If you want to make a climate scientist uncomfortable, just sidle up to them and whisper “tipping points” in their ear. Climate breakdown driven by global heating is scary enough, but so-called climate tipping points send a shiver down the spine. So, what are they and why do they have us all running scared?
We’re in the middle of a unique experiment that’s driving up the global average temperature at least 10 times faster than at any time in the geological record. The consequences are all around us: explosions of extreme weather, collapsing ice sheets and accelerating sea-level rise.
But as greenhouse gas emissions continue to climb as fast as ever and the global temperature rise (compared to pre-industrial times) for the last 12 months touches 1.64°C (a rise of almost 3°F), so the likelihood of sudden, permanent switches in dangerous elements of the climate system is becoming increasingly possible.
Because a critical threshold needs to be reached before a switch can occur, and because – like a tilting seesaw – once a switch starts, there’s no going back, they’re called tipping points.
There are plenty of definitions out there, but the one that really hits the nail on the head comes from The American Heritage Dictionary of the English Language, which describes a tipping point as: “A critical moment in a complex situation in which a small influence or development produces a sudden large or irreversible change.” Where the climate is concerned, for large, read disastrous.
How things could tip
Scientists who work to model where global heating is taking our climate struggle with tipping points for two reasons. Firstly, they’re not easy to pin down in terms of timing and impact.
Secondly, how tipping points are treated within climate models can dramatically influence the output, thereby increasing uncertainty in terms of forecasting how climate breakdown will unfold in years
to come.
Such is the complexity of the climate system, that there are likely to be countless tipping points, most having local or regional influences. Nine, however, are recognised as having the potential to trigger massive environmental changes at the global scale.
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These include the collapse of the Greenland and West Antarctic ice sheets (GIS and WAIS respectively), the dieback of the Amazon rainforest and the release of colossal volumes of methane from thawing
Arctic permafrost.
GIS and WAIS collapse would eventually result in a 12m (almost 40ft) sea-level rise that would flood all coastal cities. The replacement of white ice with dark rock would also mean that more heat was absorbed, pushing up Earth’s temperature further.
The loss of the Amazon rainforest and thawing of Arctic permafrost would add massively to carbon levels in the atmosphere, driving up the heat even more.
And we aren’t talking of the distant future here. Irreversible collapse of the GIS and WAIS will likely be triggered by a global average temperature rise of just 1.5°C, which we’ll see by the early 2030s. And the same goes for an abrupt thaw of Arctic permafrost.
The tipping point that keeps members of the climate science community awake at night, however, is the collapse of the Gulf Stream and associated oceanic currents, which help to keep the UK and northern Europe warmer than they would otherwise be.
Go back a decade or so and this was thought to be highly unlikely, something which would only happen given a global temperature rise of several degrees. But all that has changed.
The Atlantic Meridional Overturning Circulation (AMOC) is now weaker than at any time probably since the end of the last Ice Age, more than 11,500 years ago, and the authors of an analysis published in 2021 suggest that it may already have suffered an “almost complete loss of stability.”
A study published in 2024 reports that the AMOC has gone through a “noticeable reduction in strength” in the last couple of decades, and recent estimates of when shutdown might happen are getting ever closer to our time.
A 2023 study forecast sometime between 2025 and 2095, with a central estimate of 2050. Another analysis, revealed in August last year, points to collapse sometime between 2037 and 2064.
Even more concerning (if not downright terrifying): up to almost half of high-quality climate models suggest that serious weakening of Atlantic currents will see cooling across the North Atlantic region begin as early
as the 2030s.
Earth’s great engine
To understand why collapse would be a big deal requires some understanding of what the AMOC does. In a nutshell, it’s a critical component of the worldwide system of currents known as The Global Conveyor Belt, carrying heat from the tropics northwards into the Arctic.
The numbers associated with the AMOC are mind-boggling. Every second it shifts 17 million cubic metres of water – equivalent to nearly 7,000 Olympic-sized swimming pools.
At the same time, the AMOC carries around 1.2 million gigawatts of heat, equivalent to more than 150 times the energy capacity of the global electricity network, which amounts to one-quarter of all the heat transported into the northern hemisphere.
The tropical waters carried northwards by the AMOC are warm, shallow and salty. As they approach the Arctic, they cool and become more dense, causing them to sink into the deep ocean, where they feed a cold current that returns south. This explains the ‘overturning’ part of its name.
But this process is becoming harder all the time, as the shallow, northward-flowing waters are retaining more of their heat.
On top of this, water from melting Greenland ice is pouring into the North Atlantic in increasing volumes, freshening the waters and reducing salinity. Because warmer, less saline waters have relatively low densities, this hinders sinking, slows the whole system down and threatens to bring everything to a grinding halt.
So what can we expect if and when the AMOC stalls? Are we talking The Day After Tomorrow? Well, not quite. But make no mistake, it would still be a catastrophe for Earth and humankind.
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The AMOC is one of nature’s great engines, which helps to drive not only the climate, but also global weather patterns. Knock-on effects of shutdown will extend across the planet, weakening the Asian and African monsoons, and playing havoc with weather patterns across the Amazon.
But the biggest and most severe impacts are reserved for the North Atlantic region. Average temperatures will plunge across the UK and Europe by at least 10°C, while winter sea ice could reach almost as far south as the southernmost point of the UK.
Meanwhile, on the other side of the Atlantic, the eastern seaboard of North America would see a rapid hike in sea level as the northward-flowing waters backed up, leading to inundation or flooding of coastal communities.
The bottom line is that AMOC collapse would be a cataclysmic event that could result in crop yields being slashed across the planet, bringing widespread starvation and spawning civil conflict and war. And don’t expect this to be short-lived. Once a tipping point tips, it stays tipped, at least on the scale of a human lifetime.
The question is: can we stop it happening? Reducing emissions to zero as soon as possible could help, but the problem with tipping points is that we might not know we’ve passed one until it’s too late. Fingers crossed that this isn’t the case with the AMOC.
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