The seas are rising faster than ever | Science

Ask climate scientists how fast the world’s oceans are moving upward, and many will say 3.2 millimeters per year, a figure enshrined in the latest 2014 Intergovernmental Panel on Climate Change report. number, based on satellite measurements made since the early 1990s, is a long-term average. In fact, the global rate varied so much during that period that it was difficult to tell if it had been stable or accelerating.

It was accelerating, great. Faster Greenland ice melt 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] the mass loss has clearly improved, “agrees Felix Landerer, a JPL scientist at sea level. With the help of new data, new vertical earth motion patterns and, this month, a new radar satellite, oceanographers are improving their picture of how fast and where the seas are devouring the earth.

Hamlington and colleagues first reported signs of acceleration in 2018 Proceedings of the National Academy of Sciences. Since then, they and others have become more confident in trends. In a 2019 study in Nature climate change, a group led by Sönke Dangendorf, a physical oceanographer at Old Dominion University, used tide readings prior to satellite records to show that seas have risen 20 centimeters since 1900. The team’s data shows that after a period of global dam construction in the 1950s which retained surface water and slowed sea level rise, began accelerating in the late 1960s, not the late 1980s, as many scientists have speculated climate, says Dangendorf. “It was surprising,” because the main drivers of sea level rise – the thermal expansion of ocean water due to global warming, along with the melting of glaciers and ice sheets – were thought to have occurred later.

Oceanographers are about to get a sharper view of trends thanks to the Sentinel-6 Michael Freilich satellite, which NASA and the European Space Agency plan to launch on November 21 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 operate much like its predecessors, using pulses from reflected radar to measure ocean height. But his higher resolution measurements will allow him to measure the height of the ocean within 300 meters of the coast, much closer than before.

Coasts are where sea level rise hits home and where large local variations can mask the global average. In a work published last month in Scientific data, Anny Cazenave, ocean geophysicist at the International Space Science Institute, and colleagues reanalyzed the satellite record and showed that sea level rise to 20% of the coastal sites surveyed in Europe, Asia and Africa was significantly different from that. of the open ocean. “We have to explain it,” he says.

Some of the variations reflect the vertical movement of the earth itself, due to the slow oscillation of the continental plates that “float” on a viscous mantle. Coastal ocean currents, fresh water from nearby rivers, and weather conditions can also inject variability causing water to accumulate or withdraw from continents, says Cazenave.

But Dangendorf believes that currents in the open ocean drive much of this variable sloshing, directing water rising from the open ocean – where there is more water to heat and expand – towards the shores. A reconstruction of the Norwegian sea level from 1960 to 2015, for example, showed that the changing currents were the best explanation for mysterious and frequent oscillations of 20 millimeters in height. Dangendorf is now monitoring sea level rise in nine coastal regions up to their ocean sources and has found that they are typically between 500 and 1000 kilometers apart; much of the sea level rise in the northern half of the east coast of the United States, for example, comes from waters swept by the Labrador Sea.

Trends are worrying. Aimée Slangen, a climate scientist at the Royal Netherlands Institute for Sea Research, and colleagues are integrating recent climate model projections to predict when sea level will rise 25 centimeters above 2000 levels, a point at which 100 years of flooding on some coasts could be an almost annual. In an unpublished work, Slangen notes that the threshold will be reached between 2040 and 2060. Efforts to slow climate change will not do much to postpone it given the inertia of ocean warming and melting ice, although they could prevent much increases. major later in the century. And that short-term certainty, while terrible, is “good enough for decision making,” says Slangen.

Dangendorf, who joined Old Dominion late last year, is getting a front row seat for the action. The university is located in Norfolk, Virginia, a part of the United States coast where the crust is sinking at the same rate that the oceans are rising. “I watch coastal floods every week,” he says. “I see it from my balcony.”