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Rapidly reducing greenhouse gas emissions is indispensable to limit the risks of crossing tipping points in the climate system.
LONDON, 8 June, 2021 − Here is a set of circumstances that could trigger global climate catastrophe. The Greenland ice sheet could begin a process of irreversible melting.
As it does, greater quantities of fresh water would flood into the Arctic Ocean, to further slow the already slowing Atlantic meridional overturning circulation, that great flow of water sometimes called the Gulf Stream that distributes warmth from the tropics.
But as the Atlantic flow weakens, so rises the probability of increased and sustained drought and dieback in the Amazon rainforest: the entire region could begin to tip inexorably into savannah.
And the Southern Ocean would begin to warm: it could warm enough to hasten the disintegration of the West Antarctic ice sheet, to accelerate the rise of global sea levels and intensify the whole machinery of global heating.
Alarmingly, this process could begin to happen while global temperatures are still not much higher than they are now: 1.5°C has been repeatedly described as the limit beyond which global average temperatures should not rise, but the official global agreed target is a limit of 2°C.
In fact, the chance of a cascade of domino effects − of tipping points that trigger other climate tipping points − could begin somewhere between those two figures, and the probability rises thereafter.
No way back
And, researchers warn, when they say irreversible, they mean it. Once the Greenland ice sheet starts to slide into the sea, there will be no stopping it. The only question is how swiftly all these things could happen.
“Once triggered, the actual tipping process might take several years up to millennia, depending on the respective response times of the system,” the scientists write in the journal Earth System Dynamics.
It’s a scenario, not a prediction. It’s a calculation of possibilities and probabilities inherent in the process of global warming and climate change. It’s an identification of the way atmospheric warming driven by greenhouse gas emissions from human economies can and might change the climate system that drives planetary weather.
“We provide risk analysis, not a prediction, yet our findings still raise concern,” said Ricarda Winkelmann, of the Potsdam Institute for Climate Impact Research, one of the authors.
She and her colleagues base their study on computer simulations of planetary response to temperature rise. And one third of those simulations suggest that if the world reaches 2°C, then one of those elements could begin to tip towards irreversible change, and at the same time trigger other tipping points.
“We’re shifting the odds, and not to our favour − the risk is clearly increasing the more we heat our planet,” said her colleague and co-author Jonathan Donges. “It rises substantially between 1°C and 3°C.
“If greenhouse gas emissions and the resulting climate change cannot be halted, the upper level of this warming range could most likely be crossed by the end of this century. With even higher temperatures, more tipping cascades are to be expected, with long-term devastating effects.”
Climate science has been concerned with the idea of tipping points − temperatures beyond which climate change might be irreversible − for decades. There have been repeated findings that some of these might be nearer than anybody had suspected.
Greenland is in effect the reservoir of most of the Northern hemisphere’s ice − enough to raise sea levels by seven metres − and it is melting at an ever-accelerating rate.
Researchers have again and again identified a possible faltering of the Atlantic current, to warn of a paradoxical consequence: if the Gulf Stream slows, then average temperatures in western Europe could actually fall in a globally-heating world.
The Amazon rainforest − a vital part of the planet’s climate machinery since the end of the last Ice Age − has been hit not just by human degradation but by drought and forest fire, and could be about to slide into permanent savannah.
And scientists in Antarctica have been warning for a decade of thinning ice sheets, and accelerating glaciers.
The planet has already warmed by more than a degree Celsius in the last century or so. There is a high chance that some time this decade the annual average planetary temperature could pass the 1.5°C threshold, if only temporarily.
Right now, although 195 nations in Paris in 2015 committed themselves to a target of “well below” 2°C by 2100, the world is heading for a temperature rise by the end of the century of more than 3°C.
The authors concede that their results contain a lot of uncertainties: there is more research to be done. But that doesn’t mean there is no urgency.
“Our analysis is conservative in the sense that several interactions and tipping elements are not yet considered”, said Professor Winkelmann. It would hence be a daring bet to hope that the uncertainties play out in a good way, given what is at stake.
“From a precautionary perspective, rapidly reducing greenhouse gas emissions is indispensable to limit the risks of crossing tipping points in the climate system, and potentially causing domino effects.”
Tim Radford, a founding editor of Climate News Network, worked for The Guardian for 32 years, most of that time as science editor. He has been covering climate change since 1988.
First published in Climate News Network. Included in Vox Populi with permission.
Melting glaciers in Greenland may be contributing to a cascade of climate emergencies. (photo: Earth)