As sea levels rise, some scientists predict a bleak future for low-lying coral reef islands, asserting that they will be uninhabitable well before the end of the present century.
But Gerd Masselink, a professor of coastal geomorphology at the University of Plymouth in the United Kingdom, said that most of these previous studies, while accounting for factors associated with the ocean—like changing sea level, tides, and waves—have ignored one important thing: the capacity of the island itself to change.
A new study by Masselink and colleagues suggests that reef islands may be more resilient than scientists originally thought. Using physical and numerical models, the research team found that under certain conditions, rising sea levels would actually cause the island crest (the tallest point on the island) to grow taller. The process associated with such growth is called wave overtopping, which pushes sand and gravel from the beach face to the island crest.
Healthy Reef Systems Support Resiliency
Masselink said that this resiliency probably won’t be true for all islands and all conditions. He said that larger islands and those with healthy reef systems are likely to be more resilient, and the team’s modeling experiments showed that islands with larger sediments like gravel were more likely to survive than islands composed of small-sized sediments like sand.
The pace of climate change will also be a major factor impacting resiliency. “It takes time for the island to adjust, so if sea level rises too fast, the island might be left behind,” said Masselink. A rapid increase in the number or intensity of storms could also spell disaster.
Alejandra Ortiz, a coastal geomorphologist at Colby College in Maine who was not involved in the study, pointed out that climate change can also impact sediment supply for these islands. “For most of these atolls, the only sediment that’s being sourced in the system is produced locally, whether that’s from the coral reef or from microalgae or microorganisms growing in the lagoon,” she said. So if increasing ocean temperatures and ocean acidification negatively affect the health of the reef or the microalgae, the result would also be detrimental for island health.
Masselink agreed: If the reef dies, he said, “this will turn off the sediment supply to the islands necessary for their long-term survival.”
Resiliency, Not Necessarily Habitability
Even if an island itself survives, the conditions of its survival don’t always correlate to human habitability. “Reef islands need very energetic waves to form, because the only way you can build up the island is for the waves to go on top of the island,” said Masselink.
“It’s a bit of a paradox,” he said, “because what people perceive to be destructive conditions—like storm conditions, large waves, and high water levels—are actually required to build the islands up.”
Indeed, Ortiz said, for houses and many other types of human infrastructure, waves washing over the island are “the definition of destruction.”
“I do firmly believe that the evidence shows that these landscapes, over the next hundred years, are resilient,” she said. “But if you say they’re going to look the same or they’re going to be resilient with humans living on them as they do now—that’s where things start to look more negative.”
Masselink acknowledged that even on resilient islands, adaptive and flood-resistant infrastructure will likely be needed. He pointed out that in many flood-prone areas of the world, houses are built on stilts, which may be a good strategy to help people continue to live on these islands.
Masselink said that nourishment—adding sediment to the beach—is another potential way to help reef islands adapt to climate change, but we still need to study it in much greater detail to determine if it would actually be feasible. “The question is, How much sediment would you need to add to significantly enhance the ability of the island to keep up with rising sea level?”
Masselink said he planned to explore these questions in future studies, noting that these issues have important real-world implications for island communities.
“It’s a bit unfair that the communities that are least responsible for [climate change] are the ones that are most affected,” he said.
—Hannah Thomasy (@HannahThomasy), Science Writer