Climate tipping points - lessons for COP28

Improvements in climate and ecological modelling are making it possible to better anticipate so-called doom loops – climate tipping points that lead to ecosystem collapse. The bad news is that the models suggest that collapses will take place sooner than expected. Simon Willcock, John Dearing and Gregory Cooper report

We know that ecosystems under stress can reach a point where they rapidly collapse into something very different. Globally, more than 20 per cent of ecosystems are in danger of collapsing.

Rainforests are drying out and becoming savanna, increasingly arid savanna is turning into desert and swathes of icy tundra are thawing. Closer to home, the clear water of a healthy lake can turn algae-green within months.

This is happening now. Thanks to this year’s record sea temperatures, colourful coral reefs have become pockmarked with bleached-white colonies in Panama, Colombia, El Salvador, Costa Rica, Mexico, the Bahamas and Cuba, prompting fears of a large-scale bleaching event across the Americas.

These collapses could happen sooner than you might think. Humans are already putting ecosystems under stress in many different ways, ranging from deforestation to excessive cultivation.

Ecosystem collapse

When you combine these stresses with more frequent extreme-weather events and multi-decadal changes in average conditions – such as rainfall and temperature – an ecosystem collapse we might have expected to avoid until late this century could happen within decades, bringing forward the date we cross these so-called tipping points by as much as 80 per cent.

That’s the ominous conclusion of our latest research, published in Nature Sustainability. To better understand how much stress ecosystems can take before they collapse, we used computer models to simulate how they will function in the future under different conditions.

We used two general ecological models representing forests and lake water quality, and two further models representing specific locations. But we also included the actions of local people, looking, for example, at the fishery in Chilika Lagoon in the eastern Indian state of Odisha.

Each of the four models contained both balancing and reinforcing feedback-loop mechanisms. The balancing loops help to absorb weak stresses, keeping the system balanced and stable.

The reinforcing loops accelerate the impact of the stresses, making the system more likely to collapse – just as a snowball gathers mass and momentum as it rolls downhill.

For instance, fishers on Lake Chilika tend to prefer to catch adult fish while the fish stock is abundant. The total fish population can be stable as long as enough adults are left to breed. However, when stresses increase, the ecosystem may pass a point of no return – the tipping point – and collapse.

In Chilika, this might occur when fishers increase their catch of juvenile fish during perceived shortages of adult fish. This undermines fish-stock renewal, which then leads to further shortages, which increases the fishers’ dependence upon juvenile fish catches, creating a so-called doom loop.

Over more than 70,000 different computer simulations, we modelled the combined effects of more stress and extreme events. Across all four models, the combinations of stress and extreme events brought forward the date of a predicted ecological collapse by 30–80 per cent.

This suggests that an ecosystem predicted to collapse in the 2090s due to the creeping rise of a single source of stress – increasing global temperatures – could, in a worst-case scenario, collapse in the 2030s once we factor in other issues, such as extreme rainfall, pollution or a sudden spike in natural-resource use.

Amazon rainforest collapse

Worryingly, we could be observing this ecological collapse in the Amazon Basin right now. Covering 5.5 million sq km, the Amazon rainforest is the world’s largest and is home to about one in ten of all known species.

The rainforest has existed for at least 55 million years, but the Intergovernmental Panel on Climate Change suggests that a potential tipping point is likely to be reached before 2100. Our findings suggest that a breakdown may occur several decades earlier.

And other research supports this conclusion. A group of scientists from the University of Exeter used satellite data to demonstrate that more than three-quarters of the Amazon has been losing resilience since the early 2000s – predominantly in areas close to human activity and regions that receive less rainfall.

Research on the ground supports these claims. Trees are dying more often and growing back more slowly, which reduces total biomass in the Amazon.

If these changes signal the start of a tipping point, given the size of the Amazon rainforest, the entire ecosystem could collapse in less than 50 years. Put simply, the Amazon rainforest could disappear well within our lifetime.

The knock-on effects of a tipping point in the Amazon could be catastrophic. One collapsing ecosystem could have a cascade effect on other globally important ecosystems through successive feedback loops.

And so, as the Amazon dries under climate change and phenomena such as El Niño, we can expect tree mortality and fires to release large amounts of carbon dioxide into the atmosphere. This, in turn, exacerbates climate change, increasing pressure on the Amazon and other important ecosystems.

The carbon dioxide that the collapsing Amazon releases will increase global warming and enhance stress on ice sheets – which could push the West Antarctic and Greenland ice sheets over their own tipping points.

The destruction of these ice sheets could then raise global sea levels, flooding coastal areas worldwide and threatening coral reefs and the Gulf Stream. The last of these impacts would have an enormous effect on the climate of the UK and Europe.

Climate-impact modelling

Modelling studies are based on real-world systems but models, by definition, simplify reality. Of the four models we used, the Lake Chilika model is the most complex.

This model has more than 100 variables that capture hydroclimatic, ecohydrological, fishery and socio-economic dynamics, interacting to create four balancing loops and seven reinforcing loops. The behaviours that the model produces have been validated against historical data.

Of all the models, it shows the least dramatic reductions in dates for any ecosystem collapse. It’s plausible that more complex systems will have stronger mechanisms that stabilise the system. We may also see slowing rates of collapse in ecosystems whose spatial locations prove to be more resilient than others.

It’s fair to view our findings as representing worst-case scenarios for the different ecosystems. Nevertheless, supporting observations from satellite data and on-the-ground measurements suggest that the worst-case scenarios are worth considering.

Previous studies of the cascades of tipping points we’ve described have focused on large-scale systems. They point to substantial social and economic costs from the second half of the 21st century onwards. It’s difficult to imagine how much our lives would change if the Amazon rainforest, polar ice sheets, coral reefs and Gulf Stream did collapse.

But the consequences would be dire, so prudent risk management must clearly consider the factors that could lead to these worst-case scenarios. The messages are stark. They have important implications for governance of real-world systems – including those that COP28 in Dubai will discuss in coming weeks.

The exponentially increasing global trends of many social and biophysical variables over the past 65 years aren’t sustainable. Humanity must prepare now for changes in ecosystems coming at us more rapidly than our traditional, linear view of the world led us to expect.

Large iconic ecosystems such as the Amazon rainforest and the Caribbean coral reefs look set to collapse over relatively short human timescales – years and decades – once a tipping point has been triggered. We have a short window of opportunity to divert unsustainable trajectories for relatively small systems.

Halt ecosystem damage

We need to formulate in advance and prepare to implement contingency plans across localised systems that we identify as heading towards the brink. Our findings give all of us yet another reason to halt the environmental damage that’s pushing ecosystems to their limits.

Put simply, we must act faster to reduce the risk that future climate change and stresses linked to it trigger ecological doom loops. Other priorities for COP28 include developing digital and physical infrastructures to better monitor the ecosystems that supply our fresh air, water and food.

Monitoring real-world ecosystems should capture multiple potential drivers, their variability and their feedback loops to social systems. This is vital to avoid ecological collapses catching us unawares.

That’s what happened in Erhai, western China, where it took lower than expected levels of agricultural runoff to cause lake eutrophication because the waterbody was also under stress from water-level management, seasonal climate and fish farming.

Knowing a local tipping point is coming could help to shift the human decisions to avoid it completely. That includes – arguably most importantly – making it a priority to halt activities that drive ecological degradation and to work to create futures that are safe and just for humanity.

Ultimately, it’s best for human civilisation, and for all the species that share our planet, to avoid tipping points; there’s no way to restore collapsed ecosystems within any reasonable timeframe.

When financial systems collapse, governments find the financial capital to bail out the banks. But no government can conjure up the natural capital to restore a collapsed ecosystem. If we let ecological doom loops happen, we all take the hit.

Simon Willcock is a principal research scientist at Rothamsted Research and professor of sustainability at Bangor University. John Dearing is emeritus professor in physical geography at the University of Southampton. Gregory Cooper is a postdoctoral research associate in the Department of Geography and the Institute for Sustainable Food at the University of Sheffield

This story is published in the November 2023 issue of The Environment magazine