Charles Darwin wrote one of his early monographs about such atolls, and throughout his life explored how these islands could come to be in the midst of the ocean, how they could change and evolve, and what their sudden appearance might mean for the ability of their resident species' ability to adapt to new environments.
What Darwin proposed, and subsequent study has largely validated, was that these coral rings are the remnants of ancient volcanic islands. Coral reefs formed and grew at a depth adequate for their growth, not too near nor too far from the surface. As the volcanic rock eroded and subsided, the coral would continue to grow, maintaining its depths a few meters below the surface. As ocean levels rose and fell, the reef would grow higher, but would not shrink again. The portion above ground would collect sand and remain a few feet above the water, while the area where a volcano once stood would become a calm lagoon. An enclosed lagoon might even collect enough rainwater to be drinkable.
If you look at the NASA image, or click through to Google Earth, you'll see a large head of coral growing in the center of Nukuoro's lagoon, and a slight ring of coral reefs around the inner perimeter of the atoll.
The island is populated by 900 people, who mostly engage in subsistence farming. There's no airstrip, but a boat visits every month or so. The setting is idyllic, and Ms. TfK and I are already hunting for a way to move there.
Unfortunately, the atoll, and others like it, are in tremendous danger.
Darwin noted, as had earlier authors, that corals are very environmentally dependent. They require water of exactly the right temperature, and solar radiation of the wrong sort can kill the organisms that produce the calcium carbonate structure of the reef. The U.S. government recently listed several coral species as endangered because warming waters are killing reefs in the US and abroad. By listing the species, they hope to restrict trade in coral and to make it easier to protect existing reefs. Coral reefs are not just beautiful structures on their own (though they are), they also provide shelter for spawning fish, and places where many species congregate to hunt and to breed. As the corals bleach and die, the consequences for the oceans at large could be horrific.
For fun, let's let Vanuatu's Port Vila Press explain the problem:
The oceans absorbed approximately 118 billion metric tons of carbon since the early 1800s and the beginning of the Industrial Age. This interaction with carbon dioxide is making the naturally alkaline ocean waters more acidic. The higher levels of acidity lower the concentration of carbonate ion, a building block of calcium carbonate, which many marine organisms use to grow their skeletons and create coral reef structures.But the problem doesn't end there. Not only is ocean chemistry changing, but so is the temperature. The coral is highly dependent on temperature, and scientists in the Caribbean are blaming deaths of corals there on the measurably warmer waters.
"This is leading to the most dramatic changes in marine chemistry in at least the past 650,000 years," says Richard Feely, one of the authors and an oceanographer at the Pacific Marine Environmental Laboratory (PMEL) of the National Oceanic and Atmospheric Administration (NOAA).
Experimental studies have shown that the marine organisms dependent on calcium carbonate grow more slowly as the oceans become more acidic. Consequently, reef structures will become threatened because corals will be unable to build reefs at a pace to keep ahead of natural forces of erosion.
What about Nukuoro, though? It already exists, so even if the corals there bleach and die, the atoll won't go away.
The atoll's problem, one it shares with many of its neighbors, is that the highest point is only a few feet above the ocean. Storm waves sweep right across it. The Intergovernmental Panel on Climate Change estimated in 2001 that, without any massive change in the ice in Greenland or West Antarctica, sea level will rise half a foot on average. But new research suggests that a massive loss of Greenland's or West Antarctica's glaciers are more likely than once thought, and either event could produce sea level changes of dozens of feet.
Even a half a foot change would put most, if not all, of Nukuoro underwater, and much more vulnerable to storms. Ms. TfK and I will need a new vacation spot, but the Nukuoroans would need new homes, while the Nukuoroan language would probably just disappear with the island.