On September 17, a short paper in the journal Nature Climate Change caught the attention of Penn State University glaciologist Richard Alley. The article compared satellite observations of the Antarctic ice sheet between 2007 and 2017 with projections for the same period published in 2013 by the Intergovernmental Panel on Climate Change. The research showed that actual melting of the southern ice sheet has tracked the curve of the fastest change IPCC considered plausible.

In a decade of observations, the Antarctic ice sheet had shed 30 times more water than it would have had it receded at the low end of the IPCC estimates. If this trend continues, Antarctica could empty enough water into the world’s oceans to raise sea level by 1.5 meters by 2100, threatening the world’s largest cities, most of which are coastal. (Actual sea-level rise would be higher, including meltwater from the Greenland ice sheet and mountain glaciers, and the effect of expansion of warming sea water.)

Alley, a leading ice sheet expert, said in a phone interview that the results validate a fear that preoccupies him: that continued warming of the planet could propel the Antarctica to a tipping point, beyond which civilization-threatening sea-level rise could not be avoided. He said the new study contributes to an accumulating body of evidence of “mass loss that has accelerated.”

Increasing urgency over potential ‘inflection point’

Climate researchers are debating with growing urgency whether the steady release of carbon dioxide and other warming gasses from the world’s tailpipes and smokestacks could push the planet to an inflection point beyond which the climate might abruptly and irreversibly change.

Such changes might be too great, and might occur too quickly, for people to adapt. Cities, no longer suitable for the new conditions, might become uninhabitable. Nations might be unable to feed themselves. “There’s an old adage that speed kills,” said James White, dean of the College of Arts and Sciences at the University of Colorado and an expert on past climate. “The same is true for climate change.” Twenty five years ago, White himself had a hand in proving the lightning speed with which the climate can turn.

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That was in July 1993, when a team of U.S. scientists drilling an ice core in Greenland hit bedrock. In five years of summer campaigns, they’d extracted nearly two miles of fence-post-thick cylinders of ice that could be analyzed in laboratories to study global climate for the past 100,000 years with precise chronological accuracy.

One remarkable discovery made with that ice core involved a series of abrupt weather shifts transcribed in the physical and chemical properties of tree-ring-like annual layers formed from compressed snow. One particular abrupt change discovered in that ice captured the attention of researchers and has since spawned dozens of scientific papers.

About 15,000 years ago, as the last ice age retreated, Greenland made a sudden about-face, and plunged back into a deep freeze in a series of steps. Full ice-age conditions reigned again for 1,200 years – a period geologists call the Younger Dryas – until about 11,600 years ago. That’s when conditions reversed course again, this time in one giant leap.

As a young professor, Alley had logged the pearly cores of Greenland ice at a light table in a makeshift lab near the drill rig. He vividly recalls the “Oh, wow!” moment when he noticed the translucent layers corresponding to the end of the Younger Dryas. Suddenly, they thickened – indicating a huge, abrupt increase in snowfall. White – who also spent time in Greenland processing the cylinders of ice – Alley, and several other researchers went on to write a paper in Nature reporting that snowfall doubled over a period of one-to-three years. At the same time, Greenland’s temperatures rose an astounding 8 °C (14 °F) in less than a decade. This virtually instantaneous climate fluctuation has since been identified in many other parts of the world.

Past instances ‘most compelling reason’ for concerns

Timothy Lenton, director of the Global Systems Institute at the University of Leeds, points to those astonishing past instances of abrupt climate variations as “the most compelling reason” scientists are concerned that global warming could trigger such a change in years to come. Those developments prove that “a small change in the background conditions in the climate somehow triggered a larger response,” he said by phone.

Two other kinds of evidence also raise researchers’ concerns that further warming could trigger a dangerous climate jolt. First, scientists have identified self-reinforcing mechanisms that, once-initiated, might swiftly amplify the rate of warming. One frequently cited example involves the melting of once-permanently-frozen permafrost in Alaska and other high-latitude parts of the Northern Hemisphere. Carbon-rich tundra can let off methane – a powerful greenhouse gas – when permafrost thaws and decays. Some scientists theorize that once Earth warms enough, so much of the gas will be released that the extra warming it generates will, in turn, melt more permafrost, thereby releasing more gas, in a vicious – possibly accelerating – circle. (Though worrisome, many experts say that this scenario won’t pose a substantial risk this century.)

A second cause of scientific concern is that computer simulations of Earth’s climate sometimes produce abrupt events. For instance, in the early 2000s, a climate model run by researchers at the United Kingdom’s Met Office forecast a sudden and dramatic drying in the Amazon basin by the end of the century. This Amazon dieback would replace the closed-canopy rainforest with an austere grassland sprinkled with drought-tolerant trees, akin to an African savannah. (Subsequent studies have cast doubt on the hypothesis, but scientists still anticipate that extensive swaths of the Amazon will be recolonized with trees more suitable for drier conditions.)

Academy reports underscore grave risks

Since 2002, the National Academy of Sciences has published two reports on abrupt climate change, both stressing the grave risks posed. Yet both struggled to define abrupt climate change precisely. Generally scientists refer to a fundamental transition in the climate, or the impacts of the climate on society, over a period of years to decades – a timescale of human consequence. Abrupt changes, sometimes called a climate surprise, must also be irreversible on a span of decades, the duration most relevant to people’s lives.

Alley, who had chaired the first of those NAS studies, said in a phone interview that what concerns him most is the risk of a collapse of parts of the West Antarctic Ice Sheet. He’s especially concerned about the Thwaites Glacier, a stream of ice that originates deep in West Antarctica. The widely shared apprehension is not that warm air will melt the glacier’s outer surface, a relatively slow process. Rather, scientists fear that the glacier could suddenly (in a small number of decades or centuries) collapse, casting mammoth glaciers into the sea and contributing to frightfully fast sea-level rise.

Thwaites glacier concerns in Antarctica

Recent theoretical studies and observations in Greenland show that warming sea water could destabilize glaciers, such as Thwaites, that come into contact with the ocean. Icebergs could break faster than ever seen before and float away, accelerating ice seaward from the continent’s interior. A 2016 paper in Nature predicts that Thwaites Glacier alone could drain enough ice from Antarctica to raise sea level by 10 feet within 100 years, a rate likely to force the abandonment of some entire cities.

“That’s not a worst-case scenario in any sense of the word,” Alley said grimly. Last April the National Science Foundation and the UK’s Natural Environment Research Council began a $25 million research program, the International Thwaites Glacier Collaboration, to study the stability of the Thwaites glaciers. Research is to begin this month, though conclusive results could take years.

‘Tipping point’ comes when too late for canoe to avoid the nearby waterfall, not when actually plunging over it.

James White, who chaired the second NAS study, completed in 2013, said a tipping point – the point of no return beyond which an abrupt change becomes inevitable – could occur long before the unwanted transition, or even before anything seriously awry begins. “Imagine you’re in a canoe floating down a river and you hear a noise on the horizon and see mist,” White said in a telephone interview. “The tipping point is not when you go over waterfall. It is when you can no longer get to the shore.”

Could Earth have already reached a tipping point for a catastrophic change? Some computer models suggest that it might already be too late to stop Thwaites from emptying its huge ice load into the sea. It’s impossible to say, Alley said. But the consensus seems to be that, so far, “we probably have not.”

As for White, he predicts that the first climate change tipping point might be in the north, not the south. “I tell my students, ‘keep your eyes on the Arctic Ocean,'” he said. Warming and other changes in the Artic have shrunk the summer cap of sea ice by more than 40 percent since 1979. At that rate, the Arctic could be virtually ice free within the next several decades. If that happens, White anticipates “a fundamental change in weather patterns and climate.”

In April 2017, authors of a paper in Nature reported clues that another tipping point might have been reached. They wrote that their study showed the Atlantic meridional overturning circulation – AMOC, an important ocean current associated with the Gulf Stream – has slowed by 15 percent since the mid-20th century. The AMOC ferries equatorial heat to the north Atlantic, warming Europe. It’s understood that if the current were to halt, the British Isles and Scandinavia would cool dramatically, upending regional agriculture and fisheries. Sea level along the U.S. Atlantic seaboard could rise nearly an additional meter. The AMOC’s current tugs the ocean away from the U.S. coast, a benefit to coastal dwellers that would cease if the flow ended.

Scientists have long speculated that meltwater from a shrinking Greenland could make water in the north Atlantic less salty, and thus less dense, impairing the engine – powered by sinking saline water – that moves the AMOC. The important Atlantic current has shut down before in geologic history. Many researchers consider it to be safe for the rest of this century; but the evidence of slackening raises doubts. The drop-off of that current “might be within the range of natural variability,” said Lenton. But he adds, “It better start kicking up soon or we would have cause for concern.”

Authors of the 2013 NAS report had strongly urged the United States to put into place early warning systems to detect these precursors of climate tipping points. But so far no government has made a priority of keeping watch for warning signs of dangerous climate rapids ahead. Compared to how much it might save, the cost of an early warning system “is peanuts,” White said.