Posted on 2 March 2020 by Guest Author

This is a re-post from Carbon Brief by Robert McSweeney

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.

And while climate records are being routinely broken, the cumulative impact of these changes could also cause fundamental parts of the Earth system to change dramatically and irreversibly.

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.

Along with this explainer, Carbon Brief has published guest articles from experts in four of the tipping points covered here.

Tipping towers

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.

Animation by Tom Prater for Carbon Brief.

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.” 

Irreversible change?

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”.

AR4 adopted a definition of a tipping point based on a 2002 report led by Penn State scientist Prof Richard Alley for the US National Research Council. It 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 (“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 Atlantic Meridional Overturning Circulation. Reprinted by permission from Springer. Praetorius (2018) North Atlantic circulation slows down, Nature.

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. 

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