It's Good to Be on the 17th Floor

With a record number of Americans now saying that press accounts of the impact of global warming are exaggerated—41 percent say that, according to a Gallup poll released last week—I can easily imagine the reaction to this study, but here goes anyway: sea level due to global warming will be almost twice as great along the northeastern U.S. coast as it is globally. As always, it’s not merely the average level of sea-level rise that’s the problem, but the synergistic effect of sea-level rise with hurricanes and winter storm surges. When winds hurl water onto coasts from a higher starting point, the damage is that much greater.

Of all the predicted impacts of global warming, sea-level rise is where models have most underestimated what we can expect, scientists told a climate change conference in Copenhagen last week. Now a new study, focused on the eastern seaboard of the United States, suggests that the models are really off for that part of the world.

Writing in the advance online edition of Nature Geoscience, the team of Jianjun Yin of the Center for Ocean-Atmospheric Prediction Studies (COAPS) at Florida State University, Michael Schlesinger of the University of Illinois at Urbana-Champaign and Ronald Stouffer of the Geophysical Fluid Dynamics Laboratory at Princeton University analyzed 10 of the climate models used by the Intergovernmental Panel on Climate Change (IPCC). All the models show that, in a warming world, water expands and land ice such as the Greenland ice sheet melts, raising sea levels. But the models did not account for the effect of changes in ocean circulation.

When those are incorporated, the scientists predict, there will be a sea-level rise in New York of 15 to 23 centimeters (about 6 to 9 inches) on top of the rise of 36 to 51 centimeters (14 to 20 inches) globally. “The northeast coast of the United States is among the most vulnerable regions to future changes in sea level and ocean circulation,” Yin said. Much of New York City is less than 16 feet above sea level, with some parts of lower Manhattan only about 5 feet above. That means that a rise of 6 to 9 inches on top of the 14 to 20, especially with a storm surge, poses a real risk of flooding, beach erosion, destruction of wetlands and increase in the salinity of estuaries.

One thing that’s been interesting to note about climate change research over the decades is what happens to the assessments when scientists realize they forgot something. The result is almost never that the predicted impacts or severity of climate change need to be dialed back. Instead, as researchers learn more about the climate system and feedback loops, the expected impacts keep getting worse. So it is in this case, in which the researchers took a look at the effect of the Atlantic meridional overturning circulation. This current is characterized by warm and salty seawater flowing north from the Gulf of Mexico in the ocean’s upper layers and cold water flowing south at depth. Global warming could reduce or prevent the sinking of the surface water, which would slow the AMOC and raise sea levels off the eastern seaboard. Or, as the scientists put it in their paper, “future changes in sea level and ocean circulation will have a greater effect on the heavily populated northeastern United States than estimated previously.”