On Jaluit Atoll, part of the Marshall Islands, the highest elevation is just 10 feet above sea level. (Photo: Keith Polya/Flickr)

When most people picture the consequences of sea level rise, they imagine whole islands swallowed beneath the sea. For many low-lying coral atolls and islands in the tropics, where waters are rising at some of the fastest rates on the planet, that could someday become a reality. Most atolls, which are ring-shaped reefs or islands surrounding lagoons, have an average elevation of just six feet above sea level—the same amount that scientists predict sea levels will rise by the end of the century.

But a new study, published in Science Advances, suggests that low-lying islands might be uninhabitable well before that.

Though it may be another 80 years before sea level rise sinks islands, there is a more imminent threat: the contamination of soil and groundwater by salt water, which can occur as the ocean rises or when waves crash over the land. Until now, few studies considered how sea level rise and wave action might interact. In the new study, an international team of researchers looked into the impact that wave-driven flooding might have on freshwater resources as sea levels continue to increase.

In particular, the study focused on Roi-Namur Island, an atoll in the Marshall Islands where the United States military's Ronald Reagan Ballistic Missile Defense Test Site is located. The work was commissioned by the Pentagon's Strategic Environmental Research and Development Program, as the U.S. has military bases on many vulnerable atolls in the Pacific Ocean.

The researchers' models suggest that wave overwash events—like the storm in March of 2014 in which nearly 20-foot waves washed over Roi-Namur and infiltrated the fresh groundwater—are likely to become more frequent and severe in the future. And two consecutive years of wave overwash events could leave the island without a source of drinking water.

How soon that happens will depend on the rate of climate change, and the authors modeled wave dynamics under three potential scenarios: middle-range emissions, high emissions, and high emissions with the collapse of the Antarctic ice sheet, which would cause sea levels to rise drastically and rapidly. Under that severe scenario, Roi-Namur could be without potable water before 2030—well within the lifespan of the island's current residents, the authors note. Under the high emissions scenario, this tipping point could arrive before 2040, and under middle-range emissions, sometime between 2055 and 2065—which means that even the most conservative scenario would render the island uninhabitable 50 to 100 years earlier than previously thought.

Roi-Namur is "characteristic of (if not of higher elevation than) most of the thousands of atoll islands in the world’s oceans," the authors write, meaning that these tipping points could hold true for most atolls across the tropics.

The Pentagon has long warned that the effects of climate change could threaten national security, but the inundation of these islands threatens much more than just U.S. military readiness. Altogether, millions of people live on low-lying islands across the Pacific.