Sea level rise—perhaps the most consequential effect of climate change—just got a whole lot more urgent. If you live near the coast, this is your wakeup call.
In a study released Wednesday, a new estimate of how much Antarctic ice would melt in a warmer world nearly doubles previous projections of sea level rise by the end of the century. And it might be even worse than that: The study did not explore the true worst-case scenario, and its lead author said the work is still incomplete. Taken together with recent results from other research teams—most notably James Hansen’s, just last week—it’s increasingly clear that consensus projections of near-term sea level rise, about three feet in the next 85 years, are likely an underestimate.
The latest information comes via a breakthrough in simulating the behavior of Antarctica’s vast and complex network of glaciers and ice shelves. That’s brought a more complete understanding of how warmer air temperatures—projected to surpass those regularly experienced on Earth at any point during at least the last few million years—are affecting the sea level. At the same time, the study provides new certainty that—should the world act immediately to curb carbon emissions at a scale far beyond current efforts—virtually all Antarctic ice melt could be avoided.
We should take this result very seriously. The new study prompted a lapse into Ciceronian prose from the New York Times and an instant revision to sea level rise projection maps for coastal cities worldwide, with many observers noting that, at current effort levels, humanity is veering dangerously close to the worst-case scenario.
“Under the high emissions scenario, the 22nd century would be the century of hell,” Ben Strauss, a sea level scientist at Climate Central told the Washington Post. “There would really be an unthinkable level of sea rise. It would erase many major cities and some nations from the map.”
But well before then, in the lifetimes of people being born today, the new study points to a potentially existential threat for cities like Miami; Guangzhou, China; Mumbai, India; New Orleans; Boston; and Alexandria, Egypt. In a scenario in which global carbon emissions remain essentially unchecked, the study argues the world’s coastal cities could see an additional two feet of sea level by 2100 above previous estimates—about five feet total.
In an interview with Slate, lead author Rob DeConto said that his results would be “really, really bad news for the business-as-usual future.”
DeConto found that the biggest deciding factor of future sea level rise from Antarctica is near-term carbon emissions. There’s tenuous hope that we’re finally getting the message: Global emissions may have temporarily stopped rising in the last year or two, and December’s climate agreement in Paris provides the framework to steer humanity off from its present course, albeit only slightly.
“Chances are, there’s going to be a much better outcome if we end up following one of the lower emissions scenarios,” DeConto says, noting that in the lowest-emission scenario—in which global emissions peak almost immediately—his study finds there about a 90 percent chance that Antarctica will stay essentially completely frozen.
In all scenarios, DeConto found for Antarctica, the damage may already be done—at least on timescales we can imagine. “It takes literally millennia for the oceans to cool back down again. It will take thousands of years for the ice sheet to push its nose back out into the ocean. … It’s essentially a permanent change.”
In the high-emissions scenarios DeConto explored, that means Antarctica will be melting both from above—via a warmer atmosphere—and from below—via warmer oceans. Since many of the largest glaciers in western Antarctica are resting on bedrock that’s currently below sea level, ocean warming could melt them quite quickly—within decades. “Once it gets away from us, there isn’t really a geoengineering solution for that,” DeConto said. “Once the oceans warm up enough, it’s not going to matter.”
Like any single scientific study, Wednesday’s results aren’t a sure thing, but they’re much closer to a complete answer than anything we’ve had so far. The Washington Post’s Chris Mooney has a particularly illustrative look at the science that underpinned the new results.
The study’s major achievement is the addition of two key ice melting processes left out of all Antarctica-wide models to this point: Meltwater ponding and crevasse formation on the surface of ice sheets—so called “hydrofracturing”—and the tendency of ice sheets that are grounded on marine bedrock to rapidly retreat. Scientists know these processes can lead to collapse, because it’s already happening: In Greenland, the Jakobshavn glacier is in rapid retreat, with the unstable calving cliff now more than 250 feet high, about as tall as the Statue of Liberty. In 2002, the Larsen B ice shelf on the Antarctic Peninsula rapidly collapsed in just a few weeks after a particularly warm summer saw the formation of innumerable meltwater ponds and crevasses. Recent fieldwork at the Larsen C ice shelf, a much bigger one, showed that the same process may soon play out there.
Luke Trusel, a climate scientist at Woods Hole Oceanographic Institute, whose similar work was cited in the DeConto paper, says that taken together, his work and DeConto’s new work shows that “melting at the [ice shelf] surface can go from insignificant to extremely significant over a short amount of time. This is a nonlinear response to climate warming,” Trusel told me. “It’s a light switch sort of change. It’s rapid.” But Trusel says he was surprised by the new study at how rapidly those ice shelf collapses could translate into sea level rise. “As a resident of a coastal community, it leaves me very concerned,” Trusel said, who lives on Cape Cod, Massachusetts.
Eric Rignot, a climate scientist who was a co-author on the Hansen paper, said the DeConto study brings evidence of greater than expected near-term sea level rise “full circle.” While Hansen’s study focused on bottom-up melting (from the oceans), DeConto’s study focused on top-down melting (from the atmosphere). “I think these two studies are shedding new lights on the sort of terrible mess we are heading for with ice sheet melt, which most people do not realize and wishfully would like to avoid without doing anything.”
DeConto says that his study provides some “complementary” “actual numbers” to support the controversial recent result of James Hansen, who argued that truly tremendous amounts of meltwater from Antarctica later this century could trigger a feedback loop of further ice melt and throw the global climate into chaos. In the top-end scenario DeConto’s study describes, meltwater from Antarctica could surpass the equivalent of all the world’s rivers combined in about 100 years—which is similar to the scenario that Hansen says could be enough to trigger profound changes in the way global oceans transport heat.
“We’re providing mechanistic explanations for how those dramatic sorts of rates of ice sheet retreat could actually happen,” DeConto told me, “but we don’t need [Hansen’s] feedback [to reach similar conclusions.]”
Climatologist Richard Alley, who assisted with some of the initial preparation of the DeConto study, said that while Hansen and his colleagues raised the possibility that sea level rise may far exceed previous estimates, DeConto and his co-author, David Pollard, “have taken a major step forward” in showing specifically how that might happen.
Alley also expressed dismay at the current incomplete state of knowledge in the field of polar glaciology, which he thinks is leading to an environment where every new bit of knowledge makes headlines—and oversaturates people to the point that dire climate change news no longer alarms them. “Personally, I really would be happier if we had the luxury of doing the research on this, without bothering the public until we have 95 percent confidence in an answer. All of us are fully aware how wrong it is to falsely yell ‘Fire’ in a crowded theater. But, we also are fully aware how wrong it is to sit silently while a fire begins to spread in that theater. Right now, I do not believe humanity can continue with unchecked warming while confidently assuming that sea-level rise will be limited to roughly 3 feet in a century.”
Antarctica is a difficult place to do research. It’s remote, it’s cold, and studies like these are some of the first to assess some of the basic mechanisms by which global warming may profoundly change it. So, there will be plenty of surprises over the next several years as our understanding of the continent improves. Right now, though, it seems like all the surprises are bad ones.