Just when you think the news about sea level rise couldn’t get much worse for New Orleans, it has.

According to a study released this month, the city will experience one of the highest increases in sea level among 138 coastal cities around the planet because of its location on the northern Gulf of Mexico.

New Orleans could see as much as 14.5 inches of sea level rise by 2040, and 6.5 feet by 2100 if the world doesn’t act quickly to lower greenhouse gas emissions, the main driver of global warming.

The populated parts of the city, of course, are protected by levees rising to about 22 feet. The increase would be evident outside the levees, such as the land bridge between New Orleans and St. Tammany Parish, and coastal communities to the south and west.

Those projections do not include subsidence, which is a serious problem for New Orleans. Coastal Louisiana, built on sediment-starved deltas of the Mississippi River, is sinking at one of the fastest rates of any coastal landscape in the world.

The paper, “Coastal sea level rise with warming above 2° C,” was published in the Proceedings of the National Academy of Sciences. It’s aimed at giving policymakers a more accurate assessment of local risks posed by sea level rise.

Researchers ran more than 5,000 computer simulations to project sea level rise for cities under three levels of emissions.

Although atmospheric warming is causing oceans worldwide to rise, the rate is not uniform – and in some places, it is actually dropping. Local rates can be influenced by the proximity of ocean currents, differences in the pull of gravity and shifts in rock layers far below the surface. Each of those is related to warming.

Ocean currents tend to push sea levels higher as they near shorelines. Researchers have shown some currents are growing wider as they warm due to climate change.

When millions of tons of ice are removed from a landmass, it can rise in elevation, a process geologists call post-glacial rebound. However, parts of the landmass that were on the edge of the ice field may begin to sink. That happens as rock layers that had been pushed away by the weight of the ice move back to their former positions.

For example, since the removal of the last glaciers, the center of the U.S. has been rising, while the northern Gulf Coast has been sinking.

Scientists say the melting of the vast, deep ice fields covering Greenland and Antarctica also will produce some counterintuitive results.

While the meltwater pouring into the oceans from those areas will cause a dramatic rise in overall sea levels, the level along those landmasses will actually drop. That’s because as the total mass of those landscapes shrinks, so does their gravitational pull on the tides.

The land mass on the northern Gulf will remain the same, so its share of the tidal pull will increase. That will raise sea levels locally.

The U.S. has joined 193 nations in a plan to reduce emissions enough to prevent the average global temperature from rising more than 2.7 degrees Fahrenheit by the year 2100. But  President-elect Donald Trump, who denies scientific research that says humans are warming the planet, has pledged to pull out of that agreement. If he does, scientists doubt the goal can be reached.

The authors of the study concluded if emissions are not curbed,  temperatures probably would increase 3.6 degrees Fahrenheit between 2040 and 2050. If that trend goes unchecked, they see average temperatures rising 9 degrees by 2100.

In that case, Buenos Aires, about 3,000 miles from the melting Antarctic ice fields, would see sea level rise of about 4.8 feet.

New Orleans is about 8,000 miles away from Antarctica, but the area is gaining gravitational pull on the ocean as Antarctica shrinks. So, researchers concluded, the Gulf of Mexico would climb nearly 6.5 feet — not counting subsidence.

Parts of Southeast Louisiana are sinking three feet a century, while the Bird’s Foot Delta at the mouth of the Mississippi River is sinking at about twice as fast.

Bob Marshall

From 2013 to 2017, Bob Marshall covered environmental issues for The Lens, with a special focus on coastal restoration and wetlands. While at The Times-Picayune, his work chronicling the people, stories...