Earth's oceans are swelling faster than anytime in past 2,800 years, study shows

For nearly three millenniums, the Earth's ocean levels fluctuated by just three inches, rising no more than one or one and a half inches per century. 

After the Industrial Revolution, that crept up to five and a half. And since 1993, human activity has helped set a new, and worrying, record: at current rates, the seas would rise by a foot per century. Without big changes, the world could be on course to push seas up by as much as 52 inches by 2100, transforming coastal landscapes in ways we haven't seen since before the Iron Age.

"There's no question that the 20th century is the fastest," said Rutgers University earth and planetary sciences professor Bob Kopp, the lead author on one of two studies published Monday that combine historical records, present-day testing, and statistics to estimate changing ocean levels yesterday, today, and tomorrow. "It's because of the temperature increase in the 20th century which has been driven by fossil fuel use."

To begin charting that history, Professor Kopp's international team examined 24 coastlines, marshes, and archipelagos around the world, studying organisms that respond to ocean-level indicators like salinity and sediments, and other tell-tale signs of human impact: lead, for example, and certain isotopes from the atomic age. 

But "no local record measures global sea level," Kopp said in a Rutgers press release. "The statistical challenge is to pull out the global signal. That’s what our statistical approach allows us to do."

The team's databases also included 66 tide-gauge records, dating back to about 1700, to solve their "geological detective story," as co-author and fellow Rutgers professor Ben Horton calls it.

Their analysis found that, on average, sea levels were actually decreasing until the Industrial Revolution, and might have continued that trend if it weren't for influencing human factors, like fossil fuel use. As global warming heats up the seas, they're bound to rise: hot water expands, cold water contracts, and warmer temps simply melt ice faster. The authors believe that, with fewer fossil fuels, 20th century oceans would have risen by no more than half of the actual five and a half inches.

"Physics tells us that sea-level change and temperature change should go hand-in-hand," Kopp told The New York Times. "This new geological record confirms it."

The study, published Monday in the Proceedings of the National Academy of Sciences, projects that 21st century seas will rise between 22 and 52 inches, unless countries can successfully keep global warming to under 2 degrees Fahrenheit, a goal of this fall's Paris Climate Conference. In that case, oceans would still go up, but by about half as much. 

In many cases, smaller, less developed countries have been the first to suffer the impact of climate change. But coastal communities in the United States have also had a taste, particularly in increasingly-common "nuisance floods." An accompanying paper from Climate Central, a nonprofit news organization in Princeton, N.J., finds that up to 75 percent of recent East Coast tidal floods are due to rising seas, and the human activity behind them.

Charleston, S.C., for instance, experienced 34 flood days between 1955 an 1964. Between 2005 and 2014, on the other hand, there were 219. 

"I think we need a new way to think about most coastal flooding," Climate Central's Dr. Benjamin H. Strauss, the lead author, told the Times. "It’s not the tide. It’s not the wind. It’s us."

Even as flooding has increased around the world, however, the human cost has decreased, as countries adapt with new safety procedures and natural barriers. Over the past twenty years, flood-region fatalities have gone from 70 per 1 million people to 40 per million, although rates are still far higher in the world's poorest countries.

This report includes material from The Associated Press.