The persistent march of a warming climate is seen across a multitude of continuous, incremental changes. CO2 levels in the atmosphere. Ocean heat content. Global sea level rise. Each creeps up year after year, fuelled by human-caused greenhouse gas emissions.
These "tipping points" are thresholds where a tiny change could push a system into a completely new state.
Imagine a child pushing themselves from the top of a playground slide. There is a point beyond which it is too late for the child to stop themselves sliding down. Pass this threshold and the child continues inevitably towards a different state - at the bottom of the slide rather than the top.
In this article, Carbon Brief explores nine key tipping points across the Earth system, from collapsing ice sheets and thawing permafrost, to shifting monsoons and forest dieback.
And, over the coming week, Carbon Brief will be publishing guest articles from experts in four of the tipping points covered here.
- Tipping towers
- Irreversible change?
- 1. Shutdown of the Atlantic Meridional Overturning Circulation
- 2. West Antarctic ice sheet disintegration
- 3. Amazon rainforest dieback
- 4. West African monsoon shift
- 5. Permafrost and methane hydrates
- 6. Coral reef die-off
- 7. Indian monsoon shift
- 8. Greenland ice sheet disintegration
- 9. Boreal forest shift
- Other tipping points
A glance at the news media on any given week will likely highlight all sorts of climate change impacts. From declining Arctic sea ice and record-breaking heatwaves to melting glaciers and worsening droughts, the increase in global average temperature is being felt around the world.
Broadly, these impacts reflect gradual changes caused by a climate that is steadily warming. Scientists have estimated, for example, that for every tonne of CO2 emitted into the atmosphere, summer sea ice cover in the Arctic shrinks by three square metres.
However, there are parts of the Earth system that have the potential to change abruptly in response to warming. These systems have "tipping points", explains Prof Tim Lenton, director of the Global Systems Institute at the University of Exeter. He tells Carbon Brief:
"A climate tipping point, or any tipping point in any complex system, is where a small change makes a big difference and changes the state or the fate of a system."
So, rather than a bit more warming causing slightly hotter heatwaves or more melting of glaciers, it causes a dramatic shift to an entire system.
That extra bit of warming would be, as the saying goes, the straw that breaks the camel's back. Or, to use a more animal-friendly metaphor, a game of Jenga - where a particular component within the Earth system, such as an ice sheet, circulation pattern or ecosystem, is represented by the tower of blocks.
The gradual increase in global temperature sees block after block removed from the tower and placed on top. As time goes on, the tower becomes more and more misshapen and unstable. At some point, the tower can no longer support itself and it tips over.
In the game of Jenga, the tower collapses in a split second. For a component of the Earth system, the shift to one physical state to another may take many decades or centuries. But the feature they have in common is that once the collapse has started, it is virtually impossible to stop.
It is worth noting that a tipping point can be caused by natural fluctuations in the climate as well as by an external forcing, such as global warming. These are called "noise-induced" tipping points and include, for example, periods of abrupt change during the last ice age called "Dansgaard-Oeschger (D-O) events".
Natural fluctuations can also be the final nudge for a tipping point pushed to the brink by human-caused climate change, says Prof Mat Collins, joint Met Office chair in climate change at the University of Exeter and coordinating lead author on the "Extremes, Abrupt Changes and Managing Risks" chapter of the Intergovernmental Panel on Climate Change (IPCC) special report on the ocean and cryosphere in a changing climate ("SROCC"). He tells Carbon Brief:
"As you approach the edge of the cliff, a small random gust of wind is more likely to blow you over the edge. This is more prevalent in biological systems. A strong marine heatwave in one year can wipe out a large coral ecosystem for many decades - or, perhaps, even permanently. The heatwave is a result of natural fluctuations, but becomes more likely and more extreme with an increasing average trend."
The theory of potentially abrupt changes in the Earth system is not new. In a Nature commentary in 1987, for example, Prof Wally Broecker of Columbia University - who died in 2019 - warned that palaeoclimate data suggests the "Earth's climate does not respond to forcing in a smooth and gradual way. Rather, it responds in sharp jumps which involve large-scale reorganisation of Earth's system".
The term "tipping point" itself was popularised by journalist and author Malcolm Gladwell in his book of the same name, published in 2000. Gladwell describes tipping points as "the moment of critical mass, the threshold, the boiling point", and explores examples throughout human society:
"There was a tipping point for [declining] violent crime in New York in the early 1990s, and a tipping point for the reemergence of Hush Puppies, just as there is a tipping point for the introduction of any new technology."
In the years since, the term has been used increasingly in scientific circles. However, this has not been without controversy. There are, for example, many different views on how the term should be defined and used, explains Collins:
"There has been an intensive debate in the field of tipping points, abrupt change and irreversibility about the definitions of these terms. They range from the very mathematical to those which are intended to be understood by policymakers."
According to a 2009 paper on the use of the term "tipping points" in climate science and the media, a presentation (pdf) in 2005 by Dr James Hansen of Columbia University's Earth Institute helped "initiate a tipping point trend in climate change communication that was quickly reflected in public debate".
In Hansen's talk - a tribute to scientist Prof Charles Keeling, given at the American Geophysical Union (AGU) Fall Meeting - Hansen warned that "we are on the precipice of climate system tipping points beyond which there is no redemption".
By its Fall Meeting of 2008, the AGU had an entire half-day session dedicated to climate tipping points. A Science briefing about the meeting declared that "tipping points, once considered too alarmist for proper scientific circles, have entered the climate change mainstream".
A year earlier, the IPCC had published its fourth assessment report ("AR4", pdf). This was the first of its assessment reports to use the term "tipping point" - though the third assessment report ("TAR", pdf) in 2001 had discussed "large-scale discontinuities" that have the "potential to trigger large-scale changes in Earth systems". Indeed, speaking to a journalist at the time, chapter lead author Prof Hans Joachim Schellnhuber explained that "these are, more or less, tipping points".
"Technically, an abrupt climate change occurs when the climate system is forced to cross some threshold, triggering a transition to a new state at a rate determined by the climate system itself and faster than the cause."
The IPCC's definition in its fifth assessment report ("AR5", pdf), published in 2013-14, gives more detail:
"We define abrupt climate change as a large-scale change in the climate system that takes place over a few decades or less, persists (or is anticipated to persist) for at least a few decades, and causes substantial disruptions in human and natural systems."
Typically, definitions for a tipping point fall into two categories, says Dr Ricarda Winkelmann, junior professor of climate system analysis at the Potsdam Institute for Climate Impact Research (PIK). She explains to Carbon Brief:
"One is simply that one vital part of the climate system shows some kind of threshold behaviour and that means that a small perturbation around that element can cause a huge qualitative change. And then there's another definition that actually says there needs to be a positive feedback mechanism associated with the element. So that means there is something that's self-reinforcing and then that could lead to irreversible changes as well."
Passing an irreversible tipping point would mean a system would not revert to its original state even if the forcing lessens or reverses, explains Dr Richard Wood, who leads the Climate, Cryosphere and Oceans group in the Met Office Hadley Centre. He tells Carbon Brief:
"In some cases, there is evidence that once the system has jumped to a different state, then if you remove the climate forcing, the climate system doesn't just jump back to the original state - it stays in its changed state for some considerable time, or possibly even permanently."
This is known as "hysteresis". It occurs when a system undergoes a "bifurcation" - which means to divide or fork into two branches - and it is subsequently difficult, if not impossible, for the system to revert to its previous state.
For example, part of the reason that Greenland has an ice sheet today is that it has had that ice sheet for hundreds of thousands of years. If the Greenland ice sheet were to pass a tipping point that led to its disintegration, simply reducing emissions and lowering global temperatures to pre-industrial levels would not bring it back again. It would probably require another ice age to achieve that.
Similarly, returning to the Jenga analogy, the amount of energy required to rebuild the tower once it collapsed is significantly greater than the energy used to tip it over.
The extent to which the tipping points considered in this article are irreversible is just one of the many uncertainties that researchers are still exploring. Nonetheless, each of the nine - explained below - are examples of where seemingly small changes have the collective potential to pack a potent punch.
Shutdown of the Atlantic Meridional Overturning Circulation
The Atlantic Meridional Overturning Circulation (AMOC) is a system of currents in the Atlantic Ocean that brings warm water up to Europe from the tropics and beyond.
The illustration below shows the two main features of the AMOC: the first is the flow of warm, salty water in the upper layers of the ocean northwards from the Gulf of Mexico (red line). This is made up of the "Gulf Stream" to the south and the "North Atlantic Current" further north. The second is the cooling of water in the high latitudes of the Atlantic, which makes the water more dense. This denser water then sinks and returns southwards towards the equator at much deeper depths (blue line).
The AMOC forms part of a wider network of global ocean circulation patterns that transports heat all around the world. It is "driven by deep water formation", explains Prof Stefan Rahmstorf, professor of physics of the oceans at Potsdam University and co-chair of earth system analysis at PIK. This is "the sinking of dense, therefore heavy, water in the high latitudes of the North Atlantic", he explains to Carbon Brief.
Climate change affects this process by diluting the salty sea water with freshwater and by warming it up, he says:
"The dilution happens through increased rainfall and also melting of continental ice in the vicinity of mainly the Greenland ice sheet. And that makes the water lighter and, therefore, unable to sink - or at least less able to sink - which, basically, slows down that whole engine of the global overturning circulation."
Recent research suggests that the AMOC has already weakened by around 15% since the middle of the 20th century. This is in line with projections by climate models, says Dr Richard Wood. However, the question remains at what point a weakening tips over into a complete shutdown, he explains:
"Perhaps a much less likely, but larger cause for concern is whether there's a threshold beyond which the AMOC becomes unsustainable and at that point - if you pass that threshold - then over some period of time, the AMOC might reduce to zero or even potentially a reversed circulation. And that would have big impacts on the climate of, well, the whole northern hemisphere, but particularly Europe."
This shutdown could happen because the AMOC is a self-reinforcing system, explains Rahmstorf:
"The circulation itself brings salty water into the high-latitude Atlantic and the salty water increases the density. And so we can say the water is able to sink because it is salty and it is salty because there is this circulation. So it's like a self-reinforcing system."
Such a system can only be pushed "up to a limit", says Rahmstorf, after which the self-reinforcing system actually works to further weaken the circulation. Too much freshwater in the North Atlantic slows the circulation, preventing it from pulling salty water up from the south. Thus, the North Atlantic freshens even more and the circulation weakens further - and so on. It "really is an on-off system", he adds.
There is still a lot of uncertainty about where exactly this tipping point is, says Rahmstorf. To the extent that "nobody really knows", he adds:
"But, I would say, most people think that to trigger a real shutdown would require substantial global warming - like 3C or 4C [above pre-industrial levels]. And we could pretty well minimise this risk by limiting the warming to below 2C. So, if we actually take the Paris Agreement seriously, then I would feel relatively relaxed about the risk of a shutdown. But if we continue on the current path and heading for three or more degrees, then this becomes a really serious concern."
(According to Climate Action Tracker, current global climate policies put the world on track for around 3C of warming.)
And it is "important to emphasise that climate models are not suggesting a complete shutdown of the AMOC in the next 100 years or so", adds Wood: "We're looking at what we call a 'low-probability, high-impact' event".
The IPCC's special report on 1.5C of warming, for example, concludes that while "it is very likely that the AMOC will weaken over the 21st century", there is "no evidence indicating significantly different amplitudes of AMOC weakening for 1.5C versus 2C of global warming, or of a shutdown of the AMOC at these global temperature thresholds".
Were the AMOC to cross a tipping point, models suggest it would trigger a "quick decline that takes decades and then a kind of slower decline which might take even hundreds of years", says Wood.
This would be "practically irreversible" on human timescales, notes Rahmstorf:
"Depending on the exact nature of the stability of the circulation, it could be shutdown basically indefinitely for thousands of years into a new stable shutdown state. Or it could eventually recover - both things we observe in different models. But, on a timescale if you're just interested in what happens in the next 200-300 years or so, that doesn't actually make a difference because it does stay off then once it dies for quite a long time."
This "shutdown" state is an example of hysteresis, explains Wood in the video below. It means that once you go over the tipping point, even if global warming is stopped or reversed, the AMOC does not necessarily switch back on again immediately.
As the AMOC plays a crucial role in bringing heat up from the tropics, a shutdown would cause "widespread cooling around the whole of the northern hemisphere, but particularly around western Europe and the east coast of North America", says Wood. This could be in the order of "several degrees, possibly 5C", he adds.
This cooling would have knock-on impacts for rainfall patterns as there would be less evaporation from the North Atlantic, says Wood. This could either offset or magnify the changes caused by global warming, he says:
"In northern parts of Europe, we might expect from global warming to see wetter winters and then the drying would compensate. In other regions, more in southern Europe, where we would already be expected to see a drying signal from the global-warming signal, so paradoxically, the cooling would give you a further drying. So it would actually reinforce the climate-change signal."
The knock-on impacts would be considerable. For example, a recent study in the new journal Nature Food suggests that an AMOC shutdown would cause "widespread cessation of arable farming" in the island of Great Britain with "losses of agricultural output that are an order of magnitude larger than the impacts of climate change without an AMOC collapse".
In addition, there will be implications for the ocean itself, notes Rahmstorf:
"The whole North Atlantic ecosystem is adapted to the existence of this overturning circulation, which really sets the conditions - the seasonal cycle, the temperature, the nutrient conditions - in the North Atlantic, and so the intricate web of the Atlantic ecosystem will be substantially disrupted if allow such a massive change in the ocean circulation to happen."
Finally, research suggests that the collapse of the AMOC could itself trigger other tipping points. As the SROCC explains:
"For example, a collapse of the AMOC may induce causal interactions like changes in ENSO [El Niño-Southern Oscillation] characteristics, dieback of the Amazon rainforest and shrinking of the West Antarctic Ice Sheet due to seesaw effect, ITCZ [Intertropical Convergence Zone] southern migration and large warming of the Southern Ocean."
However, the SROCC notes that "such a worst-case scenario remains very poorly constrained" as a result of the large uncertainties around how systems such as AMOC will respond to warming.
West Antarctic ice sheet disintegration
The West Antarctic Ice Sheet (WAIS) is one of three regions making up Antarctica. The other two are East Antarctica and the Antarctic Peninsula, with the Transantarctic Mountain range dividing east from west.
Although much smaller than its neighbour to the east, the WAIS still holds enough ice to raise global sea levels by around 3.3 metres. Therefore, even a partial loss of its ice would be enough to change coastlines around the world dramatically.
The long-term stability of the WAIS is of particular concern because it is a "marine-based" ice sheet. As the IPCC's special report on the ocean and cryosphere in a changing climate ("SROCC") explains, this means that it sits "upon bedrock that largely lies below sea level and [is] in contact with ocean heat, making [it] vulnerable to rapid and irreversible ice loss".
The map below shows the elevation of Antarctic bedrock; the greens, yellows and reds indicate areas above sea level, while the whites and blues show areas below it - by as much as 2.5km. The WAIS itself is more than 4km thick in places.
Acting under the force of gravity, the ice of the WAIS gradually flows out from its interior towards the coast and into the Southern Ocean. Fresh snowfall on the interior of the ice sheet replenishes the lost ice. If the ice sheet loses more ice to the ocean than it gains in snow, it adds to global sea levels.
For example, analysis published in Nature in 2018 showed that the rate of ice loss from the WAIS had tripled from 53bn tonnes a year during 1992-97 to 159bn tonnes a year in 2012-2017.
Where the ice meets the ocean, floating ice shelves form. These ice shelves have a "buttressing" effect, holding back the glaciers on land that flow into them.
Sitting on the ocean surface, ice shelves are at risk of melting from above and below from warm air and water, respectively. In the Antarctic Peninsula, for example, research has shown that the collapse of the Larsen B ice shelf in 2002 was primarily driven by warm air temperatures. While the Larsen C ice shelf, which is "thinning rapidly", is being melted from above and below.
Because ice shelves float on water, their collapse does not directly cause sea level rise. But thinning and/or collapse of the WAIS's ice shelves could trigger a positive feedback loop that sees rapid and irreversible loss of land ice into the ocean - which would add to sea levels. This theory is called "marine ice sheet instability" (MISI).
The illustration below shows how it works. As an ice shelf thins, more ice lifts off the seafloor and begins to float. This pushes back (see blue arrows) the "grounding line" - the transition point between grounded and floating ice (indicated by dashed lines). Floating ice flows more rapidly than grounded ice and so the rate of ice flow near the grounding line increases (black arrows). Faster flow means thinning, which may in turn cause more ice to lift off and float. And because greater thickness also causes the ice to flow faster, grounding-line retreat into deeper sections of the ice sheet can also produce faster flow.
What makes this a positive feedback loop is the retrograde slope of the WAIS's bedrock. Not only is much of the bedrock beneath the ice sheet below sea level, large portions of it slope downwards away from the coast. This means that once ice sheet retreat reaches this point, it is self-sustaining.
(There is also an additional feedback loop mechanism that could further endanger the WAIS. This is called Marine Ice Cliff Instability (MICI), which would see towering cliffs of glacier ice collapse into the ocean under their own weight. The theory is still under debate.)
In terms of tipping-point behaviour, most research has focused on the Amundsen Sea sector of the WAIS into which six glaciers drain. As far back at the 1980s, this region was identified as the "weak underbelly" of the WAIS. Here, the grounded ice flows directly into the ocean with "no significant ice shelf barrier" to hold it back.
Antarctica's contribution to global sea levels is currently dominated by ice loss from Amundsen sea sector glaciers. Sections of the Thwaites and Pine Island glaciers, for example, are thinning at rates of 49 and 45cm per year, respectively, on average over 1992-2017.
Research indicates that glaciers in this sector are "undergoing a marine ice sheet instability that will significantly contribute to sea level rise in decades to centuries to come".
For example, model simulations in a 2014 study in Science have suggested that the "process of marine ice-sheet destabilisation is already under way on Thwaites Glacier". The study notes:
"Although [ice] losses are likely to be relatively modest over the next century (<0.25 mm/year of sea level equivalent, SLE), rapid collapse (>1 mm/year of SLE) will ensue once the grounding line reaches the basin's deeper regions, which could occur within centuries."
This rapid collapse "would probably spill over to adjacent catchments, undermining much of West Antarctica", the study adds.
The SROCC is also a little more circumspect in its conclusions. It says that rapid mass loss due to glacier flow acceleration in this region "may indicate the beginning of MISI". However, it also notes that "observational data are not yet sufficient to determine whether these changes mark the beginning of irreversible retreat".
Prof Tim Lenton tells Carbon Brief that whether all or part of the WAIS has already passed a tipping point for irreversible loss is "the big concern at the moment" because of the sea level rise it would cause.
Overall, the SROCC assessment of "partial West-Antarctic Ice sheet collapse" is that it is potentially abrupt and would be "irreversible for decades to millennia". It ascribes "low confidence" to a collapse during the 21st century.
A Nature Climate Change review paper published in 2018 concluded that "under sustained warming, a key threshold for survival of Antarctic ice shelves, and thus the stability of the ice sheet, seems to lie between 1.5 and 2C mean annual air temperature above present".
This temperature threshold refers to regional warming in Antarctica, rather than a global average figure. However - as lead author Prof Frank Pattyn explains to Carbon Brief - because the poles warm more quickly than the global average, 2C of warming on Antarctica from present is approximately equivalent to 2C of global warming since pre-industrial levels.
Pattyn, a glaciologist and co-director of the Laboratoire de Glaciologie at the Université libre de Bruxelles, also notes that a tipping point for the WAIS "is not sharply defined". Referring to the different "Representative Concentration Pathway" emissions scenarios, he adds:
"Studies show that under RCP2.6 the ice sheets continue to lose mass but seem stable, while for RCP4.5, in some cases irreversible mass loss is encountered. However, only a few studies consider the full range of RCPs and most of them only compare RCP2.6 to RCP8.5."
Evidence from Earth's distant past also suggests the WAIS has collapsed before. For example, a Nature Geoscience review paper from 2011 notes:
"The palaeo record strongly suggests that the WAIS largely disappeared, perhaps during the past few hundred thousand years and more confidently during the past few million years, in response to warming similar to or less than that projected under business-as-usual CO2 emission scenarios for the next few centuries."
Amazon rainforest dieback
The Amazon rainforest is the largest rainforest in the world. Spanning nine countries in South America, it is twice the size of India. The lush vegetation is a haven for millions of species of plants, insects, birds and animals.
As its name suggests, a rainforest is sustained by very wet conditions. But the forest itself plays a critical role in the local climate. As the forest is saturated with heavy rains, much of this moisture is returned to the atmosphere through evaporation. In addition, transpiration of moisture from plant leaves transfers water from the soil into the atmosphere. These two processes combined are called "evapotranspiration".
These processes keep the atmosphere moist, but also help drive convection - strong upward motion of the air - which, ultimately, creates clouds and more rainfall. Research published in the 1970s showed that the Amazon generates around half of its own rainfall.
The result is that either reducing the amount of rainfall or the amount of forest can shift the climate into a drier state that cannot support a rainforest. There are three potential causes of this, explains Prof Richard Betts, head of climate impacts at the Met Office Hadley Centre and chair of climate impacts at the University of Exeter.
The first is a decline in rainfall in response to a warming climate. Model projections suggest this would be a result of "particular patterns of sea surface temperature (SST) change in the tropical Atlantic and Pacific", says Betts, but there is a lot of variation between models as to how strong the impact would be on the Amazon. The second is a response to reduced transpiration in response to higher CO2, Betts says:
"Microscopic pores in plant leaves open less widely under higher CO2. So the plants lose less water and less transpiration means less water going back into the atmosphere."
Finally, the third cause would be the direct impact of deforestation - fewer trees mean less evapotranspiration and less moisture entering the atmosphere.
There is only so much drying the Amazon could tolerate before the rainforest would no longer be able to support itself. Beyond this point, the forest would see widespread "dieback" and transition to savannah - a drier ecosystem dominated by open grasslands with few trees.
In the clip below, Betts summarises how the Amazon could be pushed "beyond the point of no return".
Dr David Lapola - a research scientist at the University of Campinas in Brazil - cautions that, while it is "reasonable to think that deforestation and fire could of course contribute to reach that [Amazon dieback] tipping point", the hypothesis is predominantly based on model simulations. He tells Carbon Brief:
"It happens that the same model simulations show that if the so-called 'CO2 fertilisation effect' - as a basic input to photosynthesis, when atmospheric CO2 increases it theoretically enhances plant productivity - really exists and expresses itself in the Amazon, then it would counteract the bad effects of higher temperature and lower rainfall, leaving the forest basically the way it is now. The problem is that we don't have experimental evidence proving the existence, magnitude and duration of such CO2 fertilisation effect in the tropics."
If there is indeed a threshold, where might it lie? Betts says that "3C is the lowest level of warming that might trigger it, but it might need much higher warming".
A Science Advances editorial last year by Prof Carlos Nobre of the University of Saõ Paulo's Institute for Advanced Studies and Prof Thomas Lovejoy of George Mason University noted that "many studies show that in the absence of other contributing factors, 4C of global warming would be the tipping point to degraded savannahs in most of the central, southern and eastern Amazon".
One of those contributing factors is deforestation, which could hasten a shift to savannah as a "fragmented forest is probably more sensitive to rainfall reductions driven by global heating", says Betts.
In a recent interview with Yale Environment 360, Nobre explains that he "published a paper about this in Science in 1990 that said if we deforest parts of the Amazon, it will become a savannah". He adds:
"The post-deforestation climate will no longer be a very wet climate like the Amazon. It will become drier, it will have a much longer dry season, like the long dry seasons in the savannahs in the tropics in Africa, South America and Asia."
Without global warming, a tipping point for Amazon dieback could be reached "if you exceed 40% total deforested area in the Amazon", says Nobre:
"About 60 to 70% of the Amazon forest would turn into a dry savannah, especially in the southern and northern Amazon, areas that now border savannahs. Only the western Amazon near the Andes, which is very rainy, the forest will still be there."
Nobre estimates that approximately 17% of the Amazon rainforest has been cleared so far - principally for cattle ranching and soy plantations. While rates of deforestation slowed in the early 21st century, they have recently rebounded. In the Brazilian Amazon, for example, tree clearance fell by two-thirds between 2005 and 2011, but 2018 saw annual rates rise to their highest levels in a decade. In 2019, deforestation rose again - with rates 85% higher than in 2018.
Reports suggest that a change in policy under Brazilian president Jair Bolsonaro is encouraging development at the expense of the rainforest.
Factoring in climate change and "widespread use of fire" brings the tipping point closer, say Lovejoy and Nobre in their editorial. They estimate that a "tipping point for the Amazon system to flip to non-forest ecosystems in eastern, southern and central Amazonia [lies] at 20-25% deforestation". Nobre recently told the Guardian that this could happen "in 15 to 20 years".
There is "no point in discovering the precise tipping point by tipping it", their editorial says, but instead to "build back a margin of safety...by reducing the deforested area to less than 20%".
The impacts of losing the Amazon rainforest would be felt locally and globally. As well as being an ecological "catastrophe" for wildlife, the socioeconomic damage to the region could amount to $0.9-3.6tn over a 30-year period.
"The reduced evaporation and reduced convection would alter atmospheric circulation worldwide," says Betts, which would influence weather patterns around the world.
Amazon dieback would also make it more difficult to tackle climate change, he notes:
"Increased release of CO2 from forest fires and tree death would accelerate CO2 rise, and with the forest gone we would also have lost an important carbon sink which would mean that deeper emissions cuts would be needed to stop the rise in atmospheric CO2."
Unpublished results from a new study by Nobre, seen by BBC's Newsnight programme, suggest that the south-eastern part of the forest - around 20% of the Amazon basin - has become a net source of CO2.
There are already "ominous signals" of changes in the Amazon, say Lovejoy and Nobre in another Science Advances editorial published in December 2019:
"Dry seasons in Amazonian regions are already hotter and longer. Mortality rates of wet climate species are increased, whereas dry climate species are showing resilience. The increasing frequency of unprecedented droughts in 2005, 2010 and 2015/16 is signaling that the tipping point is at hand."
Lapola agrees that "we may be already observing" a shift in the Amazon system. He explains:
"First evidence: a study has shown that the dry season is already getting longer - by a few days in the last decade - in south Amazonia (Mato Grosso and Rondonia). Second evidence: a recent study showed that forest composition is already changing towards tree species that are more resistant to drought. This suggests that the dieback may be more subtle than previously thought, but not less catastrophic."
The IPCC's fifth assessment report ("AR5", pdf) describes dieback of tropical forests as "potentially abrupt", but "reversible within centuries". Whereas in a recent Nature "world view" piece, Nobre writes that a recovery from an Amazon tipping point would be "probably impossible".
Whether a reversal would be achievable, at the very least it would be slow, adds Betts:
"Reforestation or natural regrowth in places with less severe drying could help increase rainfall levels again. Loss of forests on passing the tipping would be quicker though - forest loss can be quite rapid through fire and tree death, but return is slower because it is limited by how fast new trees grow."
West African monsoon shift
The term "monsoon" in its strictest sense refers to the seasonal reversal of winds and its accompanying rainfall. Along with India, West Africa is one of the few places on Earth where this happens.
The West African monsoon (WAM) brings rainfall to West Africa and the Sahel - a band of semi-arid grassland sandwiched between the Sahara desert to the north and tropical rainforests to the south. The Sahel stretches from the Atlantic coast of Mauritania and Senegal through to Sudan, Eritrea and the Red Sea.
The WAM is a feature of the northern hemisphere summer. West Africa's dry season, which runs from November through to May, sees prevailing winds "come from the desert, so they're dry, dusty winds", says Dr Alessandra Giannini, a senior research scientist at Columbia University (currently at the Laboratoire de Météorologie Dynamique in Paris as part of a "Make Our Planet Great Again" grant). The shift to the wet season sees this system switch, she explains to Carbon Brief:
"When the system reverses, the low pressure over the Sahara - or the land [more generally] - drives winds from the southwest inland and those are moist winds because they're from the ocean."
The moisture that the winds bring to the region is part of the Intertropical Convergence Zone (ITCZ), a huge belt of low pressure that encircles the Earth near the equator. Ultimately, the monsoon is being driven by insolation, says Giannini, as the ITCZ wanders north and south across the tropics each year, roughly tracking the position of the sun through the seasons.
(Giannini emphasises that, while the ITCZ and monsoon are "part of the same season and the same latitudinal migration of the rain band", some researchers prefer to "distinguish between the ITCZ over the ocean from the monsoon inland" and so do not use the terms interchangeably.)
The Sahel marks the ITCZ's most northerly position and the monsoon brings rain to the region from around June to September.
But the West African monsoon is notoriously unreliable. Between the late 1960s and 1980s, a lack of rain hit much of the Sahel, with average rainfall declining by more than 30% over most of the region compared to the 1950s. This plunged the region into an extended drought, contributing to a famine that killed tens of thousands of people and triggering an international aid effort.