By John Upton
Feb. 22, 2016
The oceans have heaved up and down as world temperatures have waxed and waned, but as new research tracking the past 2,800 years shows, never during that time did the seas rise as sharply or as suddenly as has been the case during the last century.
The new study, the culmination of a decade of work by three teams of farflung scientists, has charted what they called an “acceleration” in sea level rise that’s triggering and worsening flooding in coastlines around the world.
The findings also warn of much worse to come.
The scientists reported in a paper published Monday in Proceedings of the National Academy of Sciences that they have greater than 95 percent certainty that at least half of more than 5 inches of sea level rise they detected during the 20th century was directly caused by global warming.
“During the past millennia, sea level has never risen nearly as fast as during the last century,” said Stefan Rahmstorf, a physics professor at Potsdam University in Germany, one of 10 authors of the paper. “That was to be expected, since global warming inevitably leads to rising seas.”
By trapping heat, rising concentrations of atmospheric pollution are causing glaciers and ice sheets to melt into seas, lifting high tides ever higher.
Globally, average temperatures have risen about 1°C (nearly 2°F) since the 1800s. Last year was the hottest recorded, easily surpassing the mark set one year earlier. The expansion of warming ocean water was blamed in a recent studyfor about half of sea level rise during the past decade.
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“The new sea level data confirm once again just how unusual the age of modern global warming, due to our greenhouse gas emissions, is,” Rahmstorf said. “They also demonstrate that one of the most dangerous impacts of global warming, namely rising seas, is well underway.”
Were it not for the effects of global warming, the researchers concluded that sea levels might actually have fallen during the 20th century. At the very least, they would have risen far less than was actually the case.
A report published by Climate Central on Monday, the result of an analysis based in part on the findings in Monday’s paper, concluded that climate change was to blame for three quarters of the coastal floods recorded in the U.S. from 2005 to 2014, mostly high tide floods. That was up from less than half of floods in the 1950s.
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Even If humans quickly stop polluting the atmosphere, potentially keeping a global temperature rise to well below 2°C (3.8°F) compared with preindustrial times — a major goal of the Paris climate agreement — seas may still rise by an additional 9 inches to 2 feet this century, the study concluded. That would trigger serious flooding in some areas, and worsen it in others.
Under the worst-case scenario investigated, if pollution continues unabated, and if seas respond to ongoing warming by rising at the fastest rates considered likely, sea levels could rise more than 4 feet this century alone, wiping out coastal infrastructure and driving communities inland.
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