice wall. As temperatures warmed the ice wall collapsed, depositing the fresh water from the lake into the North Atlantic and interrupting the AMOC. A major cooling at this time can be seen in palaeoclimatic records across North America, Greenland and Europe.
Comprehensive climate models generally do not project a complete shutdown of the AMOC in the 21st century, but recently models have been run further into the future. Under scenarios of continued high greenhouse gas concentrations, a number of models project an effective AMOC shutdown by 2300.
Model projections of the future AMOC do range widely, though. As a result, on the question of what level of global warming would result in an AMOC shutdown, it is unlikely that the scientific community will see any convergence in the near future.
While the fundamental mechanism that destabilises the AMOC in Stommel’s original model appears to be important in climate models, there are other processes that are trying to stabilise the AMOC. Many of these processes are difficult to model quantitatively, especially with the limited resolution that is possible with current computing power. So our AMOC projections will continue to be subject to quite some uncertainty for some time to come.
Taking all the evidence into account, the IPCC’s AR5 and SROCC concluded that an AMOC collapse before 2100 was “very unlikely” (pdf). However, the impacts of passing an AMOC tipping point would be huge, so it is best viewed as a “low probability, high impact” scenario.

What would be the impacts of a collapse?
Climate models can be used to assess the impact on climate if the AMOC were to shut down completely. By adding large amounts of fresh water to the North Atlantic in a model, scientists artificially lighten the cold, dense water that forms the lower branch of the loop. This stops the AMOC and we can then look at the impact on climate.
The figure below illustrates the changes that result in one such experiment. Shutdown of the AMOC results in a cooling (blue shading) of the whole northern hemisphere, particularly the regions closest to the zone of North Atlantic heat loss (the “radiator” of the North Atlantic central heating system). In these regions the cooling exceeds the projected warming due to greenhouse gases, so a complete shutdown in the 21st century, while very unlikely, could result in a net cooling in regions such as western Europe.

Modelled change in surface temperature (C) following an artificially induced collapse of the AMOC. Shading indicates cooling (blue) or warming (orange and red). Reprinted by permission from Springer. Jackson et al. (2015) Global and European climate impacts of a slowdown of the AMOC in a high resolution GCM, Climate Dynamics.

Other impacts include major shifts in rainfall patterns, increases in winter storms over Europe and a sea level rise of up to 50cm around the North Atlantic basin. In many regions these effects would exacerbate the trends due to global warming. While such model experiments are artificial “what if?” scenarios, they illustrate the magnitude of the changes that could result from an AMOC collapse. The impacts on agriculture, wildlife, transport, energy demand and coastal infrastructure would be complex, but we can be certain that there would be major socioeconomic consequences. For example, one study showed a 50% reduction in grass productivity in major grazing regions of the western UK and Ireland.

What can be done about the risk of a collapse?
As explained above, scientists are some way from being able to confidently define a level of global warming at which the AMOC would be at risk of crossing a tipping point.
However, it may be possible to manage the risk of AMOC collapse, even without knowing how likely it is.
To take a domestic analogy: I know that it is possible, but unlikely, that my house will burn down – it is a low-probability, high-impact event. I don’t have much idea of how probable it is that I will have a fire, but I can manage the risk anyway by getting the electrical wiring checked and by installing smoke alarms. The wiring check reduces the chance of a fire, while the smoke alarm gives me early warning if a fire starts so that the impact can be reduced – by evacuating the house and calling the fire brigade.
Recently, along with colleagues at the University of Exeter, we have been exploring the possibility of developing an early-warning system for AMOC tipping.
Using a simple model, we have shown that the way the salinities of the subtropical and subpolar Atlantic evolve over time can give an early indication if the AMOC is on the path to a collapse, possibly decades before any major weakening has been seen in the AMOC itself.
It is early days for this research, but by monitoring such an indicator it may be possible to give more time to prepare for the consequences of an AMOC collapse, or to adopt more aggressive climate change mitigation measures to get the AMOC onto a more stable pathway.

Outstanding questions
As the world gets to grips with the challenges of meeting the targets of the Paris climate accord, interest is increasing in climate pathways that temporarily overshoot the final target level. It is important that such overshoots do not cross any irreversible thresholds on the way to the final destination, so research on tipping points needs to link theoretical results to these more practical questions.
Much of the modelling of AMOC tipping points to date has used idealised scenarios of freshwater input to the North Atlantic. This is relevant to some past AMOC changes, but to model future climate change we need to understand what happens when warming and freshening are taking place together.
This is a more challenging problem because the number of relevant processes and feedbacks is increased. Some of these processes operate at small scales that models struggle to resolve with current computing power. Improving the modelling of key AMOC processes needs patience and long-term commitment, but will eventually pay dividends in more confident AMOC predictions.
Research on early warning of AMOC collapse is in its infancy, but may be a fruitful way to respond to the risk. One thing is for sure: early warning will require continuous observations of key aspects of the AMOC.
AMOC monitoring entered a new era in 2004 with RAPID-MOCHA, an array of moored instruments that spans the width of the Atlantic at latitude 26.5 degrees north and provides continuous monitoring of the AMOC. Before this there had only been five snapshots of the circulation spread over 47 years.
Results have already changed our understanding of how the AMOC varies in time: for example, an unexpected dip in the AMOC – observed in Autumn 2009 – is thought to have played a role in the unusually cold European winters of 2009-10 and 2010-11.
More recently, a similar monitoring array has been installed further north in the subpolar Atlantic. Along with continuous measurements of temperature and salinity from drifting Argo floats, oceanographers now have an unprecedented database to study this crucial element of our climate system and give the world a chance to prepare for any nasty surprises.

Links

• Explainer: Nine ‘Tipping Points’ That Could Be Triggered By Climate Change
• Guest Post: How Close Is The West Antarctic Ice Sheet To A ‘Tipping Point’?
• Arctic sea ice summer minimum in 2019 is ‘joint-second lowest’ on record
• Northern hemisphere’s extreme heatwave in 2018 ‘impossible’ without climate change
• Cold-war spy photos reveal ‘doubling’ of glacier ice loss in Himalayas
• Climate change has influenced global drought risk for ‘more than a century’
• State of the climate: How the world warmed in 2019
• Mapped: How every part of the world has warmed – and could continue to warm