The world may be teetering on the brink of a potentially irreversible cascade of climate tipping points that scientists say could lead to an "existential threat to civilization."

According to a new commentary published in the prestigious science journal Nature, we are so close to at least one or two of these tipping points — the thresholds beyond which a slow change tips over into big and abrupt planetary transformations — that we can't rule out the process has already begun.

"What's happened now is we've got some direct physical evidence of the kind of accelerating changes that we expect at a tipping point," said Timothy Lenton, a professor of Climate Change and director of the Global Systems Institute at the University of Exeter, in conversation with Quirks & Quarks host Bob McDonald.

With action on climate change far behind political rhetoric, we're now in an urgent situation. - Timothy Lenton, University of Exeter

One troubling example is the potential collapse of the West Antarctic ice sheet, which could lead to more than three metres of sea level rise.

"We see the glacier retreat accelerating. And as it happens, this speeds up the outflow of ice in a way that's just propelling the change faster," said Lenton. "If we are past it, the saving grace is — at the moment — the rates of melt or ice sheet collapse are relatively slow."

If we continue on our current trajectory of greenhouse gas emissions, the relatively slow change will accelerate, "and not just in proportion to warming it either," he added.

(Timothy M. Lenton et al.)

The latest United Nations Environment Programme's report on the Emissions Gap said that if we don't triple our efforts to reduce emissions to meet the Paris Accord target, we may be looking at a 3.2 C of warming by 2100.

"With action on climate change far behind political rhetoric, we're now in an urgent situation," said Lenton. "This is a climate emergency."

Feedback loops feeding into tipping points

One troubling possibility is that one climate tipping point may trigger others, like dominoes, piling disaster upon disaster.

As ice caps melt and the sea ice shrinks, polar oceans heat up because without the ice, it no longer reflects as much of the sun's energy back to space. Darker water also absorbs more heat, melting ice more quickly in a feedback loop.

Increasing heat also leads more Arctic permafrost melting, which releases even more heat-trapping gases into the atmosphere, and further exacerbating Earth's warming.

Aerial view of melting permafrost tundra and lakes near the Yupik Eskimo village of Quinhagak on the Yukon Delta in Alaska on April 12, 2019. - According to scientists, Alaska has been warming twice as fast as the global average, with temperatures in February and March shattering records. "From 1901 to 2016, average temperatures in the mainland United States increased by 1.8 degrees Fahrenheit (one degree Celsius), whereas in Alaska they increased by 4.7 degrees," said Rick Thoman, a climate expert at the Alaska Center for Climate Assessment and Policy. (Photo by Mark RALSTON / AFP) (Photo credit should read MARK RALSTON/AFP via Getty Images) (Mark Ralston/AFP via Getty Images)

Increasing temperatures also contribute to increasing wildfires in boreal forests in Canada, Alaska and Eurasia. As the forests burn, the carbon they contain is released into the atmosphere.

"A great ecosystem that was taking up carbon from the atmosphere and helping us out is is now turning on us, if you like, releasing that carbon back. And adding to the problem, the soot from those fires sometimes is landing on the ice on top of the Arctic Ocean and is accelerating its melt," said Lenton. "These things are starting to connect in damaging ways."

Tipping over into a hothouse Earth

Lenton said the most dangerous tipping point on the horizon is what's called "overturning circulation" in the Atlantic Ocean, partly because of the way it is fed by, and feeds into, other climate transformations.

This circulation acts like a conveyor belt transporting warm surface water from the south to north, and cold deep water in the other direction. Massive currents like the Gulf Stream are part of this phenomenon.

In the last half a century this circulation, has already slowed by 15 per cent, possibly due to melting Arctic ice which 'caps' the ocean with cold, fresher water and blocks the circulation. If it were to slow down even more or stop, Lenton said Earth's past shows us that it would lead to drastic changes in climate.

"The heat that wouldn't be getting transported from the southern hemisphere to the northern hemisphere, that would translate into seeing rainfall and monsoon systems in the tropics shifting southwards."

If that happens, it could disrupt rainfall patterns over the Amazon rainforest and cause "catastrophic droughts in West Africa" and even more warming in the Southern Ocean.

My glimmer of hope lies in the thought that there is clearly the possibility of tipping points in our social, economic [and] technological systems. And we're going to need to tip some of those positive changes if we want to avoid the really bad climate tipping points. - Timothy Lenton, University of Exeter

Some of these individual systems and feedback loops are well understood, but the extent of their interaction and how they drive each other is still a subject of active research. But Lenton and his colleagues are concerned that it could lead to a "hothouse Earth" scenario in which there is little ice left at the poles and sea levels are tens of metres higher.

"One way to get that kind of state is through a series of these tipping points cascading together," he said. "So we're not sure yet, but that seems like a plausible scenario and it's one we definitely need to get a handle on because that's what we'd call an existential threat to civilization tipping into our Earth."

One nightmare scenario involves mass migration of climate refugees. Populist backlash to moderate numbers of economic migrants is already a destabilizing political force. "What's it going to be like when [the refugees are] in the tens or hundreds of millions?"

Boreal forest wildfires like the 2016 Fort McMurray fire add carbon into the atmosphere and can leave dark soot on Arctic ice, further exacerbating climate warming. (Jason Franson / The Canadian Press)

Different kinds of tipping points

Lenton said that while he said it's hard to be optimistic as a climate scientist, he remains hopeful.

He points out that technological advances and economies of scale have started to make renewable power systems cheaper to install than fossil fuel power station in some places — China in particular. This may represent a positive kind of tipping point in carbon neutral energy systems that could continue to accelerate in a virtuous, rather than vicious, cycle.

"My glimmer of hope lies in the thought that there is clearly the possibility of tipping points in our social, economic [and] technological systems," Lenton said.

"And we're going to need to tip some of those positive changes if we want to avoid the really bad climate tipping points."