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Die-off of coral reefs, collapsing ice sheets and thawing permafrost are among the runaway processes that are likely to kick in if we exceed 1.5°C of warming
The collapse of the West Antarctic ice sheet and the abrupt thawing of permafrost are among six tipping points in Earth’s climate that are now likely to be reached if global warming exceeds 1.5°C, the goal set by the Paris Agreement in 2015.
In 2008, researchers identified nine tipping points in the planet’s climate system: processes such as ice melt that would become irreversible and self-perpetuating and that could speed up climate change. Now David Armstrong McKay at the University of Exeter, UK, and his colleagues have completed the first major assessment of those possible shifts, and how much global warming it might take to trigger them.
While it was previously thought that most tipping points would occur when the global average temperature rose around 3°C above that in pre-industrial times, the new study found that some could occur at much lower temperatures.
The number of tipping points has also expanded to 16. Some new ones have been added – including changes in the Labrador Sea, part of the North Atlantic, which could cool Europe – while others have been dropped, such as loss of Arctic sea ice, as it is no longer seen as having a tipping point dynamic.
The world has already warmed by 1.1°C since the industrial revolution, at which point there is a low possibility of triggering some tipping points. However, at 1.5°C, six of them become likely, including the collapse of the Greenland ice sheet and the die-off of coral reefs. A further four become possible, from abrupt loss of ice in the Barents Sea to the collapse of the vital Atlantic Ocean conveyor belt, a large system of currents that carries warmer, tropical water north, disruption of which could lead to more extreme heat and cold on both sides of the ocean.
“This provides really strong scientific support for rapid cutting of emissions in line with the 1.5°C goal,” says Armstrong McKay. “But the closer you get to 2°C, the more likely some of these tipping points get. Where we’re heading at the moment is something like 2.6°C — that’s definitely going to hit lots of tipping points.”
Last year’s Intergovernmental Panel on Climate Change (IPCC) report highlighted the risk of tipping points, but didn’t outline the temperatures at which each might be triggered. Armstrong McKay and colleagues trawled scientific literature and asked experts to provide estimates of how much warming might be required to set off the tipping points.
The reason the temperature thresholds have come down since 2008 is a subsequent explosion of research. Better modelling has been key, particularly of ice sheets. Bubbles of air thousands of years old captured in ice cores and other palaeoclimate records have helped us learn how ice sheets responded in the past when the world was 1.5°C hotter. Recent years have also provided observations showing early signs of destabilisation of the Greenland ice sheet and weakening of the Atlantic conveyer belt.
“The science of climate change has advanced hugely in the intervening 14 years and [the study authors] now provide a reassessment based on the latest science. And it is not good news,” says Mark Maslin at University College London, who wasn’t involved in the research.
The tipping points can now be expected much sooner than thought. Many are considered likely or possible at around 1.5°C of warming, which the IPCC has said could happen in the 2030s. “It’s all a lot closer than we were feeling like they were before,” says Armstrong McKay.
One crumb of comfort is that the most imminent tipping points, such as the collapse of the West Antarctic ice sheet — which some scientists believe has already begun — won’t have a huge feedback effect that leads to runaway warming. Armstrong McKay says: “Some people will look at this and go, ‘well, if we’re going to hit tipping points at 1.5°C, then it’s game over’. But we’re saying they would lock in some really unpleasant impacts for a very long time, but they don’t cause runaway global warming.”
Nonetheless, he says it is urgent that societies act to stop tipping points being reached, to prevent impacts such as huge sea level rise, which could result from losing Antarctic and Greenland ice sheets over millennia or centuries. “We’d be locking in future generations to an extremely different planet with 10 metres or more sea level rise. It would completely reshape the coasts of every continent,” says Armstrong McKay.
As his team notes, most of the systems they assess “contribute significantly to human welfare”. Maslin says events such as an abrupt thawing of permafrost would “be devastating for human society and should be avoided at all costs”.
One thing the new research doesn’t consider is how the tipping points might interact with each other. Some could exacerbate others, while some will have a cooling effect offsetting the warming effect of others.
Armstrong McKay says the tipping point that concerns him most is the Amazon transforming from rainforest into savannah, which would release more carbon dioxide. Models predict that this isn’t expected unless warming exceeds 2°C, but that doesn’t account for the deforestation there.
Ice sheet collapse is more of a far-future issue that can seem abstract, while the Amazon’s collapse could unfold in our lifetimes – and there are signs that the transition has already begun in some areas. “That’s the one that you would actually see happening in real time,” he says.
Journal reference: Science, DOI: 10.1126/science.abn7950
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