Looming Floods, Threatened Cities

The risk is clear: Antarctica’s collapse has the potential to inundate coastal cities across the globe.

Over tens of millions of years, thin layers of snow falling on the continent — in many places, just a light dusting every year — were pressed into ice, burying mountain ranges and building an ice sheet more than two miles thick. Under its own weight, that ice flows downhill in slow-moving streams that eventually drop icebergs into the sea.

If that ice sheet were to disintegrate, it could raise the level of the sea by more than 160 feet — a potential apocalypse, depending on exactly how fast it happened. Recent research suggests that if society burns all the fossil fuels known to exist, the collapse of the ice sheet will become inevitable.

Improbable as such a large rise might sound, something similar may have already happened, and recently enough that it is still lodged in collective memory.

In the 19th century, ethnographers realized that virtually every old civilization had some kind of flood myth in its literature.

In the Epic of Gilgamesh, waters so overwhelm the mortals that the gods grow frightened, too. In India’s version, Lord Vishnu warns a man to take refuge in a boat, carrying seeds. In the Bible, God orders Noah to carry two of every living creature on his ark.

“I don’t think the biblical deluge is just a fairy tale,” said Terence J. Hughes, a retired University of Maine glaciologist living in South Dakota. “I think some kind of major flood happened all over the world, and it left an indelible imprint on the collective memory of mankind that got preserved in these stories.”

That flooding would have occurred at the end of the last ice age.

Ice ages occur when wobbles in Earth’s orbit change the distribution of sunlight, allowing huge ice sheets to build up, mainly in the Northern Hemisphere. At the peak of the last ice age, about 50,000 years ago, the ice sheets grew so large and locked up so much of the world’s water that the sea level fell by an estimated 400 feet.

Beginning perhaps 25,000 years ago, after the orbit shifted again, the ice sheets began to melt and the sea level began to rise. Over several thousand years, coastlines receded inland by as much as a hundred miles.

Human civilization did not yet exist, but early societies of hunters and gatherers lived along most of the world’s shorelines, and they would have watched the inundation claim their lands.

Remnants of that ice age remain. A little bit still clings to mountains, but the main survivors are the two great ice sheets covering Greenland and Antarctica.

Scientists once thought that further destruction of those ice sheets was likely to take thousands of years. But, starting in the 1970s, lone scientific voices warned that the ice sheets could be vulnerable much sooner if greenhouse emissions were not checked. A scientist at Ohio State University, John H. Mercer, pointed in particular to the western part of Antarctica.

Because the West Antarctic ice sheet sits in a giant bowl, much of it below sea level, it is fundamentally unstable.

Extensive satellite monitoring began in the 1990s and, within a decade, evidence emerged that the ice sheet was already starting to speed up, retreat and destabilize. Since then, the rate at which some of the glaciers are dumping ice into the ocean has tripled. More than 100 billion tons are lost every year.

In 2016, Robert M. DeConto of the University of Massachusetts, Amherst, and David Pollard of Pennsylvania State University published a study, based on a computer analysis of Antarctica, that raised alarms worldwide.

Incorporating recent advances in the understanding of how ice sheets might break apart, they found that both West Antarctica and some vulnerable parts of East Antarctica would go into an unstoppable collapse if the Earth continued to warm at a rapid pace.

In their worst-case scenario, the sea level could rise by six feet by the end of this century, and the pace could pick up drastically in the 22nd century. Dr. DeConto and Dr. Pollard do not claim that this is a certainty — they acknowledge that their analysis is still rough — but they argue that the possibility should be taken seriously.

When the seas rushed inland thousands of years ago, human communities were still small, primitive and presumably able to move with relative ease. Today, the world population is seven billion and growing, and hundreds of millions of people and trillions of dollars of property are within a few feet of sea level.

If the rise turns out to be as rapid as the worst-case projections, it could lead to a catastrophe without parallel in the history of civilization.

The National Science Foundation in Washington and the Natural Environment Research Council in Britain have decided to make a huge push over the next few years to get the data needed to refine the forecasts and get ahead of the alarming changes already occurring in Antarctica.

Some scientists point out that during the last ice age, ice sheets similar to West Antarctica’s formed in other ocean basins. But as the ice age ended and the oceans warmed, all of them collapsed. These experts have started to think that West Antarctica, as a fragile holdover, is basically a disaster waiting to happen — and that if human-caused global warming has not already set the calamity in motion, it may soon do so.

“We could have a substantial retreat on a time scale of 10 years,” said Robert A. Bindschadler, a retired NASA climate scientist who spent decades working in Antarctica. “It would not surprise me at all.”