Dying of coral reefs, collapsing ice sheets and thawing permafrost are among the runaway processes likely to occur if we exceed 1.5°C warming
September 8, 2022
The collapse of the West Antarctic Ice Sheet and the abrupt thawing of the permafrost are six turning points in Earth’s climate that are now likely to be reached if global warming exceeds 1.5°C, the target set in 2015 in the United States. Paris Agreement was made.
In 2008, researchers identified nine tipping points in the planet’s climate system: processes such as ice melting that would become irreversible and self-perpetuating and could accelerate climate change. Now David Armstrong McKay of the University of Exeter, UK, and his colleagues have completed the first major assessment of those potential shifts, and how much global warming it could take to trigger them.
While it was previously thought that most tipping points would occur when global average temperatures rose about 3°C above those in pre-industrial times, the new study found that some could occur at much lower temperatures.
The number of tipping points has also been 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 scrapped, such as the loss of Arctic sea ice, as it is no longer seen as a dynamic tipping point.
The world has already warmed by 1.1°C since the industrial revolution, and at that point there is a small chance that some tipping points will arise. At 1.5°C, however, six become likely, including the collapse of the Greenland ice sheet and the death of coral reefs. Four more may become possible, from abrupt ice loss in the Barents Sea to the collapse of the vital conveyor belt of the Atlantic Ocean, a vast system of currents that carries warmer, tropical waters northward, whose disruption could lead to more extreme heat and cold. on both sides of the ocean.
“This provides really strong scientific support for a rapid reduction in emissions in line with the 1.5°C target,” Armstrong McKay said. “But the closer you get to 2°C, the more likely some of these will become tipping points. What we’re going for right now is something like 2.6°C – that’s definitely going to lead to a lot of tipping points.’
Last year’s report from the Intergovernmental Panel on Climate Change (IPCC) emphasized the risk of tipping points, but did not outline the temperatures at which each could be triggered. Armstrong McKay and colleagues scoured scientific literature and asked experts to provide estimates of how much warming might be needed to cause the tipping points.
The reason temperature thresholds have dropped since 2008 is a subsequent explosion of research. Better modeling has been key, especially of ice sheets. Air bubbles thousands of years old trapped in ice cores and other paleoclimate data have helped us learn how ice sheets reacted in the past when the world was 1.5°C hotter. Recent years have also yielded observations showing early signs of destabilization of the Greenland ice sheet and weakening of the Atlantic conveyor belt.
“The science of climate change has advanced tremendously in the intervening 14 years and [the study authors] now offer a reassessment based on the latest science. And that’s not good news,” said Mark Maslin of University College London, who was not involved in the study.
The tipping points are now to be expected much sooner than expected. Many are considered likely or possible with warming around 1.5°C, which could happen in the 2030s, according to the IPCC. “It’s all much closer than we felt before,” Armstrong McKay says.
One crumb of consolation 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 massive feedback effect leading to runaway warming. Armstrong McKay says, “Some people will look at this and say, ‘Well, if we get to tipping points at 1.5°C, then it’s game over’. But we’re saying they’ll hold some really unpleasant effects for a very long time, but they’re not causing runaway global warming.”
Nevertheless, he says there is an urgent need for societies to take action to prevent tipping points from being reached, to prevent effects such as massive sea level rise, which could result from the loss of Antarctic and Greenlandic ice sheets over millennia or centuries. “We would lock up future generations on a very different planet with a sea level rise of 10 meters or more. It would completely reshape the coasts of every continent,” Armstrong McKay said.
As his team notes, most of the systems they review “contribute significantly to human well-being.” Maslin says events such as the abrupt thawing of permafrost “would be devastating to 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 can aggravate others, while some have a cooling effect that offsets the warming effect of others.
Armstrong McKay says the tipping point that worries him most is the Amazon turning from rainforest to savanna, releasing more carbon dioxide. Models predict this is not expected unless warming exceeds 2°C, but that doesn’t explain the deforestation there.
The ice sheet collapse is more of a distant future issue that may seem abstract, while the Amazon collapse could happen in our lifetime – and there are signs that the transition has already begun in some areas. “That’s the one you’d see happening in real time,” he says.
Reference magazine: ScienceDOI: 10.1126/science.abn7950
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