The oceans have risen and fallen throughout Earth’s history, following the planet’s natural temperature cycles. Twenty thousand years ago, what is now New York City was at the edge of a giant ice sheet, and the sea was roughly 400 feet lower. But as the last ice age thawed, the sea rose to where it is today.
Now we are in a new warming phase, and the oceans are rising again after thousands of years of stability. As scientists who study sea level change and storm surge, we fear that Hurricane Sandy gave only a modest preview of the dangers to come, as we continue to power our global economy by burning fuels that pollute the air with heat-trapping gases.
This past summer, a disconcerting new scientific study by the climate scientist Michiel Schaeffer and colleagues — published in the journal Nature Climate Change — suggested that no matter how quickly we cut this pollution, we are unlikely to keep the seas from climbing less than five feet.
More than six million Americans live on land less than five feet above the local high tide. (Searchable maps and analyses are available at SurgingSeas.org for every low-lying coastal community in the contiguous United States.) Worse, rising seas raise the launching pad for storm surge, the thick wall of water that the wind can drive ahead of a storm. In a world with oceans that are five feet higher, our calculations show that New York City would average one flood as high as Hurricane Sandy’s about every 15 years, even without accounting for the stronger storms and bigger surges that are likely to result from warming.
Floods reaching five feet above the current high tide line will become increasingly common along the nation’s coastlines well before the seas climb by five feet. Over the last century, the nearly eight-inch rise of the world’s seas has already doubled the chance of “once in a century” floods for many seaside communities.
We hope that with enough time, most of our great coastal cities and regions will be able to prepare for a five-foot increase. Some will not. Barriers that might work in Manhattan would be futile in South Florida, where water would pass underneath them by pushing through porous bedrock.
According to Dr. Schaeffer’s study, immediate and extreme pollution cuts — measures well beyond any discussion now under way — could limit sea level rise to five feet over 300 years. If we stay on our current path, the oceans could rise five feet by the first half of next century, then continue rising even faster. If instead we make moderate shifts in energy and industry — using the kinds of targets that nations have contemplated in international talks but have failed to pursue — sea level could still climb past 12 feet just after 2300. It is hard to imagine what measures might allow many of our great coastal cities to survive a 12-foot increase.
We might find comfort in the fact that this is just one set of projections, and projections are notoriously tough to get right. But a second study that also came out this past summer erases any such comfort.
Led by the geochemist Andrea Dutton and published in the journal Science, the second paper uses deep history, not model projections, for clues to the future. About 125,000 years ago, before the last ice age, there was a warm period that lasted 10,000 to 15,000 years. It was perhaps a little warmer than today, but cooler than the temperatures that climate scientists expect later in this century without sharp pollution cuts. Dr. Dutton’s research strongly reinforces a prior study led by one of us, which found that the warm-period sea levels rose roughly 20 to 30 feet higher than those of today. We just don’t have a clear picture of how fast that could happen again.
Any sea level forecast must be interpreted carefully: things could be better, or worse.
The Schaeffer study uses the relationship between global temperature and sea level over the past 1,000 years — when it was cool, and the great ice sheets were generally stable — to extrapolate over the next 300 years — when it will be hot, and the ice sheets in Greenland and Antarctica may behave differently. Other scientific teams have tried the notoriously difficult task of forecasting ice sheet decay in physical detail, and this has tended to produce slower estimates of sea level rise than the Schaeffer team’s method. But any projection is compromised by the fact that we are sending heat-trapping carbon dioxide into the atmosphere far faster than anything the planet has seen for at least 55 million years.
The Dutton study comes with caveats, too. Earth’s orbit was different during the last warm period, bringing more sunshine to the Arctic and complicating the analogy with today. But today we are on a path to a planet that will be much hotter than it was in the period Dr. Dutton studied.
There are two basic ways to protect ourselves from sea level rise: reduce it by cutting pollution, or prepare for it by defense and retreat. To do the job, we must do both. We have lost our chance for complete prevention; and preparation alone, without slowing emissions, would — sooner or later — turn our coastal cities into so many Atlantises.