A team of U.S. Geological Survey scientists recently discovered that the sea level along the East Coast of the United States, particularly a 600-mile stretch from Cape Hatteras, N.C., to Boston, has risen at an annual rate three to four times faster than the global average since 1990.
When the global sea level rose by 2 inches, Norfolk, Va., saw a rise of 4.8 inches, Philadelphia answered with 3.7 inches, and New York City 2.8 inches, according to the study, which was published in the journal Nature Climate Change. Peter Howd, an oceanographer and co-author of the study for the U.S. Geological Survey, helped illuminate just why the East Coast is in the wrong place at the wrong time. It's complicated, but the short version is: The Gulf Stream might be slowing down.
Early European explorers didn't cut straight across the Atlantic Ocean in their quest for the New World because it was impossible, for the same reasons that flight paths destined for the United States from Europe are northward and curvilinear. The Earth's spin and temperature gradients cause the winds that travel over the Northern Atlantic Ocean to move in a clockwise pattern, dragging the waters below along a similar pathway.
Because of the Earth's rotation and friction in the water column, Howd says, water on the ocean's surface is transported approximately 90 degrees to the right of the direction the wind is blowing. This rerouting of the water toward the center of the ocean basin is called Ekman transport. If the wind is blowing to the north alongside the East Coast, for example, the water transport will be to the east. For winds blowing westward from North America to Europe, the water will move to the south. For winds blowing south along the European coastline to Africa, the water moves west, and so on.
The result of this physical churning might defy human imagination: It creates a huge pile of water, roughly 700 miles wide and 2000 miles long, that stands about 3 feet taller than the waters in coastal areas. "This pile of water roughly coincides with the Sargasso Sea," Howd says. The Sargasso, located in the middle of the Atlantic, is the only sea without a shoreline; it's created by these ocean currents and accumulates a high concentration of non-biodegradable plastic waste, similar to the Great Garbage Patch in the Pacific.
It seems counterintuitive for the ocean to have a big hump out in the middle. So what's keeping that pile of water in the center of the ocean and away from our coastlines is a fine balance between the velocity of the circular ocean currents and the Coriolis effect, which describes how objects move as a result of the earth's spin. "Water likes to be flat," said Dr. Howd. "As gravity pulls that water down and tries to get that bump to smooth out, Coriolis takes over. Water will want to flow downhill to the East Coast, but Coriolis will divert its flow to the north, producing a western boundary current known as the Gulf Stream."
You've no doubt seen the Gulf Stream on TV weather maps. It's a fast plume of warm water that moves from its starting point in the Caribbean northward along the eastern coast of the United States and tapers out near Newfoundland, heading in the general direction of Europe. According to the National Oceanic and Atmospheric Administration, the Gulf Stream flows 300 times faster than the Amazon River. The flow is fastest near the surface, peaking at 5.6 mph, and reaching 4 mph on average. It transports 4 billion cubic feet of water per second, surpassing all the rivers in the world combined.
The Gulf Stream is a powerful shield for the East Coast. The faster it moves, the more water will get deflected to the Sargasso Sea and sucked away from the coast. But if the Gulf Stream slows down—and scientists believe this is happening as climate change progresses—then more water will travel down from the Sargasso Sea to that 600-mile swath of coastline from North Carolina to Boston. Howd says the slowing of the Gulf Stream is a major reason East Coast sea levels have been rising at an annual rate three to four times faster than global averages.
The pile of water in the center of the North Atlantic Ocean Basin is more or less at the mercy of global temperature distribution, which drives wind speeds and wind circulation patterns, Howd says. "If the temperature goes up, it results in enough changes that the Gulf Stream slows down, surprisingly enough, and sea level will rise on the East Coast as a result."
Another reason that the East Coast is a victim of accelerated sea level rise is because the land is still readjusting from the Last Glacial Maximum. About 20,000 years ago, an ice sheet that was up to 2 miles thick in some places covered most of Canada, extended over much of the Midwest, and stretched east all the way to New York City. "The weight of that ice pushed coastal New England down. When it went down, the area to the south went up. It's kind of like squeezing a water balloon," Howd says.
Relieved of that ice-sheet load, the land is puffing itself back out in some regions north of New York City and settling back down in locales south of the city. Howd says this glacial isostatic adjustment is still occurring along the East Coast, inviting accelerated sea level rise, "stress on salt marshes and on areas with chronic beach erosion problems," and more frequent "winter storm flooding from the ocean." The simultaneous rise in sea level and sinking of the landmass south of New York City may account for this accelerated rate of change.