Climate tipping points

Tim Lenton, Professor of Earth System Science at the University of Exeter, explains climate tipping points, when we may hit them and warning methods.

Tim Lenton

Professor of Climate Change and Earth System Science

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Tim Lenton

Professor of Climate Change and Earth System Science

22 Jan 2022

Key Points

Like any complex system, the parts of the Earth system are interconnected. A tipping point is where a small nudge to a system has a huge outcome. It changes the state or the fate of that system.
There are three main kinds of climate tipping points: a first includes the melting of the great ice sheets; a second involves a major loss of the biosphere; a third involves the circulation of the ocean and the atmosphere, and how they’re coupled together.
When various tipping points interact in a domino effect, it’s called a “tipping cascade” – and this is what we must avoid at all costs.
We expect some tipping points to be triggered at different levels of warming and that’s the strongest reason, to try and limit global warming, which means stopping fossil fuel burning as soon as possible.

Tipping points are something that can happen in a whole range of complex systems, including the human body and the entire Earth climate system, or “Earth system”. A tipping point is where a small nudge to a system has a huge outcome. It changes the state or the fate of that system. You could say that a heart attack or death were the bleakest tipping points for the human body; but, in all cases, the crucial thing is that within complex systems, there are cycles. There are closed loops of causality; there are things that scientists call feedbacks. Some of these feedbacks amount to amplifiers.

And that’s what’s driving change.

Identifying the different kinds of climate tipping points

There are many potential tipping points in the climate system that human activities are currently causing and could potentially tip for the rest of this century.

A first class of tipping points includes the melting of the great ice sheet on Greenland, on West Antarctica and parts of the East Antarctic ice sheet. A second category of tipping points involves the biosphere: the possibility that we could tip a dieback of the Amazon rainforest or the great forests that cloak the northern high latitudes – the boreal forests – or a major loss of coral reefs in the tropical oceans. Finally, there’s a third class of tipping points, which would involve reorganising aspects of the circulation of the ocean and the atmosphere, and how they’re coupled together.

Like any complex system, the parts of the Earth system are interconnected. That’s also true of the climate tipping points that I and others have identified. We know, for example, that as the Greenland ice sheet is melting and fresh water is pouring into the North Atlantic Ocean on either side of Greenland, that this makes the surface waters of the ocean less dense, less salty and less prone to sinking to the bottom of the ocean. We call this extraordinary process: "deep convection of ocean waters".

The deep convection of ocean waters

It’s the deep convection of ocean waters that propels what we call the Atlantic overturning circulation. So, the Greenland melt is weakening the Atlantic overturning circulation, and we know from the record of Earth’s climate past that this affects the monsoons in West Africa and in South America, and in India.

The other thing that happens is that you leave heat behind in the southern oceans and the southern hemisphere. That heat can contribute to melting the West Antarctic and the edge of the East Antarctic ice sheet. These are just some examples of couplings between the tipping elements of the climate system.

"Cold polar water is drawn down from higher latitudes and sinks to the ocean bottom, pulled down toward the equator as lighter, warmer water rises to the ocean's surface."

What tipping points could mean for life on Earth

Perhaps the biggest thing we need to be concerned about with respect to climate tipping points is whether they interact in a way that’s like dominoes, where you stick the dominoes upright and if you tip one of them over, it tips over the next, and so on. That’s what we would describe as a “tipping cascade” – and a very bad one if it was to happen in the climate. Now, we don’t see that happening yet, thankfully, but we do have some concerning evidence that several of the tipping points do connect in that way.

There are some counterexamples where, for example, weakening the overturning circulation of the Atlantic Ocean will cool Greenland and suppress its melt. But overall, unfortunately, they look to be connections that are in this cascading form – and that’s a real risk we’re running, with changing the climate. We must avoid triggering that kind of tipping cascade, because if it isn’t obvious, it could be highly destructive and fatal for civilisation and many other things we value.

The bottom line

As the global temperature increases with human-caused global warming, we expect the likelihood of particular tipping points to be crossed to increase. What’s alarming is that we’re currently at just over one degree centigrade of warming and we’re already seeing strong evidence that we might have tipped part of the West Antarctic ice sheet into an irreversible retreat. It drains enough ice to raise the global sea level by over a metre. If that bit of the ice sheet goes, physical models of the ice sheet suggest that the other parts of the West Antarctic ice sheet would eventually collapse and that would give over three metres of sea level rise.

The same dynamics have been happening for parts of the East Antarctic ice sheet, where, again, it’s hard to rule out that we aren’t already at a tipping point for a part of East Antarctica, draining the Wilkes Basin. The Greenland ice sheet is melting at an accelerating rate. We can’t rule out that it’s shrinking irreversibly. So, we might have already made a commitment in the long term to 10 or more metres of sea level rise.

When and where the tipping points will occur

Scientifically, it’s a really interesting and difficult challenge to predict where and when tipping points are going to happen. We have a general understanding that, as we warm the world up, they will become more likely. In the last 20 years, a range of evidence we’ve got from the Earth's climate, as well as improving models, has been bringing our assessment of where the tipping points lie down to lower temperatures nearer to the present.

So, we would still expect some tipping points to be triggered at lower levels of warming and some at higher levels. That’s a strong reason, perhaps the strongest reason, to try and limit global warming, which in simple terms means stopping fossil fuel burning as soon as possible.