29/11/2019

'Dangerously Close': Tipping Points May Trigger Climate Cascade

Sydney Morning Herald - Peter Hannam

The planet faces a "global cascade of tipping points" that could lead to an abrupt shift to a warmer world and cause huge disruption to human societies and ecosystems unless nations slash their greenhouse gas emissions.
The warning is contained in an article appearing on Thursday in Nature. The authors identified several "large-scale discontinuities" in the climate system that may be underway, and which could trigger further warming.

Countries need to cut their greenhouse gas emissions much faster than agreed under the Paris Climate Agreement, the UN has warned, or risk "wide-ranging and destructive climate impacts." 

Countries need to cut their greenhouse gas emissions much faster than agreed under the Paris Climate Agreement, the UN has warned, or risk "wide-ranging and destructive climate impacts."
"If damaging tipping cascades can occur and a global tipping point cannot be ruled out, then this is an existential threat to civilisation," the scientists said.
The ice sheets in West Antarctica may be one of several cryosphere tipping points that were "dangerously close", if they hadn't already begun an irreversible retreat. These alone would raise sea levels by three metres if melted.
Active global warming tipping points
Evidence that tipping points are under way has mounted in the past decade.
Source: Nature
Illustration: Matt Golding
Those in the Wilkes Basin of eastern Antarctica may be similarly unstable, with another four metres of potential sea-level rise if they disintegrated.
"The time to act decisively is now. Any more dithering is irresponsible, as the risks are increasing year by year," said Stefan Rahmstorf, head of Earth system analysis at Germany's Potsdam Institute for Climate Impact Research, and one of the authors.
"But even once we pass a tipping point – and probably we have done so for the West Antarctic Ice Sheet – we will need to reduce emissions even more urgently, to slow down the unfolding effects and to avoid passing further tipping points."

Running AMOC
The interconnected nature of the giant mixing processes that distribute heat around the world's oceans is a key reason why one region's changes could reinforce other shifts.
For instance, the melting of Greenland's ice sheets is driving an influx of fresh water into the North Atlantic, slowing the Gulf Stream – also known as the Atlantic Meridional Overturning Circulation (AMOC) – by 15 per cent since the middle of the 20th century.
Another outcome is a further heating of the Southern Ocean, resulting in more Antarctic ice melt.
A view from a NASA aircraft of large icebergs that have broken from the calving side of Thwaites Glacier in Antarctica. Credit: NASA
Other effects include drought for the Sahel region because of disruptions to the West African monsoon, and worse fires in the Amazon as that region dries out.
Will Steffen, an emeritus professor at the Australian National University and another author, said some of the processes would add to warming by releasing more carbon dioxide or methane to the atmosphere.
Amazon dieback alone had the potential to release about 90 billion tonnes of CO2 while boreal forests could add another 110 billion tonnes. Even without including the methane, emissions from melting permafrost could total 100 billion more tonnes of CO2, the report said.
Fires in the northern hemisphere's boreal forest will add to the atmosphere's carbon dioxide emissions. Credit: Jason Franson/The Canadian Press
By contrast, humans directly contribute to about 40 billion tonnes of CO2 a year. We also have a total emissions budget of 500 billion tonnes if the world is to have a 50:50 chance of hitting the Paris climate target of keeping warming to 1.5 degrees.
Entire ecosystems, such as the Great Barrier Reef, were also facing tipping points. Half the reef's coral cover had been lost in recent bleaching and only a tiny fraction would remain if warming reached 2 degrees, the paper said.
A red sun among heavy smoke caused by the fires in the Amazon forest in the state of Rondonia, Brazil, in August 2019. Credit: Joedson Alves
Australia is among the most exposed nations given it is already exposed to droughts and heatwaves.
"The warning signs are clear," Professor Steffen said. "It will be a much tougher climate for us to live in."
Mustering of sheep in a paddock of a failed wheat crop at Rebecca and Dan Reardon's property near Moree, NSW, which has been affected by years of drought. Credit: Alex Ellinghausen
The risk was not of runaway global warming but rather of a world stabilising at perhaps 5 degrees warmer.
"The schoolchildren are right," he said. "We indeed have a climate emergency, and an emergency-level response is now needed to ensure that we don’t activate the tipping cascade."

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