Ocean levels are rising much faster than at any time in 28 centuries, sobering new studies show

The Earth is exiting a long period of stable ocean and climate levels during which human civilization grew and flourished, and it's almost certainly due to human activity, scientists in the U.S. and Germany said in a pair of papers published Monday in the journal Proceedings of the National Academy of Sciences. One study, led by Rutgers climate scientist Robert Kopp, mapped out changes in sea levels around the globe over the past 2,800 years; oceans rose or fell no more than 1.5 inches a century from ancient Rome's founding until the Industrial Age in the 1800s, the study found, but rose 5.5 inches in the 20th century alone, accelerating to a rate of 12 inches a century by 1993.

The researchers blamed the increasing sea levels on rising global temperatures they and almost all other scientists attribute to the burning of fossil fuels. "Physics tells us that sea-level change and temperature change should go hand-in-hand," Kopp said. "This new geological record confirms it." Kopp and his team estimate that sea levels will rise 22 to 52 inches by 2100 at the current rate, or 11 to 22 inches if nations fully enact the global climate change treaty negotiated in Paris last year.

The second paper, led by Matthias Mengel of Germany's Potsdam Institute for Climate Impact Research, similarly estimated that sea levels will rise three to four feet by 2100 if humans don't curb carbon emissions — roughly the same range predicted in 2013 by the United Nations Intergovernmental Panel on Climate Change. Both papers acknowledged that there were significant unknowns in their analyses, but not in a way that should make humanity in general and coastal dwellers in particular feel any safer: If the massive ice sheets of Greenland and Antarctica melt, as seems likely, most bets are off.

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If that seems distant and theoretical, a third, unpublished study released Monday found that rising temperatures are responsible for a sharp increase in "nuisance floods" in seaside towns along the southern U.S. East Coast over the past 50 years, causing millions of dollars of damage due to incursions of a few feet of saltwater. Most of those floods wouldn't have happened without manmade global warming, the team, from Climate Central, reported. "I think we need a new way to think about most coastal flooding," said lead author Benjamin Strauss. "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."