When it comes to swelling oceans that threaten coastal communities around the world, it's bad, and it's going to get worse.
Sea levels are rising faster than they have in the last three millennia, and that rate continues to accelerate due to the burning of fossil fuels, according to new research published Monday.
"Our study is for sea level what the now well-confirmed famous 'hockey stick' diagram was for global temperature."
—Stefan Rahmstorf, Potsdam University
One study appearing in the journal Proceedings of the National Academy of Sciences states that "almost certainly, more than half of the 20th century rise has been caused by human activity, possibly even all of it."
Employing a database of geological sea-level indicators from marshes, coral atolls, and archaeological sites around the world, the paper shows that global sea levels stayed fairly steady for about 3,000 years. Then, from 1900 to 2000, the seas rose 5.5 inches—a significant increase, especially for low-lying coastal areas. And since 1993, the rate has soared to a foot per century.
"The new sea-level data confirm once again just how unusual the age of modern global warming due to our greenhouse gas emissions is—and they demonstrate that one of the most dangerous impacts of global warming, rising seas, is well underway," said Stefan Rahmstorf, a physics professor at Potsdam University in Germany and one of 10 authors of the paper.
As John Upton explains at Climate Central:
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 study for about half of sea level rise during the past decade.
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Changes in sea level vary around the world and over time, because of the effects of ocean cycles, volcanic eruptions and other phenomenon. But the hastening pace of sea level rise is being caused by climate change.
As the Washington Post reports, "[t]he new work is particularly significant because, in effect, the sea level analysis produces a so-called 'hockey stick' graph—showing a long and relatively flat sea level 'handle' for thousands of years, followed by a 'blade' that turns sharply upwards in very recent times."
Indeed, said Rahmstorf: "Our study is for sea level what the now well-confirmed famous 'hockey stick' diagram was for global temperature. We can confirm what earlier, more local sea-level data already suggested: during the past millennia sea-level has never risen nearly as fast as during the last century."
Meanwhile, a separate study also published Monday warns that without a sharp reduction of greenhouse gas emissions, sea levels worldwide will likely rise by one to four feet by the end of this century.
This study, led by the Potsdam Institute for Climate Impact Research, combined the two most important estimation methods for future sea-level rise to show that "increasingly routine tidal flooding" events, as the New York Times wrote, "are just an early harbinger of the coming damage."
Furthermore, it found that "even if ambitious climate policy follows the 2015 Paris agreement," sea levels are still projected to increase by 20 to 60 centimeters by 2100, necessitating coastal adaptation such as building dikes, designing insurance schemes for floodings, or mapping long-term settlement retreat.
But most important will be to follow experts' warnings and "keep it in the ground."
"With all the greenhouse-gases we already emitted, we cannot stop the seas from rising altogether, but we can substantially limit the rate of the rise by ending the use of fossil fuels," said study co-author Anders Levermann, of the Potsdam Institute for Climate Impact Research and Columbia University's Lamont-Doherty Earth Observatory. "If the world wants to avoid the greatest losses and damages, it now has to rapidly follow the path laid out by the UN climate summit in Paris."