It's still uncertain how much, exactly, sea levels will rise by 2100 as a result of climate change. A lot depends on whether the world actually manages to curtail its greenhouse-gas emissions before then and slow the rate of temperature increase. A good deal also depends on what the ice sheets in Greenland and Antarctica do: The faster they melt and slide into the ocean, the higher sea levels will rise; and, as we've discussed before, there are a lot of complicating factors here. (Read Rob's old post on the various unexpected consequences of West Antarctic Ice Sheet melt, for one.)

In recent months, more and more climatologists have been settling on a prediction of a roughly one-meter rise by 2100, assuming greenhouse emissions stay their current course. NASA's Eric Rignot, one of the world's top ice-sheet experts, has warned that we're in for a one-meter rise by 2100 only if the rate of ice-sheet melt stays on its current course—it gets worse if the pace of melting accelerates (which is hardly improbable if temperatures keep rising).

So that's the current state of play. Now here's even more fodder for contemplation. According to a new study by Jianjun Yin of Florida State University and other colleagues, however much the sea levels do rise by 2100, you can add about eight inches to that number when figuring out how it'll affect the Northeast United States. If global sea levels rise an average of three feet by 2100, say, then bump that up another eight inches for New York City or Boston. It sounds bizarre, but here's the logic:

The explanation involves complicated ocean currents. Computer models forecast that as climate change continues, there will be a slowdown of the great ocean conveyor belt. That system moves heat energy in warm currents from the tropics to the North Atlantic and pushes the cooler, saltier water down, moving it farther south around Africa and into the Pacific. As the conveyor belt slows, so will the Gulf Stream and North Atlantic current. Those two fast-running currents have kept the Northeast's sea level unusually low because of a combination of physics and geography, Yin said.

Slow down the conveyor belt 33 to 43 percent as predicted by computer models, and the Northeast sea level rises faster, Yin said.

This also means that New York City, for one, will become a lot more vulnerable to severe flooding from storm surges long before 2100, since many parts of that city sit only slightly above sea level, especially in lower Manhattan. It'll also, no doubt, put further strain on the city's subway system, whose pumps already get overloaded during heavy rains. Maybe it's time to start looking into dikes...

--Bradford Plumer