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To what degree we will be able to adapt to new coastlines is primarily a matter of just how fast the water rises. Our understanding of that timeline has been evolving disconcertingly fast. When the Paris Agreement was drafted, those writing it were sure that the Antarctic ice sheets would remain stable even as the planet warmed several degrees; their expectation was that oceans could rise, at most, only three feet by the end of the century. That was just in 2015. The same year, NASA found that this expectation was hopelessly complacent, suggesting three feet was not a maximum but in fact a minimum. In 2017, the National Oceanic and Atmospheric Administration (NOAA) suggested eight feet was possible—still just in this century. On the East Coast, scientists have already introduced a new term, “sunny day flooding”—when high tide alone, aided by no additional rainstorm, inundates a town.
In 2018, a major study found things accelerating faster still, with the melt rate of the Antarctic ice sheet tripling just in the past decade. From 1992 to 1997, the sheet lost, on average, 49 billion tons of ice each year; from 2012 to 2017, it was 219 billion. In 2016, climate scientist James Hansen had suggested sea level could rise several meters over fifty years, if ice melt doubled every decade; the new paper, keep in mind, registers a tripling, and in the space of just five years. Since the 1950s, the continent has lost 13,000 square miles from its ice shelf; experts say its ultimate fate will probably be determined by what human action is taken in just the next decade.
All climate change is governed by uncertainty, mostly the uncertainty of human action—what action will be taken, and when, to avert or forestall the dramatic transformation of life on the planet that will unfold in the absence of dramatic intervention. Each of our projections, from the most blasé to the most extreme, comes wrapped in doubt—the result of so many estimates and so many assumptions that it would be foolish to take any of them, so to speak, to the bank.
But sea-level rise is different, because on top of the basic mystery of human response it layers much more epistemological ignorance than governs any other aspect of climate change science, save perhaps the question of cloud formation. When water warms, it expands: this we know. But the breaking-up of ice represents almost an entirely new physics, never before observed in human history, and therefore only poorly understood.
There are now, thanks to rapid Arctic melt, papers devoted to what are called the “damage mechanics” of ice-shelf loss. But we do not yet well understand those dynamics, which will be one of the main drivers of sea-level rise, and so cannot yet make confident predictions about how quickly ice sheets will melt. And even though we now have a decent picture of the planet’s climatological past, never in the earth’s entire recorded history has there been warming at anything like this speed—by one estimate, around ten times faster than at any point in the last 66 million years. Every year, the average American emits enough carbon to melt 10,000 tons of ice in the Antarctic ice sheets—enough to add 10,000 cubic meters of water to the ocean. Every minute, each of us adds five gallons.
One study suggests that the Greenland ice sheet could reach a tipping point at just 1.2 degrees of global warming. (We are nearing that temperature level today, already at 1.1 degrees.) Melting that ice sheet alone would, over centuries, raise sea levels six meters, eventually drowning Miami and Manhattan and London and Shanghai and Bangkok and Mumbai. And while business-as-usual emissions trajectories warm the planet by just over 4 degrees by 2100, because temperature changes are unevenly distributed around the planet, they threaten to warm the Arctic by 13.
In 2014, we learned that the West Antarctic and Greenland ice sheets were even more vulnerable to melting than scientists anticipated—in fact, the West Antarctic sheet had already passed a tipping point of collapse, more than doubling its rate of ice loss in just five years. The same had happened in Greenland, where the ice sheet is now losing almost a billion tons of ice every single day. The two sheets contain enough ice to raise global sea levels ten to twenty feet—each. In 2017, it was revealed that two glaciers in the East Antarctic sheet were also losing ice at an alarming rate—eighteen billion tons of ice each year, enough to cover New Jersey in three feet of ice. If both glaciers go, scientists expect, ultimately, an additional 16 feet of water. In total, the two Antarctic ice sheets could raise sea level by 200 feet; in many parts of the world, the shoreline would move by many miles. The last time the earth was four degrees warmer, as Peter Brannen has written, there was no ice at either pole and sea level was 260 feet higher. There were palm trees in the Arctic. Better not to think what that means for life at the equator.
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As with all else in climate, the melting of the planet’s ice will not occur in a vacuum, and scientists do not yet fully understand exactly what cascading effects such collapses will trigger. One major concern is methane, particularly the methane that might be released by a melting Arctic, where permafrost contains up to 1.8 trillion tons of carbon, considerably more than is currently suspended in the earth’s atmosphere. When it thaws, some of it will evaporate as methane, which is, depending on how you measure, at least several dozen times more powerful a greenhouse gas than carbon dioxide.
When I first began seriously researching climate change, the risk from a sudden release of methane from the Arctic permafrost was considered quite low—in fact so low that most scientists derided casual discussion of it as reckless fearmongering and deployed mockingly hyperbolic terms like “Arctic methane time bomb” and “burps of death” to describe what they saw as a climate risk not much worth worrying about in the near term. The news since has not been encouraging: one Nature paper found that the release of Arctic methane from permafrost lakes could be rapidly accelerated by bursts of what is called “abrupt thawing,” already under way. Atmospheric methane levels have risen dramatically in recent years, confusing scientists unsure of their source; new research suggests the amount of gas being released by Arctic lakes could possibly double going forward. It’s not clear whether this methane release is new or just that we finally began to pay attention to it. But while the consensus is still that a rapid, sudden release of methane is unlikely, the new research is a case study in why it is worthwhile to consider, and take seriously, such unlikely-but-possible climate risks. When you define anything outside a narrow band of likelihood as irresponsible to consider, or talk about, or plan for, even unspectacular new research findings can catch you flat-footed.
Today, all do agree that that permafrost is melting—the permafrost line having retreated eighty miles north in Canada over the last fifty years. The most recent IPCC assessment projects a loss of near-surface permafrost of between 37 and 81 percent by 2100, though most scientists still believe that carbon will be released slowly, and mostly as less-terrifying carbon dioxide. But as far back as 2011, NOAA and the National Snow and Ice Data Center predicted that thawing permafrost would flip the whole region from being what is called a carbon sink, which absorbs atmospheric carbon, to a carbon source, which releases carbon, as quickly as the 2020s. By 2100, the same study said, the Arctic will have released a hundred billion tons of carbon. That is the equivalent of half of all the carbon produced by humanity since industrialization began.