The oceans are rising faster than at any point in the past 28 centuries, and human emissions of greenhouse gases are primarily responsible, scientists reported Monday.
They added that the flooding that is starting to make life miserable in many coastal towns — like Miami Beach, Fla.; Norfolk, Va.; and Charleston, S.C.— was largely a consequence of those emissions, and that it is likely to grow worse in coming years.
The scientists confirmed previous estimates, but with a larger data set, that if global emissions continue at a high rate over the next few decades, the ocean could rise as much as 3 or 4 feet by 2100, as ocean water expands and the great ice sheets in Greenland and Antarctica begin to collapse.
Experts say the situation will grow far worse in the 22nd century and beyond, likely requiring the abandonment of many of the world’s coastal cities.
“I think we can definitely be confident that sea-level rise is going to continue to accelerate if there’s further warming, which inevitably there will be,” said Stefan Rahmstorf, a professor of ocean physics at the Potsdam Institute for Climate Impact Research in Germany and co-author of a paper released Monday by the Proceedings of the National Academy of Sciences.
“Ice simply melts faster when the temperatures get higher,” Rahmstorf added. “That’s just basic physics.”
In a report issued at the same time as the scientific paper, a climate research and communications organization, Climate Central, used the new findings to calculate that roughly three-quarters of the tidal floods now occurring in towns along the American East Coast would not be happening in the absence of sea-level rise caused by human emissions.
The lead author of that report, Benjamin H. Strauss, said the same was likely to be true on a global scale, in any coastal community that has seen an increase of saltwater flooding in recent decades.
Local factors do come into play, though: Communities on land that is sinking, as in the Chesapeake Bay region of the United States, are being hit especially hard by the rising sea level.
Tidal floods are occurring more frequently, and are becoming a strain in many towns by killing lawns and trees, polluting supplies of fresh water, blocking streets in the middle of sunny afternoons and sometimes stranding entire island communities for hours by covering the roads to the mainland.
“I think we need a new way to think about most coastal flooding,” Strauss said in an interview. “It’s not the tide. It’s not the wind. It’s us. That’s true for most of the coastal floods we now experience.”
The new research was led by Robert E. Kopp, an earth scientist at Rutgers University who has won respect from his colleagues by bringing elaborate statistical techniques to bear on longstanding problems, like understanding the history of global sea level.
Scientists knew that the sea level rose drastically at the end of the last ice age, by almost 400 feet, causing shorelines to retreat by up to 100 miles in places. They also knew that the sea level had basically stabilized, like the rest of the climate, over the past several thousand years, the period when human civilization arose.
The new paper confirms a central finding of earlier research, that the sharp increase of sea level in the 20th century was unprecedented over thousands of years, but does so with a larger data set that may add to the confidence scientists place in the results.
The paper confirms that the ocean is exquisitely sensitive to small variations in the earth’s temperature — a portentous finding, given that human emissions are inducing a large temperature rise.
The researchers found that when the average global temperature fell by a third of a degree Fahrenheit in the Middle Ages, for instance, ice started to build up on land, and the volume of ocean water contracted, causing the average surface of the ocean to fall about 3 inches over 400 years. When the climate warmed slightly, that trend reversed.
“Physics tells us that sea-level change and temperature change should go hand in hand,” Kopp said. “This new geological record confirms it.”
In the 19th century, as the Industrial Revolution took hold, the oceans began to rise, and have gone up by about 8 inches since 1880. That may sound small, but the increase has caused extensive erosion worldwide, and governments are spending billions of dollars to try to shore up beaches and other coastal defenses.
Largely because of human emissions, global temperatures have jumped about 1.8 degrees Fahrenheit since the 19th century. Land ice has started to melt all over the planet.
“There’s no question that the 20th century is the fastest,” Kopp said. “It’s because of the temperature increase in the 20th century which has been driven by fossil fuel use.”
One of the authors of the new paper, Rahmstorf, had previously published estimates suggesting the seas could rise as much as 5 or 6 feet by 2100. But with the improved calculations from the new paper, his latest upper estimate is 3 to 4 feet.
That means Rahmstorf’s estimate is now more consistent with calculations issued in 2013 by the Intergovernmental Panel on Climate Change, a U.N. body that reviews climate research.
Rahmstorf said, however, that the rise would eventually exceed 3 feet — the only question is how long it will take.
“Sea level is going to continue going up for many centuries,” Rahmstorf said.
The Associated Press contributed to this report.