Climate Tipping Points

The 8 climate tipping points we could reach this century.

This is what climate tipping points will do to Florida by 2050 as sea levels rise. Image: ClimateCentral.org

Abrupt Collapse of Climate System

The phrase “climate tipping point” describes how parts of Earth’s climate system could abruptly collapse and trigger an irreversible sequence of warming.

The example most commonly used to illustrate how a climate tipping point works, is a tall tower of bricks. Removing bricks from various locations in the lower part of the tower will gradually weaken its stability. But there comes a point when the removal of just one more brick will send the tower crashing to the ground.

Often, it takes time for the effects of the tip-over to become visible. So it’s possible we may have breached one or more of these critical thresholds without realizing it. As we shall see, an example of this may be the ocean, which takes time to respond to climatic variations.

Although commonly used to describe thresholds exceeded due to man-made global warming, a climate tipping point can also be caused by naturally occurring problems. Known as “Dansgaard-Oeschger events”, these natural periods of abrupt change occurred during the last ice age and were named after the two scientists who identified them from Greenland ice cores.

Some tipping points may be driven by global warming but given the final push by natural fluctuations.

Common to all these abrupt transitions, is that they lead to a structural change of the system. The tower has collapsed, so to speak, so the bricks cannot be returned to their original location. In some cases, this means it may not be possible to reverse course because the system has become irreversibly altered. (1)

Scientists call this phenomenon “hysteresis”. It happens when a system undergoes a “bifurcation” — which means to separate or divide into two branches — making it difficult, if not impossible, for the system to return to its previous state.

For instance, if climate change leads to the wholesale thawing of northern permafrost and a consequent mass release of carbon dioxide (CO2) into the atmosphere, no amount of climate action will put it back into the ground. The thawing process is irreversible. This is an example of “hysteresis”.

It’s worth noting that nothing in this article about climate tipping points alters the fact that the main climate forcing — the main influence on global warming — is our use of fossil fuels and their emissions of greenhouse gas.

In This Article

• Abrupt Collapse of Climate System
• What’s the Biggest Problem About Climate Tipping Points?
• Who First Coined the Term Climate Tipping Points?
• What Does the IPCC Say About Climate Tipping Points?
• 8 Major Climate Tipping Points
• 1. Collapse of the West Antarctic Ice Sheet
• 2. Drying Out of the Amazon Rainforest
• 3. Slowdown of the Atlantic Meridional Overturning Circulation (AMOC)
• 4. Thawing of Northern Permafrost
• 5. Collapse of the Greenland Ice Sheet
• 6. Ecological Shifts in the Northern Boreal Forests
• 7. Loss of Tropical Coral Reefs
• 8. Hothouse Earth: A Cascade of Climate Tipping Points
• References

What’s the Biggest Problem About Climate Tipping Points?

The worst thing about these critical thresholds is that they can be crossed without us realizing it. In fact, we may have already crossed several tipping points.

A possible example is ocean warming. So far, the ocean has absorbed around 93 percent of all the extra heat caused by global warming. This has spared the terrestrial planet from suffering the full effects of humanity’s fossil fuel binge. (2)

But studies show that the ocean has been heating faster and deeper than we thought. And now there are signs that it may be starting to release some of that pent-up thermal energy, which could rewrite global temperature projections for the future.

It won’t happen overnight: the ocean’s heat capacity is huge and a significant portion will be locked away for thousands of years. But an increasing amount of heat will be released into the air at the ocean surface, and the atmosphere is bound to heat up. Given the ocean’s enormous thermal capacity, even a relatively small release of heat energy is likely to have a big impact. See also: How Do Oceans Influence Climate Change?

Who First Coined the Term Climate Tipping Points?

The phrase “climate tipping point” was first used nearly 20 years ago by IPCC lead author Hans Joachim Schellnhuber in press comments following the publication of the IPCC’s Third Assessment Report. Later, in 2005, when director of the Potsdam Institute for Climate Impact Research in Germany, he repeated the term in writing when describing how parts of Earth’s climate system could abruptly collapse or trigger a cascade of events.

It wasn’t the first use of the term, which was invented in 2000 by Canadian journalist Malcolm Gladwell, who published “The Tipping Point: How Little Things Can Make a Big Difference.” Gladwell’s use of the term was inspired by his coverage of the AIDS epidemic. The term “tipping point” describes the moment in an epidemic when a virus reaches critical mass and begins to spread at a much higher rate.

Also, in 2005, NASA scientist James Hansen gave a talk — “Is There Still Time to Avoid Dangerous Anthropogenic Interference with Global Climate? — in which he warned that “we are on the precipice of climate system tipping points beyond which there is no redemption”.

Schellnhuber himself used the tipping point metaphor as a rhetorical device, to warn the public as well as his scientific colleagues of sudden, possibly irreversible, changes in the climate system. Journalists, too, adopted the idea of a tipping point in global warming as a metaphorical concept with societal implications. As a result, the phrase became popular as a metaphorical model for research in the climate sciences.

Later, from about 2011, “tipping point” ceased being metaphorical and was widely used in media coverage as a scientific term for important impending changes in certain climatic processes and events. 3 The rate of melting of polar ice sheets, and the frequency of extreme weather events like hurricanes, are among numerous effects of global warming which are said to signal the impending crossing of a tipping point.

Just as Schellnhuber understood that his effectiveness at the Potsdam Institute required him to stimulate public awareness of the climate crisis, journalists came to prefer the more evocative metaphorical term “tipping point” to the more scientific terminology like “critical threshold” and “regime shift”.

Back in 2008, Timothy Lenton, Professor of Climate Change at Exeter University in Britain, warned in a landmark study paper about the dangers of climate tipping points which he thought would occur once global warming exceeded 5° Celsius (9° Fahrenheit) above pre-industrial levels. In November 2019, a new study by Lenton and six co-authors, now warns that tipping points are “much more likely and much more imminent” and that some “may already have been breached.”

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What Does the IPCC Say About Climate Tipping Points?

The Intergovernmental Panel on Climate Change (IPCC) mentioned the existence of “large-scale discontinuities” with the “potential to trigger large-scale changes in Earth systems” in its Third Assessment Report in 2001, although the report didn’t actually use the words “climate tipping points”. This came in its Fourth Assessment Report (2007), which was the first of its assessment reports to use the term.

The definition of “climate tipping point” used in the Fourth Assessment Report states: “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, published in 2013–14, goes further: “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.” It goes on to state that the exact degree of global warming necessary to trigger a tipping point is not known, but that the risk of crossing tipping points increases with rising temperature. See also: Why Does Half A Degree Rise In Temperature Make Such A Difference To The Planet?

In its Special Report on the Ocean and Cryosphere in a Changing Climate (2019), the IPCC describes a tipping point as “a level of change in system properties beyond which a system reorganises, often in a nonlinear 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 when global or regional climate changes from one stable state to another.”

8 Major Climate Tipping Points

The main active tipping points include:

1. Collapse of the West Antarctic Ice Sheet
2. Drying Out of the Amazon Rainforest
3. Slowdown of the Atlantic Meridional Overturning Circulation
4. Thawing of Northern Permafrost
5. Collapse of the Greenland Ice Sheet
6. Ecological Shifts in the Northern Boreal Forests
7. Loss of Tropical Coral Reefs
8. Hothouse Earth

1. Collapse of the West Antarctic Ice Sheet

The West Antarctic Ice Sheet (WAIS) is one of three geographical regions that make up Antarctica. The other two are the Antarctic Peninsula and the East Antarctic Ice Sheet. All three regions are shedding ice more rapidly than expected.

Temperatures are also rising. In West Antarctica, the average temperature at the 1,550-meter high Byrd Station in the heart of the ice cap, rose by 2.4 degrees Celsius from 1958 to 2010. (4)

But the big danger isn’t rising temperatures, it’s warmer ocean waters. It’s this warm water that’s melting the ice in the region’s Amundsen Sea embayment, home to the huge Thwaites and Pine Island glaciers. Pine Island drains 10 percent of the entire West Antarctic Ice Sheet, while Thwaites occupies an area of 181,000 square kilometers (70,000 sq mi) These ice giants appear to be melting from the bottom up, as warm ocean water seeps beneath the ice.

What’s the cause of this ocean warming? Wind. Back in the 1920s, the winds over the Amundsen Sea mainly blew outwards to the west, keeping the warm ocean water at bay. But climate change has caused a long-term change in the winds, so that today the wind alternates between blowing eastward and westward. And when the wind blows in an easterly direction, the warm ocean water creeps forward to warm the ice. (5)

The West Antarctic Ice Sheet (WAIS) is particularly vulnerable to ocean warming because it’s a ‘marine-based’ ice sheet. It sits on bedrock that is mostly below sea level, which means that the bottoms of its glaciers are more accessible to the ocean water as they retreat. This leads to a structural weakness known as ‘marine ice sheet instability’ (MISI), and maybe also to the more controversial climate feedback known as ‘marine ice cliff instability’ (MICI).

According to recent studies, the rate of ice loss from the WAIS has tripled from 53 billion tonnes annually during the period 1992–97, to 159 billion tonnes a year in 2012–2017. (6)

The area of most concern is the Amundsen Sea embayment into which six glaciers drain, including Thwaites and Pine Island glaciers which have been thinning at rates of 49 and 45 centimeters annually, over the past three decades. Here, the bedrock lies significantly below sea level which is why the area has long been seen as the “weak underbelly” of the WAIS.

The Amundsen Sea embayment is seen as the most likely location for the WAIS to reach a “tipping point” that could trigger a major ice collapse. The big question is: when is this tipping point likely to be reached?

A recent study in Nature Climate Change states 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°C and 2°C of global warming above pre-industrial levels. (7)

In its Special Report on the Ocean and Cryosphere in a Changing Climate (2019), the IPCC took a conservative line, saying that a “partial West-Antarctic Ice sheet collapse” while potentially abrupt and “irreversible for decades to millennia”, was unlikely to occur during the 21st century.

Other studies are more pessimistic, pointing out that in 1980 both the Thwaites and Pine Island glaciers were in balance — ice losses into the sea were equalled by ice gains from snowfall. Since then, they have lost billions of tonnes of ice. In fact, since 1992, ice thinning has spread across one-quarter of West Antarctica and over most of its major ice streams, which are now losing ice five times more rapidly than they were at the start of the 1990s. (8)

Furthermore, evidence from Earth’s past suggests the WAIS has collapsed before. A study from 2011 stated that the paleoclimate record indicated that the WAIS largely disappeared, during the past few hundred thousand years or (even more likely) during the past few million years, in response to warming we are likely to face for the next few centuries.” (9)

If both the Pine Island and Thwaites glaciers collapsed, sea levels could rise by 1–2 meters (3 ft 3 inches to 6 ft 6 inches), due to the probable knock-on effects across West Antarctica, and possibly even across the eastern half of the continent as well. (10)

According to Paul Cutler, program director for Antarctic glaciology at the U.S. National Science Foundation, Thwaite serves as a “keystone for the other glaciers around it. By itself, Thwaites could raise sea levels about 65 cm (2 ft) as it melts. But if Thwaites goes, the knock-on effect across the western half of Antarctica would lead to 2–3 meters of sea level rise”.

Floods caused by storm surges on top of the relatively small amount of sea level rise to date, are a growing feature of coastal life across Asia, South America and the Southern United States. So any significant increase in sea levels is bound to be disastrous.

2. Drying Out of the Amazon Rainforest

The Amazon Rainforest is the world’s largest tropical rainforest, occupying more than three quarters of the Amazon Basin in South America. It consists of 2.1 million square miles (1.3 billion acres) of moist broadleaf tropical rainforest. Alive with biodiversity, its lush, dense plant mass makes it one of the world’s largest and most precious carbon sinks.

The rainforest ecosystem is sustained by extremely warm and wet conditions, with the forest itself creating its own water cycle to make full use of local rainfall.

In addition to the normal evaporation of water, plants and trees pull up water from the ground which they then transpire into the air. This water vapor rises to form rain clouds. Rain then falls and the whole process — known as evapotranspiration — is repeated. In fact, rain that falls on the eastern coast of Amazonia can be recycled up to six times before the clouds reach the Andes mountains in the west.

There the clouds empty themselves onto the slopes of the Andes, resupplying the Amazon River which carries the water all the way back to the Atlantic. This outflow is full of nutrients that fertilize a vast expanse of surrounding water and stimulate the growth of phytoplankton — a key organism in the ecology of the ocean.

In this way, the Amazon rainforest creates its own regional climate. Not only does it generate about half of its own rainfall, recycling part of the moisture through the soil, but also its rain clouds block the sunlight, exerting a cooling effect across the region.

Unfortunately, the natural climate of the Amazon basin is coming under pressure from two sources — both man-made. The first is the growing deforestation in the Amazon Rainforest, because fewer trees mean less transpiration and less moisture entering the atmosphere and creating clouds. This causes the forest to gradually dry out. Also, the fewer clouds to block the sunlight, the hotter the forest becomes.

The second factor is global warming. This impacts on the Amazonian rainforest in two ways. To begin with, it alters patterns of sea surface temperature change in the tropical Atlantic and Pacific. This causes a decline in precipitation. Secondly, as atmospheric carbon dioxide levels increase, plant leaves tend to open less widely. This means less transpiration which leads to fewer rain clouds.

Climate models show there’s a tipping point when the Amazon rainforest biome becomes so dry that trees start dying. This signals a gradual transition to savannah — a drier, grassland ecosystem with few trees. This savannization process (also called savannification) could have a major impact on regional climates throughout Central and South America. In addition to being catastrophic for wildlife, the socioeconomic damage to the region could amount to US$3.6 trillion, over a 30-year time span.

The effects of savannization on the carbon cycle are incalculable. The Amazon Rainforest, which is more than twice the size of the Indian subcontinent, stores around 150 billion to 200 billion tons of carbon in its trees and other plants. Scientists are concerned that, if the Amazon turns into grassland, a large part of this carbon reservoir will be released into the atmosphere, giving a huge boost to global warming. (11)

The only hope is that the extra CO2 in the air might drive more photosynthesis and cause higher levels of plant growth. This CO2-fertilization mechanism might counter-balance some of the other effects. Unfortunately, scientists don’t have enough evidence as to the magnitude or duration of such a CO2 fertilization effect in the tropics.

The situation is further complicated by the increasing levels of deforestation in the area, since a fragmented forest is more prone to drying out. (12)

So when might the Amazon Rainforest reach this tipping point? Scientists aren’t sure. Some say that it will happen when global warming reaches 4° Celsius. Others say more warming may be necessary.

In 2018, Experts Carlos Nobre and Thomas E. Lovejoy published a paper stating that the Amazon tipping point could happen in eastern, southern and central Amazonia when total deforestation reaches 20–25 percent — a situation not expected for 20 to 25 years. 13 Nobre has since brought forward his prediction by about five years. (14)

The latest research suggests that, once it reaches the tipping point, the Amazon rainforest could shift to savannah within roughly 50 years. (15)

Whatever the timeline, the signs are ominous. Dry seasons in Amazonia are already getting hotter and longer. Wet climate species are suffering increased mortality rates, while drier climate species are doing better. The increasing frequency of unprecedented droughts is another sign that a tipping point may be close.” (16)

Bottom line: climate action is urgently needed to contain the spread of deforestation in order to conserve the rainforest for future generations, and to ensure that its value as an economic and a climate resource remains intact. In the end, a degraded rainforest ecosystem is no use to anyone.

3. Slowdown of the Atlantic Meridional Overturning Circulation (AMOC)

AMOC is a branch of the thermohaline circulation of ocean currents in the Atlantic Ocean, that brings warm water up to Europe from the tropics, and takes away cold polar water for redistribution to warmer areas.

The system is driven by the downwelling of cold, salty water in the Arctic. Unfortunately, the momentum of this process is weakened by global warming, that melts Arctic sea ice as well as Greenland’s glaciers, thus diluting the salty sea water with freshwater, and warming it up with higher ambient temperatures. The warmer and less saline the water, the lighter it is and the less able it is to sink into the depths. As a result, the AMOC slows down.

Recent studies indicate that the AMOC has weakened already by about 15 percent since the middle of the 20th century. (17) But the real question is, at what point does this slowing process tip over into a complete shutdown? Because even a very slow AMOC would have a major impact on the climate of the whole northern hemisphere, especially Europe.

At present, climate models suggest that no shutdown of the AMOC is likely in the next century or so, making the prospect a low-probability, high-impact event. The IPCC’s Special Report on the Ocean and Cryosphere in a Changing Climate (2019) stated that the AMOC — described as an important potential tipping point — was very likely to weaken over the next 100 years, but was very unlikely to collapse — although it stated it was “physically-plausible.” (18)

The report went on to say that a weakening of the AMOC would result in a decrease in marine productivity in the North Atlantic, further reduced rainfall in the already semi-arid Sahel region, and an increase in sea-levels around the Atlantic especially along the northeast coast of North America.

Since the AMOC plays a vital role in bringing heat up from the tropics, a shutdown would cause widespread cooling of up to 5° Celsius around the northern hemisphere, especially in western Europe and the eastern coast of North America. This could lead to reduced rainfall because there would be less evaporation from the North Atlantic. In southern Europe, this would lead to further drying. So it would actually reinforce the climate-change signal. In North Africa, it would result in further reduced rainfall in the semi-arid Sahel region.

A new study in the journal Nature Food finds that an AMOC shutdown would cause “widespread cessation of arable farming” in the United Kingdom. (19)

Other research shows that a shutdown of the AMOC could itself trigger other tipping points. (20)

4. Thawing of Northern Permafrost

Permafrost is the name given to permanently frozen ground whose temperature remains at 0°C (32°F) or below, for at least two years in a row. It is found mainly in sub-surface ground in the Northern Hemisphere, notably across large parts of Siberia as well as northern Canada, Alaska and the Tibetan plateau.

The thickness of permafrost varies enormously according to temperature and local conditions, and ranges in depth from 1 meter (3 feet) to more than 1,000 meters (3,281 feet).

Being frozen, permafrost is part of Earth’s cryosphere as well as an important part of Earth’s climate system. But its main characteristic is the fact that it contains large amounts of organic material made up of the remains of dead plants and animals. This makes permafrost one of the largest carbon reservoirs in the global carbon cycle. According to the NOAA’s Arctic Report Card (2019) soils in the northern permafrost region contain 1,460–1,600 billion tonnes of organic carbon, double the amount presently in the atmosphere. (21)

But, if permafrost continues to thaw — as many climate models suggest it will — the resulting emissions of carbon dioxide (CO2) and methane (CH4) would act as powerful climate feedbacks, raising temperatures around the world.

In recent years, the circumpolar region — which contains the largest share of carbon-rich permafrost — has been warming twice as fast compared to the rest of the world. Since the 1970s, the average temperature of the region has increased by 2.3°C.

As a result, Arctic sea ice is at its lowest extent, and melting of the Greenland ice sheet continues apace. In addition, we have seen record-breaking Arctic fires in forests and peatlands across northern Russia, Canada, Alaska and Greenland. (22)

And as the ground warms, the danger of CO2 and methane emissions becomes a growing threat. (23)

This is because, as temperatures rise microbes in the soil become active and start to decompose the organic carbon around them. Depending on whether or not oxygen is present, these microbes emit the greenhouse gases CO2 or (to a lesser extent) CH4 (via methanogenesis). And the signs are that it’s already happening on a large scale.

The IPCC’s Special Report on the Ocean and Cryosphere (SROCC) confirms that record high temperatures have been documented in permafrost up to 20 meters deep, at many sites in the Northern Hemisphere.

NOAA’s 2019 Arctic Report Card states that thawing permafrost in the Arctic is already estimated to be releasing 300–600 million tonnes of net carbon annually.

The SROCC affirms that widespread disappearance of Arctic near-surface permafrost is probable during the 21st century, as a result of global warming. The report says that by 2100, the area of permafrost will decrease by up to 66 percent under RCP2.6, and up to 99 percent under RCP8.5. This is estimated to emit up to 240 billion tonnes of permafrost carbon as CO2 and CH4.

So while there is no sudden and irreversible tipping point for permafrost thawing in sight, there are a series of factors that could accelerate the process significantly by the end of the century.

For example, wildfires, soil subsidence or thermokarst, soil erosion, a continuing rise in Arctic temperatures or heatwaves — all these factors are likely to accelerate permafrost thaw.

Another form of permafrost are methane hydrates, or “clathrates”. This ice-like material, which is formed when methane and water combine together at low temperatures and under reasonably high pressure, is mostly found under the sea bed on shallow continental shelves on the fringes of a land mass.

A few years ago, widespread seepage of methane was detected from seafloor sediments off the Norwegian archipelago of Svalbard. Some scientists thought this seepage was emanating from undersea methane hydrate deposits, as a result of ocean warming. They believed that other similar deposits under the Arctic Ocean — containing as much as 1,400bn tonnes of carbon — could also be released.

However, the latest research has more or less refuted the idea that the undersea thawing of methane hydrates could constitute an impending tipping point. Any bubbles of methane emanating from the sea bottom are more likely to be coming from permafrost than hydrates.

5. Collapse of the Greenland Ice Sheet

The Greenland ice sheet is the second largest body of ice on Earth, after the continental Antarctic Ice Sheet. It has a total volume of 2.9 million cubic kilometers (695,000 million cubic miles) — storing enough water to raise mean sea levels by more than 7 meters (23 ft). (24)

Melting of the Greenland ice sheet is accelerating and it currently adds about 0.7mm to global sea level rise annually.

At the annual meeting of the American Geophysical Union in 2019, scientists examined satellite imagery which revealed that Greenland’s glaciers have retreated about 3 miles (5 kilometers) between 1985 and 2018. And since 2000, the pace of retreat has accelerated, resulting in the loss of an extra 50 billion metric tons of ice per year.

The drivers for the ice melt vary according to region. In the southeast of Greenland, for example, it is caused mainly by warming ocean waters that melt the front of the glaciers. (25) The Kangerlussuaq Glacier, for example, the largest tidewater glacier on the east coast of Greenland has been retreating rapidly and now occupies its most inland position since the early 20th-century.

Elsewhere, the driver is surface melting, caused by rising temperatures due to climate change. Surface melt is aggravated by several different feedback loops. The most important of these are elevation feedbacks. Basically, as the ice sheet melts and loses height, the air gets warmer, which causes more melt and an even lower ice sheet. At some point, the temperature rises so much that it triggers irreversible surface melting.

Another feedback involves the terrain’s albedo — its capacity to reflect sunlight away from Earth. As the highly reflective white surface snow melts, it exposes more of the darker less reflective grey ice. Thus, the ice receives more sunlight and undergoes more melting.

In its Fifth Assessment Report (2013), the IPCC stated it was “exceptionally unlikely” that the Greenland ice sheet would collapse in the 21st century.

Most research suggests that the ice sheet is at risk on longer timescales. For example, a new study conducted by IMBIE, supported by NASA and the European Space Agency (ESA) used satellite imagery to track the effects of global warming on the Greenland ice cap, and its impact on rising sea levels. It represents the most accurate measurements of ice loss to date.

The study found that since 1991 Greenland’s ice mass loss has accelerated from 25 billion tons per year to a current average of 234 billion tons per year. So, Greenland is losing ice on average seven times faster today, than in 1991. And the loss is getting faster. Over the past 10 years, for example, the rate has increased to 254 billion tonnes annually. (26)

Since the Greenland Ice Sheet does not suffer from structural weaknesses like those of the West Antarctic Ice Sheet, whether or not it reaches a disastrous tipping point depends largely on future levels of global warming.

Studies suggest that about 1.6°C to 1.8°C of warming above pre-industrial levels would be enough to trigger more serious acceleratory feedbacks. Overall, however, models shows that ice loss is extremely variable — varying between 80 percent loss after 10,000 years, and total loss after as little as 2,000 years — depending on future warming.

In its Special Report on the Ocean and Cryosphere in a Changing Climate (2019) the IPCC notes that decline of the ice sheets is not going to be rapid, although it concedes that the decay will be irrevocable for thousands of years once it starts.

6. Ecological Shifts in the Northern Boreal Forests

Boreal forests (known as the “taiga” in Russia) are found mostly just south of the Arctic Circle in the cold dry climate of the far north. They form a circular swathe across Norway, Sweden, Finland, Russia, Alaska, Canada and Greenland, where tree growth is restricted by year-round freezing or near-freezing temperatures and a lack of rain.

The Boreal forest is the largest biome on Earth and constitutes almost one-third of the world’s forests. It’s a major carbon sink, although estimates as to the size of its carbon content are varying and uncertain. For example, one 2015 study says that boreal forests contain around 1,000 billion tons of carbon, mostly in their soils and peatlands. (27) In contrast, Nature United estimates the carbon content at about 208 billion tons of carbon. (28)

Boreal forests throughout the Northern Hemisphere are undergoing rapid changes due to the warming climate. The rising temperatures are forecast to cause drier conditions (favoring drought-resistant tree species), leading to less above-surface biomass and a widespread warming of the subsurface permafrost. Wildfires will contribute their own positive feedback loop, further warming the ground and release more greenhouse gases in the process.

The Russian forest — the world’s largest expanse of forest — has experienced unusually high temperatures over the last several decades, as illustrated by the huge number of fires across the region. As a result, species that are less tolerant of a warmer, drier climate are declining, while those that are more tolerant are advancing northward.

In Siberia, for example, there has been an important shift from deciduous conifer trees like larch, to evergreen conifers such as pines. (29)

This is because evergreens — such as pines and spruces — use water more efficiently, which helps these trees survive over birches, larches, oaks and other deciduous varieties.

Researchers say this switch is likely to promote additional warming and vegetation change, especially in forests with low species diversity. This is because deciduous conifers drop their needles in the autumn, exposing the vast snow-covered ground surface. This whiteness reflects sunlight and heat back into space, keeping the climate in the region very cold. But evergreen conifers retain their needles all year round, and their unbroken dark-green canopy absorbs rather than reflects sunlight. This heat retention warms and dries out the soil and biomass, which creates even more ideal conditions for evergreens — hence their proliferation.

But the Russian decline in deciduous larches is not corroborated in the northernmost forests of northeast China. Here, warming soils have caused a phenomenal growth of Dahurian Larch, which is a worrying indicator of growing climate change and permafrost thaw. Under these conditions, if the Russian experience holds true, the future of the Chinese larch looks uncertain. (30)

Paradoxically, in Alaska, where the larch is less common due to an outbreak of disease in the late 1990s, large areas of forest are becoming inhospitable to the dominant white and black spruce, which is an evergreen conifer.

Also confusing, is a 2019 study — part of NASA’s Arctic Boreal Vulnerability Experiment (ABoVE) — which states that warmer, drier conditions actually favor leaf-shedding deciduous trees like larches, over spruce trees. Researchers say that as it gets drier, the landscape becomes better suited to the deciduous species that can outcompete spruce trees, which are slower-growing and stressed by drought. The study says that under climate change, the conifers may die off, leaving a deciduous forest. (31)

The big concern among climatologists is that the warmer, drier climate may cause the boreal biome to ‘tip over’ into a grassland environment.

For example, we may see a situation where a major forest fire, or series of fires, makes it so difficult for parts of the forest biome to recover, that it tips over into a sparsely wooded or grassland biome. This possibility is more likely to happen in the hotter and drier southern edges of the forest.

Even if the boreal forest manages to avoid this kind of dramatic tipping point, scientists still expect to see widespread changes in tree species composition and fire regimes. (32)

At the same time, the boreal/taiga biome is projected to encroach northwards into the hitherto treeless tundra zone, which might lead to increased heat absorption in the Arctic Circle and more rapid permafrost thaw. Meantime, a recent study forecasts major climate change impacts for more than 80 percent of the tundra and more than 40 percent of all boreal forests. (33)

In its Special Report on Global Warming of 1.5°C, the IPCC concedes that a major biome shift from tundra to boreal forest is possible if global temperature rises by 2°C, but regards it as unlikely. Even at 4 degrees of warming, it describes the possibility as uncertain, due to the complexities of the different climate feedbacks involved.

A less visible but potentially more dangerous tipping point concerns the release of the massive amount of carbon stored in the region’s wetlands, peat bogs and subsurface permafrost. (Note: Roughly one-third of the boreal forest stands on permafrost soil.)

7. Loss of Tropical Coral Reefs

Coral reefs teem with biological diversity, especially in the tropics. As many as 1 in 4 of all ocean species rely on the reef ecosystem for food and shelter, at some point in their lives. Coastal reefs, like mangrove forests, offer significant shoreline protection during storms as they absorb wave energy, thus minimizing erosion as well as damage to property and land. More than half a billion people depend on coral reefs for their livelihood, while scientists assess the total global value of coral reefs at between 30 billion and 172 billion U.S. dollars annually. (34)

Unfortunately, as we have seen, the ocean absorbs more than 90 percent of global warming resulting in significant chemical changes and loss of biodiversity throughout the marine world. In recent years, extreme weather events, such as marine heatwaves, have caused a series of “mass bleaching” events in warm water corals, caused mainly by exposure to high sea temperatures.

Once sea temperatures reach a certain point, corals get so stressed that they expel the zooxanthellae (microalgae) that live inside their tissues. Zooxanthellae are single-celled phytoplankton that give corals their color. So, once they are expelled, the coral takes on a bleached look. In return for shelter, the phytoplankton transfer up to 90 percent of the food they create during photosynthesis to their hosts, which is why corals are able to achieve the enormous growth needed to build reefs.

Corals don’t always die after a bleaching episode but they become significantly weaker and therefore more vulnerable to other threats. Very often, they slowly starve to death. Which is why all mass bleaching events are ecological tipping points — they tip the reef ecosystem into a degraded state.

Back in the 1980s, episodes of severe coral bleaching occurred every 25–30 years. Now they happen about every six years. The first genuinely mass coral-reef bleaching event took place in 1998. This initial mass bleaching was followed by others in 2010 and 2014–17. All were caused by extended periods of unseasonably high temperatures aggravated by man-made warming and its impact on the El Niño (ENSO) regional weather system.

A recent paper describes the 2014–2017 bleaching as “a watershed for the Great Barrier Reef, and for many other seriously affected reefs in the Indo-Pacific Ocean”. (35)

Previously, dead coral skeletons on the reef could be recolonized by juvenile corals. Today, many of these empty homes are more likely to be taken over by fast-growing tropical macroalgae or other types of seaweed. These usurpers can rapidly occupy entire reef systems, causing an abrupt shift in the characteristics of the community. (36) The habitat engineering species are, in effect, being replaced by passive space-holder species who are incapable of developing and growing the reef system.

The switch from coral to microalgae is well-documented in areas like the Caribbean and Seychelles. A new study indicates that, once such a shift happens, the reef is likely to collapse within 15 years. This is consistent with reports that as many as 80 percent of Caribbean reefs were lost during the period 1977–2001, and that the remainder may be lost by 2035, depending on levels of global warming, ocean acidification and overfishing. (15)

Overfishing of herbivorous fish is another important contributor to coral reef decline. The herbivores keep microalgae in check and play a vital role in helping ecologically intact reefs to recover. But they can’t reverse a takeover by microalgae.

Overall, studies indicate that coral reefs have a better chance of recovering from a bleaching event in situations where they are structurally complex and lie in deeper water, and where there is a relative abundance of juvenile corals and herbivorous fish. Under these conditions, a reef can recover within 10–15 years of bleaching. (37)

The big unknown is climate change. In 2016, a ground-breaking study found that under 1.5 degrees of global warming, 90 percent of tropical reefs would likely suffer severe degradation from temperature-induced bleaching, from 2050 onwards. Under 2 degrees of warming, 98 percent of reefs would be affected. (38)

According to temperature projections used by the IPCC, 1.5 degrees of warming will be fatal for 70–90 percent of all warm water coral reefs, and 2 degrees will wipe out 99 percent. (39)

To summarize: a temperature increase of 1.5°C is a likely tipping point for more than three-quarters of all tropical coral reefs. An increase of 2°C could finish off the rest. Given the fact that global temperature projections currently indicate a likely increase of 2–3 degrees Celsius, all we can do is hope that new strains of heat-resistant coral polyps will emerge in time to recolonize what might remain of today’s reef structures.

8. Hothouse Earth: A Cascade of Climate Tipping Points

A controversial 2018 study, published in the Proceedings of the National Academy of Sciences, warns of a major climate tipping point which is likely to be reached following a cascade of smaller tipping points and feedback mechanisms. If this threshold is crossed, say researchers, it risks pushing the Earth into a lasting and dangerous “hothouse” state. (40)

The study’s findings are based on a cascade of climate events, which are estimated to start once global warming reaches about 2 degrees Celsius. It suggests 2 °C “because of the risk that a 2°C warming could activate important tipping points, intensifying heatwaves, prolonging droughts and raising the temperature even more, to activate other tipping points in a domino-like cascade that could create even higher temperatures.

The climate tipping points mentioned in the study, most of which are interconnected, involve:

(a) The melting of the Greenland and West Antarctic ice sheets, loss of the Arctic summer ice pack, loss of coral reefs and Alpine glaciers — which are estimated to occur between 1°C and 2°C.

(b) Deforestation in the Amazon, ecological changes in the boreal forest, decline of thermohaline circulation, as well as shifts in the Jet Stream, the El Nino Southern Oscillation, and the Indian Monsoon — which are estimated to occur between 3°C and 5°C.

Note that climate change has already begun to amplify the effects of regional weather cycles, such as the El Niño-Southern Oscillation, the Indian Ocean Dipole and the Southern Annular Mode — witness the catastrophic Australian Bushfires (2019–2020) in Victoria and New South Wales.

(C ) Disappearance of the Arctic winter ice pack, wholesale thawing of permafrost and the disintegration of the East Antarctic ice sheet — which are estimated to occur over centuries once warming exceeds 5°C.

This cascade of events can tip the entire climate system into a new mode of working, says co-author Hans Joachim Schellnhuber, Director of the Potsdam Institute for Climate Impact Research. And he should know. He was the person who coined the phrase “climate tipping point” in the first place.

Melting polar glaciers would lead to sea levels that are 10–60 meters (30–200 ft) higher than today, temperatures would reach 4–5 degrees Celsius (7–9° F) higher than pre-industrial times, coral reefs would disappear, regional weather systems in the Indo-Pacific and Southern oceans would see-saw out of control and storms would destroy coastal communities around the globe. And it could happen within a matter of decades.

If ‘Hothouse Earth’ becomes the reality, places on Earth will become uninhabitable, says co-author Johan Rockstrom, executive director of the Stockholm Resilience Centre.

References

1. “TPs. Early warning and wishful thinking.” Peter D. Ditlevsen, Sigfus J. Johnsen. Geophysical Research Letters. October 2010.
2. “How fast are the oceans warming?” Lijing Cheng, John Abraham, Zeke Hausfather, Kevin E. Trenberth. Science 11 Jan 2019: Vol. 363, Issue 6423, pp. 128–129.
3. “Tipping Points and Climate Change: Metaphor Between Science and the Media.” Journal Environmental Communication. Volume 12, 2018 — Issue 5. Sandra van der Hel et al.
4. “Central West Antarctica among the most rapidly warming regions on Earth.” Bromwich, D., Nicolas, J., Monaghan, A. et al. Nature Geosci 6, 139–145 (2013).
5. “West Antarctic ice loss influenced by internal climate variability and anthropogenic forcing.” Holland, P.R., Bracegirdle, T.J., Dutrieux, P. et al. Nat. Geosci. 12, 718–724 (2019)
6. “Mass balance of the Antarctic Ice Sheet from 1992 to 2017.” Shepherd, A., Ivins, E., Rignot, E. et al. Nature 558, 219–222 (2018).
7. “The Greenland and Antarctic ice sheets under 1.5°C global warming.” Pattyn, F., et al. Nature Clim Change 8, 1053–1061 (2018).
8. “Trends in Antarctic Ice Sheet Elevation and Mass.” Andrew Shepherd, et al. Geophysical Research Letters. 16 May 2019.
9. “Stability of the West Antarctic ice sheet in a warming world.” Joughin, I., Alley, R. Nature Geosci 4, 506–513 (2011).
10. “Collapse of the West Antarctic Ice Sheet after local destabilization of the Amundsen Basin.” Johannes Feldmann et al. PNAS November 17, 2015 112 (46) 14191–14196; November 2, 2015.
11. “The Amazon used to be a hedge against climate change. Those days may be over.” Oct. 2, 2018. pri.org
12. Fires in the Brazilian Amazon in October 2020 were more than double those in October 2019. “Amazon fires: Year-on-year numbers doubled in October.” BBC News.
13. “Amazon Tipping Point.” Thomas E. Lovejoy and Carlos Nobre. Science Advances 21 Feb 2018
14. See also: “Amazon rainforest close to irreversible tipping point.”
15. “Regime shifts occur disproportionately faster in larger ecosystems.” Cooper, G.S., Willcock, S. & Dearing, J.A. Nat Commun 11, 1175 (2020).
16. “Amazon tipping point: Last chance for action.” Thomas E. Lovejoy and Carlos Nobre. Science Advances 20 Dec 2019
17. “Anomalously weak Labrador Sea convection and Atlantic overturning during the past 150 years.” Thornalley, D.J.R., et al. Nature 556, 227–230 (2018).
18. Chapter 6: Extremes, Abrupt Changes and Managing Risks (PDF). IPCC (Report). Special Report on the Ocean and Cryosphere in a Changing Climate (SROCC). September 25, 2019
19. “Shifts in national land use and food production in Great Britain after a climate tipping point.” Ritchie, P.D.L., Smith, G.S., Davis, K.J. et al. Nat Food 1, 76–83 (2020).
20. “Risk of multiple interacting tipping points should encourage rapid CO2 emission reduction.” Cai, Y., Lenton, T. & Lontzek, T. Nature Clim Change 6, 520–525 (2016).
21. “Permafrost and the Global Carbon Cycle.”
22. “The Arctic is burning and Greenland is melting, thanks to record heat.” Carolyn Gramling. Science News. Aug 2, 2019.
23. “Climate change and the permafrost carbon feedback”. Nature. 520 (7546): 171–179. Schuur, E.A.G.; et al. (2015)
24. IPCC. Fifth Assessment Report (2013)
25. “Shrinking of Greenland’s glaciers began accelerating in 2000, research finds.”
26. “Mass balance of the Greenland Ice Sheet from 1992 to 2018.” Shepherd, A., Ivins, E., Rignot, E. et al. Nature 579, 233–239 (2020).
27. “Global estimates of boreal forest carbon stocks and flux.” Corey J.A.Bradshaw. Global and Planetary Change. Volume 128, May 2015, Pages 24–30.
28. “Primer on Forest Carbon.”
29. “Sensitivity of Siberian larch forests to climate change.” Shuman, Jacquelyn & Shugart, Herman & O’Halloran, Thomas. (2011). Global Change Biology. 17. 2370–2384.
30. “Warmer Winter Ground Temperatures Trigger Rapid Growth of Dahurian Larch in the Permafrost Forests of Northeast China.” Xianliang Zhang et al. JGR Biogeosciences. 12 March 2019 [↩]
31. “Warmer, Drier Climate Could Transform Alaskan Forests.”
32. “Nine tipping points that could be triggered by climate change.”
33. “The Biosphere Under Potential Paris Outcomes.” Sebastian Ostberg et al. January 2018.
34. “Corals and Coral Reefs.” Smithsonian.
35. Hughes, T.P., Kerry, J.T., Baird, A.H. et al. Global warming transforms coral reef assemblages. Nature 556, 492–496 (2018).
36. Holbrook, S., Schmitt, R., Adam, T. et al. Coral Reef Resilience, Tipping Points and the Strength of Herbivory. Sci Rep 6, 35817 (2016).
37. “Predicting climate-driven regime shifts versus rebound potential in coral reefs.” Graham, N., Jennings, S., MacNeil, M. et al. Nature 518, 94–97 (2015).
38. “Differential climate impacts for policy-relevant limits to global warming: the case of 1.5°C and 2°C.” Carl-Friedrich Schleussner et al. Earth Syst. Dynam., 7, 327–351, 2016.
39. IPCC Special Report on Oceans. Summary for Policymakers. FAQ Chapter 5.1: How is life in the sea affected by climate change?
40. “Trajectories of the Earth System in the Anthropocene.” Will Steffen et al. PNAS August 14, 2018 115 (33) 8252–8259; first published August 6, 2018