Seven scientists say an "emergency response" must keep warming limited to 1.5°C, based on a roundup of Earth’s tipping points. The commentary piece in Nature, titled Climate Tipping Points – Too Risky To Bet Against, is a call to action.
More than half the tipping points first identified by the Intergovernmental Panel on Climate Change (IPCC) decades ago have been activated, according to the authors.
"In our view, the evidence from tipping points alone suggests that we are in a state of planetary emergency: both the risk and urgency of the situation are acute," they write.
Even if taken as a possibility, the threats can no longer justify a wait-and-see approach and response is needed. As they say, "To err on the side of danger is not a responsible option."
"As science advances, we must admit that we have underestimated the risks of unleashing irreversible changes, where the planet self-amplifies global warming," said co-author Johan Rockström, director of the Potsdam Institute for Climate Impact Research, in a statement.
"This is what we now start seeing, already at 1°C global warming."
Some of the tipping points include the West Antarctic, East Antarctic, and Greenland ice sheets. The Amundsen Sea embayment of West Antarctica is a region where ice, ocean, and bedrock meet, with rapidly thinning ice streams contributing towards sea level rise. If it collapses, it "could destabilize the rest of the West Antarctic ice sheet like toppling dominoes," write the authors. A collapse could result in a 3-meter sea-level rise in the next centuries to millennia.
Previous modeling for the East Antarctic ice sheet suggests this region of the world could add another 2 to 4 meters of sea-level rise, with Greenland's ice sheet adding up to 7 meters over thousands of years if its threshold is surpassed.
Altogether, these tipping points would commit future generations to a sea-level rise of around 10 meters over thousands of years. This rate of melting depends on our ability to keep warming below a set temperature; at 2°C it would take less than a millennium, while at 1.5°C it would take around 10,000 years to unfold, according to estimates. More data, they note, is needed to help narrow down these timescales.
Mass coral bleaching from ocean heatwaves is another tipping point, with the loss of half of shallow-water corals seen on Australia’s Great Barrier Reef. The team say a global temperature rise of above 2°C could result in a 99 percent loss of tropical corals.
The Amazon rainforest biosphere tipping point lies between 20 and 40 percent forest cover lost, with about 17 percent gone since 1970. If levels are reached above these points, the consequence could result in even more greenhouse gases.
The team also points to large-scale insect disturbances and fires that have potentially shifted some regions into a carbon source rather than a carbon sink. As the authors note, atmospheric carbon dioxide is already at levels seen 4 million years ago during the Pliocene epoch.
Other tipping points mentioned in the commentary include melting permafrost, Arctic sea ice loss, and the Atlantic meridional overturning circulation.
The team concludes on these words: "We argue that the intervention time left to prevent tipping could already have shrunk towards zero, whereas the reaction time to achieve net zero emissions is 30 years at best. Hence we might already have lost control of whether tipping happens. A saving grace is that the rate at which damage accumulates from tipping – and hence the risk posed – could still be under our control to some extent. The stability and resilience of our planet is in peril. International action – not just words – must reflect this."
The commentary was written by Timothy M. Lenton, Johan Rockström, Owen Gaffney, Stefan Rahmstorf, Katherine Richardson, Will Steffen, and Hans Joachim Schellnhuber.