How high will it go?
“Sea-level rise isn't going to stop in 2100,” says Church. “I think that's something that people don't really take on board.” Eventually, they will. Projections of sea-level rise far into the future jump from tens of centimeters to tens of meters.
For the past few years, Maureen Raymo, a marine geologist at the Lamont-Doherty Earth Observatory in Palisades, New York, has traipsed around abandoned diamond mines in South Africa, visited quarries in Australia and examined road cuts on the east coast of North America, looking for shells and other remnants of beaches from 3 million years ago. She hopes to reconstruct sea levels from the Pliocene epoch, the last time when carbon dioxide concentrations were as high as they are today: about 400 parts per million of the atmospheric volume. That, in turn, should provide a glimpse of what the world might look like in thousands of years, once the planet has had time to react fully to today's emissions.
Current estimates of sea-level rise in the Pliocene range from very little to 40 meters, says Raymo. “But that's not very helpful,” she says. The difference between the lower and higher estimates is the difference — crucially — between much of the vast East Antarctic ice sheet melting and staying frozen. Whether or not it melted in the Pliocene, in turn, provides insight for modelers who are trying to work out whether — and how fast — ice sheets might collapse in the next few hundred years.
The trick to pinning this down is not just finding Pliocene beaches, but also working out how the land has moved since they were laid down, as a result of both rebound from the loss of ice sheets and the ongoing movement of mantle rock under the continents. To estimate how such processes have played out over millions of years, researchers rely on models of how much ice covered the continents and how viscous the mantle is — factors that are subject to extreme debate. “Today's models all assume a viscosity of the mantle that is untestable, highly controversial and differs between groups,” says Raymo.
This motion of Earth makes a big difference in estimating past events. Previous work, for example, in Bermuda and the Bahamas hinted that the coastline there was 20 meters higher than it is today during a warm period 400,000 years ago. In 2012, however, Raymo and Mitrovica calculated how the ground there had moved — and concluded that half of the apparent sea-level rise was attributable not to rising waters but to sinking land, cutting the sea-level-rise estimate in half.
Given the large error bars, the only way to pinpoint Pliocene sea levels is to get data from many sites and to calculate one best-fit answer for global sea level. Raymo and her team have so far surveyed thousands of kilometers of coast, gathering evidence from dozens of beach sites. She says she needs perhaps eight more locations and five years to finish the job.
But, she admits, whatever she finds will not be a worst-case scenario because greenhouse-gas concentrations are already climbing beyond where they were in the Pliocene. “The real worst-case scenario is we don't limit fossil-fuel combustion,” she says. “Then it's 'Hello Eocene'” — returning to a world akin to a warm period 55 million years ago, with maybe just a trace of ice at the poles.
Nearly 70 meters of sea-level rise would drown all of Florida and much of Brazil, and swamp the Statue of Liberty up to her waist. But that might not happen until so many thousands of years from now that humanity has time to adapt — even if that means surrendering much of the land to the waves.