Chris Mooney reports on science and the environment.
Writing in Nature Geoscience, John Krasting and three colleagues from the Geophysical Fluid Dynamics Laboratory of the National Oceanic and Atmospheric Administration find that “Atlantic coastal areas may be particularly vulnerable to near-future sea-level rise from present-day high greenhouse gas emission rates.” The research adds to recent studies that have found strong warming of ocean waters in the U.S. Gulf of Maine, a phenomenon that is not only upending fisheries but could be worsening the risk of extreme weather in storms like Winter Storm Jonas.
“When carbon emission rates are at present day levels and higher, we see greater basin average sea level rise in the Atlantic relative to the Pacific,” says Krasting. “This also means that single global average measures of sea level rise become less representative of the regional scale changes that we show in the study.”
In the new research, the scientists used a high powered climate change model based at the Geophysical Fluid Dynamics Laboratory in Princeton, N.J., that simulates the ocean, the atmosphere and the cycling of carbon throughout the Earth system. The goal was to determine how much sea level rise would occur in the Atlantic, versus the Pacific, under a variety of global carbon emissions scenarios.
And the simulation found that at high emissions scenarios similar to current rates, the Atlantic sea levels rise considerably faster than the Pacific, with particularly noteworthy impacts for the U.S. East Coast. (Other recent research by scientists with the U.S. Geological Survey has suggested this increased rate of sea level rise is already happening — finding sea level rise rates “~ 3–4 times higher than the global average” along a large stretch of the U.S. East Coast, which the researchers dubbed a sea level rise “hotspot.”)
The reason for the difference, the researchers say, is that the Atlantic, more than the Pacific, is characterized by a strong “overturning” ocean circulation — technically known as the Atlantic Meridional Overturning Circulation, or AMOC — that spans the north-south length of the globe and ultimately connects waters off New York with those at the tip of Antarctica. This means that waters circulate through the entire Atlantic much faster than they do throughout the Pacific: A “parcel” of water that sinks beneath the surface in the Atlantic will generally make it back to the surface again in 200 to 300 years, versus about three times as long for the Pacific, Krasting explains.
For this reason, scientists sometimes say that Atlantic waters are “younger” than Pacific waters.
Another way of putting it is that the Atlantic waters “ventilate” more, plunging from the surface to great depths before eventually making their way back to the surface again. But if this circulation slows due to climate change, the study finds, less cold water will dive to ocean depths in the North and far South Atlantic (technically called “deep water formation”), leading to warmer water pooling below the surface and, ultimately, greater warming overall.
“The average temperature of the basin actually goes up, because you’re not bringing that cool water,” says Krasting. Warm water expands, and that’s the cause of the sea level rise expected in the study.
Indeed, the research finds that with global emissions rates of carbon greater than 5 gigatons (or billion metric tons) per year — current emissions from industry are about 10 gigatons per year — the Atlantic circulation would slow down considerably (and sea level rise would increase there more than in the Pacific).
That could be bad news for the U.S. “The patterns of sea level rise that we show tend to show enhanced sea level rise along the U.S. east coast, and that is associated with, or is consistent with the weakening of the overturning circulation that we demonstrate in the study,” says Krasting. It’s important to note that sea level rise shows regional variations based on topography and other factors – so the change won’t be uniform even within this particular geographical area.
“Local effects such as land subsidence, changes in offshore winds, and ocean circulation changes (that is, AMOC) lead to sea levels that are rising faster than the global average along the US East Coast,” the study says. “Our results suggest that higher carbon emission rates also contribute to increased [sea level rise] in this region compared to the global average.”
This isn’t the first study to point to reasons why the Atlantic off the U.S. East Coast could be a particular climate change “hotspot,” as another NOAA researcher, Vincent Saba, put it to me recently.
A recent study by Saba and a group of NOAA researchers found that global warming, by slowing down the Atlantic ocean’s circulation, would lead to warmer waters off the East Coast. This change in circulation appears to shift the warm waters of the Gulf Stream northward, allowing them to enter the Gulf of Maine, which in recent years has seen dramatic warming.
And it’s not just warmer seas, but higher ones as well. For instance, one recent study found that in 2009-2010, there was a sudden “extreme” sea level spike off the U.S. East Coast. Seas rose 4 inches all of a sudden thanks to an apparently abrupt change in the Atlantic’s circulation. (Independent NASA research using satellite measurements also found that the circulation slowed during this time period).
In other words, it’s a consistent picture — slowing down the ocean circulation means more warmth and more expansive Atlantic waters near the U.S.
And as if that’s not enough, there’s another factor that could also punish the U.S. with extra sea level rise: Gravity.
Right now, Antarctica is so massive that it draws the ocean toward it. Sea level slopes upward toward this largest ice mass on Earth, meaning that there is less ocean elsewhere, and particularly off of U.S. shores, than there might be otherwise.
But there are fears that Antarctica could be on course to lose a sizeable chunk of ice — roughly 3.3 meters or 11 feet worth, measured in globally averaged sea level rise terms, from West Antarctica. If that happens, a substantial part of the ocean would flow back toward the rest of the globe, due to the lessened gravitational pull. And researchers have shown that this would produce a disproportionately high sea level rise for the U.S. East Coast — and indeed, in this case, for the West Coast as well. Instead of a mere 11 feet of sea level rise (assuming a total loss of West Antarctica, of course), the east might get more like 14.
So in sum: Researchers are homing in on one regional consequence of climate change, occurring at the intersection of changes in the ocean and the atmosphere. And the upshot is that right off of the U.S. East Coast is where some of the most consequential changes could occur.
Chris Mooney reports on science and the environment.
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