Ask climate scientists how fast the world’s oceans are creeping upward, and many will say 3.2 millimeters per year—a figure enshrined in the last Intergovernmental Panel on Climate Change report, from 2014. But the number, based on satellite measurements taken since the early 1990s, is a long-term average. In fact, the global rate varied so much over that period that it was hard to say whether it was holding steady or accelerating.
It was accelerating, big time. Faster melting of Greenland’s ice has pushed the rate to 4.8 millimeters per year, according to a 10-year average compiled for Science by Benjamin Hamlington, an ocean scientist at NASA’s Jet Propulsion Laboratory (JPL) and head of the agency’s sea level change team. “The [Greenland] mass loss has clearly kicked into higher gear,” agrees Felix Landerer, a JPL sea level scientist. With the help of new data, new models of vertical land motion, and—this month—a new radar satellite, oceanographers are sharpening their picture of how fast, and where, the seas are gobbling up the land.
Hamlington and colleagues first reported signs of the speedup in 2018 in the Proceedings of the National Academy of Sciences. Since then, they and others have become more confident about the trends. In a 2019 study in Nature Climate Change, a group led by Sönke Dangendorf, a physical oceanographer at Old Dominion University, used tide gauge readings that predate satellite records to show seas have risen 20 centimeters since 1900. The team’s data show that, after a period of global dam building in the 1950s that held back surface water and slowed sea level rise, it began to accelerate in the late 1960s—not the late 1980s, as many climate scientists assumed, Dangendorf says. “That was surprising,” because the main drivers of sea level rise—the thermal expansion of ocean water from global warming, together with melting glaciers and ice sheets—were thought to have kicked in later.
Oceanographers are about to get a sharper view of the trends thanks to the Sentinel-6 Michael Freilich satellite, which NASA and the European Space Agency plan to launch on 21 November from Vandenberg Air Force Base in California. Named after the former head of NASA’s earth science program, who died this year, the satellite will work much like its predecessors, using pulses of reflected radar to measure the ocean’s height. But its higher resolution measurements will allow it to gauge ocean height within 300 meters of the coastline, far closer than before.
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The coasts are where sea level rise hits home—and where large local variations can mask the global average. In work published last month in Scientific Data, Anny Cazenave, an ocean geophysicist at the International Space Science Institute, and colleagues reanalyzed the satellite record and showed sea level rise at 20% of the coastal sites they surveyed across Europe, Asia, and Africa was significantly different from that of the open ocean. “We have to explain that,” she says.
Some of the variation reflects the vertical motion of the land itself, due to the slow bobbing of continental plates that “float” on a viscous mantle. Coastal ocean currents, freshwater from nearby rivers, and weather patterns can also inject variability by causing water to pile up or retreat from the continents, Cazenave says.
But Dangendorf believes currents in the open ocean drive much of this variable sloshing, routing rising water from the open ocean—where there is more water to warm and expand—to the coastlines. One reconstruction of Norwegian sea levels from 1960 to 2015, for example, showed shifting currents were the best explanation for mysterious, and frequent, 20-millimeter swings in height. Dangendorf is now tracing sea level rise in nine coastal regions to their ocean sources, and has found them to be typically 500 to 1000 kilometers away; much of the sea level rise in the northern half of the U.S. east coast, for example, comes from waters swept out of the Labrador Sea.
The trends are worrisome. Aimée Slangen, a climate scientist at the Royal Netherlands Institute for Sea Research, and colleagues are integrating recent projections from climate models to predict when sea levels will rise 25 centimeters above 2000 levels, a point when 100-year floods on some coastlines could be a near annual occurrence. In unpublished work, Slangen finds that the threshold will be reached sometime between 2040 and 2060. Efforts to slow climate change won’t do much to postpone it given the inertia of ocean warming and ice melt, though they could forestall much greater increases later in the century. And that near-term certainty, though dire, is “quite good for decision-making,” Slangen says.
Dangendorf, who joined Old Dominion late last year, is getting a front row seat to the action. The university is in Norfolk, Virginia, a part of the U.S. coast where the crust is sinking about as fast as the oceans are rising. “I watch coastal flooding every week,” he says. “I see it from my balcony.”
sciencemag.org, 18 November 2020