By far one of the most important impacts of global warming in the decades ahead will be sea-level rise. As the Earth heats up and ice sheets in Greenland and Antarctica melt, ocean levels will creep upward, flooding coastal cities and forcing large-scale relocations around the world.

But there’s a troubling asterisk here: We still don’t know exactly how high oceans are likely to rise this century. Studies have suggested it could be anywhere from 2 to 6 feet or more — with newer evidence leaning toward the higher end, depending on emissions and how quickly parts of West Antarctica’s massive ice sheet disintegrate. Worse, climate scientists probably won’t be able to pin down an exact number anytime soon, because getting a handle on ice-sheet dynamics is inherently tricky.

That’s not reason for complacency, though. It actually makes preparation more urgent and difficult, because coastal cities will have to start mounting defenses in the face of considerable uncertainty. (This map, for instance, shows how different levels of sea-level rise could put different parts of New York City underwater.)

That means, as climate scientists Michael Oppenheimer and Richard Alley explain in a new paper in Science, that coastal areas will have to learn to master the art of flexibility — developing sea walls and other defenses that can evolve over time — and be ready for a wide array of plausible outcomes. Meanwhile, scientists themselves need to get much better at conveying the “deep uncertainties” around ice sheets and sea levels.

“The response can’t just be to wait until the science clears up,” says Oppenheimer, a climate and geosciences expert at Princeton University. “Because it’s unlikely we’re going to get a sharp answer anytime soon. And if policymakers sit around waiting for a definitive answer, they could find it’s too late to avoid disastrous levels of change.”

Why it’s so hard to figure out exactly how high sea levels will rise

Global warming has a few well-known effects on the oceans. As water gets warmer, it expands. And as glaciers worldwide and ice sheets in places like Greenland and Antarctica melt, they add water to the ocean. That all causes global sea levels to rise.

This process is already underway: On average, global sea levels have risen more than 7.5 inches since 1900 after 2,000 years of relatively little change. And the rate of sea-level rise has continued to increase in recent decades.

The tricky part comes in forecasting future change under a given emissions scenario — mainly because of uncertainties around those massive ice sheets that sit atop Greenland and Antarctica.

For thousands of years, these ice sheets were roughly stable. Ice from the top of the sheets would flow gradually into the oceans, where it formed ice shelves that floated on water and eventually broke off into icebergs. This process was all counterbalanced by snow that was falling back on top of the sheets, replenishing them. On net, there was little change in mass:

But now, thanks to global warming, these massive sheets are losing more ice than they’re gaining. Warmer ocean water is thinning the shelves and speeding up the rate at which ice flows into the sea. On net, Antarctica is losing 147 billion tons of ice each year, mainly from the West Antarctic Ice Sheet. Greenland is losing about 269 billion tons of ice per year. Because this ice isn’t replaced, ocean levels are rising higher.

How this process speeds up going forward is tricky to model, though. Earlier projections in the 1990s and 2000s assumed that ice shelf friction would slow the rate of overall ice loss — which meant that global sea levels would likely rise less than 3 feet this century, even in high emissions scenarios.

Yet newer research has suggested that ice sheets could lose mass much faster than that. Evidence from the distant past, when the Earth was a few degrees warmer than it is today, suggests that sea levels were 20 to 30 feet higher — which can only be explained by very rapid collapse of ice sheets. What’s more, scientists are currently witnessing startling changes in Antarctica and Greenland, as ice shelfs crumble and glaciers retreat far more rapidly than expected. Researchers have found, for instance, that meltwater on the surface of ice sheets can open up crevasses that break apart ice shelves entirely, causing further destabilization and faster ice flow into the ocean.

Newer studies that put all this evidence together suggest we could see as much as 6 feet of rise this century in high-emissions scenarios — with ocean levels continuing to rise after 2100. Here is a chart from Oppenheimer and Alley showing the evolution of forecasts over time for various emissions scenarios. Recent work has favored higher sea-level rise, but there’s still a considerable range:

“The big uncertainty is how fast the ice sheets will shed their ice as the world warms,” Oppenheimer explains. “And the West Antarctic Ice Sheet in particular has a history of fast changes in both directions. But we don’t know whether these processes happen in a couple hundred years or a couple thousand years, so that’s where we turn to models. And the models lately have been favoring the possibility of relatively rapid loss from West Antarctica. On top of that, from observations we can see that parts of Antarctica are now changing quickly. But we’re still not 100 percent sure how much of that response we’re seeing is due to past recent warming or just general instability in the system. So we really need a research program to narrow that down quickly.”

Scientists are racing to better understand West Antarctica. In October, US and British scientific agencies announced a multimillion-dollar mission to study Antarctica’s Thwaites Glacier, a glacier the size of Pennsylvania that is thought to be the most likely place that unexpectedly large ice loss could occur in the near future.

Unfortunately, says Oppenheimer, even with these efforts, it’s unlikely that scientists will come up with a single definitive prediction for sea-level rise anytime soon. One obstacle here is that collecting data on Antarctica is just plain hard: “Antarctica is too remote, too cold, too expensive, and too complicated to have the kind of detailed observing program that we really need,” Oppenheimer says. That hinders accurate modeling of ice sheet behavior. And, the authors note in the Science paper, there are “numerical challenges” in solving the full fluid dynamic equations for ice flow.

Making things even more complicated, sea level rise won’t be uniform around the world. Strong wind and ocean currents can warp the waters and affect local sea levels. The melting of the ice sheets will also have odd gravitational effects that will affect regional sea levels unevenly. Broadly, scientists know that sea levels will rise more along the Atlantic coast than the Northwest coast of the United States, but there are still some questions here.

That means local policymakers need to accept they probably won’t get a single firm number to work with anytime soon — at best, they’ll get a range of possible outcomes that changes over time as research improves. But they can’t just ignore the possibility of high sea-level rise, either. Instead, they’ll have to be flexible.

Coastal cities will have to plan for uncertainty

Whatever the exact number, it’s a safe bet that sea-level rise will menace cities along the coast throughout the century. We’re already seeing some of those effects now, as high tides have increasingly led to “nuisance flooding” in places like Annapolis, Maryland; Norfolk, Virginia; Charleston, South Carolina; and Miami, Florida. This will only get worse in the future — and some areas, like Miami Beach, could eventually find themselves underwater if sea levels rise 3 feet or more.

There are three main ways that coastal areas can respond. First, they can build dikes, sand dunes, or other defenses to fend off the rising oceans. Second, they can elevate some of their coastal infrastructure (like roads). Third, they can retreat inland, abandoning some areas to the sea. (The White House is currently working with coastal communities to develop a framework for “managed retreat.”)

Yet these actions take time — which means cities need to start thinking about them now. Oppenheimer and Alley note that both the Thames storm surge barrier in England and the Netherlands’ sea defenses took nearly 30 years of negotiating, planning, and building after a catastrophic 1953 storm and flood. By extension, any preparations for sea-level rise in mid-century will have to start getting underway today.

Uncertainty around ice sheets will make this more challenging, but not impossible. In their paper, Oppenheimer and Alley point out that the Netherlands has built flexible dikes and sand dunes that can be augmented over time, as forecasts evolve. If a city is building a surge barrier or seawall, it might want to make the base big enough so that it can grow over time as science evolves.

The authors also recommend that local governments rethink policies like flood insurance or zoning so that they’re not continuing to build in areas that could soon be underwater.

Meanwhile, the authors note that the climate science community needs to get better at conveying the wide range of possible sea level rise. This would entail “presenting policy-makers with projections that are as fully probabilistic as possible, while also characterizing deep uncertainties, rather than just handing the worst-case or most-likely estimates.”

In an interview, Oppenheimer says that it would be helpful to have some entity that was continually assessing and synthesizing the fast-moving science around ice sheets and sea-level rise, so that cities could plan. Right now, the IPCC only comes out every six to seven years, which is too infrequent to be useful. The UN panel could possibly issue special reports that come out every year or two — so that local planners don’t have to sift through every new study on Antarctica and try to figure out what to expect.

Further reading: Should we try to fight rising sea levels — or abandon the coasts?