A recent Nature Geoscience study, “High rates of sea-level rise during the last interglacial period,” ($ubs. req’d) finds that sea levels could rise twice what the IPCC had project for 2100. This confirms what many scientists have recently warned (also see here), and it matches the conclusion of a study (PDF) earlier this year in Science.

[As an aside, in one debate with a denier -- can't remember who, they all kind of merge together -- I was challenged: "Name one peer-reviewed study projecting sea-level rise this century beyond the IPCC." Well, now there are two from this year alone!]

For the record, five feet (PDF) of sea level rise would submerge some 22,000 square miles of U.S. land just on the Atlantic and Gulf Coasts (farewell, southern Louisiana and Florida) — and displace more than 100 million people worldwide. And, of course, sea levels would just keep rising some six inches a decade — or, more likely, even faster next century than this century.

The researchers base their finding on their analysis of the rate of sea-level rise during the last warm or interglacial period (the Eemian, about 120,000 years ago), when seas rose 1.6 meters (five feet) per century. Why look at the rate of Eemian sea level rise? Becaause that’s the last time the planet was as warm as it soon will be again: “such rates of sea-level rise occurred when the global mean temperature was 2 °C higher than today, as expected again by AD 2100.”

Indeed, if we don’t reverse emissions trends very soon (and stay below 450 ppm of carbon dioxide), the planet might well warm 3°C or more by 2100. The Eemian warming was driven by “changes in orbital parameters from today (greater obliquity and eccentricity, and perihelion), known as the Milankovitch cycle.” Current warming is driven by human emissions of greenhouse gases.

Here is the entire abstract from the article — note that the Eemian is also called “Marine Isotope Stage 5“:

The last interglacial period, Marine Isotope Stage (MIS) 5e, was characterized by global mean surface temperatures that were at least 2 °C warmer than present. Mean sea level stood 4-6 m higher than modern sea level, with an important contribution from a reduction of the Greenland ice sheet. Although some fossil reef data indicate sea-level fluctuations of up to 10 m around the mean, so far it has not been possible to constrain the duration and rates of change of these shorter-term variations. Here, we use a combination of a continuous high-resolution sea-level record, based on the stable oxygen isotopes of planktonic foraminifera from the central Red Sea and age constraints from coral data to estimate rates of sea-level change during MIS-5e. We find average rates of sea-level rise of 1.6 m per century. As global mean temperatures during MIS-5e were comparable to projections for future climate change under the influence of anthropogenic greenhouse-gas emissions, these observed rates of sea-level change inform the ongoing debate about high versus low rates of sea-level rise in the coming century.

If we don’t act now, we are clearly risking catastrophic sea level rise for many, many generations to come.