Coral Reef Islands May Not Be Doomed by Sea Level Rise, Scientists Say

Coral reef islands around the world—home to hundreds of thousands of people—may be capable of adapting naturally to sea level rise as the world warms, researchers have suggested.

Previously, scientists have predicted that many of these vulnerable low-lying islands will likely become uninhabitable within decades as a result of increased flooding due to sea level rise (SLR) and will eventually be completely submerged by water.

However, the projections used to make these predictions generally assume that coral reef islands, which are defined as sandy or gravel islands sitting on top of coral reef platforms, are inert landforms that are not capable of changing shape or structure.

A study published in the journal Science Advances challenges this idea, suggesting that reef islands composed of gravel material may be able to adapt to rising seas thanks to a process in which sediment from the beach is transferred to the island's surface during flooding events.

"We show that there is a mechanism available to the islands to adjust to rising sea level by retreating and building up elevation during coastal flooding events that bring sand and gravel in from the seaward side," Gerd Masselink, lead author of the study from the University of Plymouth in the U.K., told Newsweek.

"This flooding of islands during energetic wave conditions and/or high water levels is referred to as overwash and is well-known from sandy and gravel barrier systems. The resulting increase in island elevation means that the islands will be habitable for longer," he said.

For their study, Masselink and colleagues constructed a scale model of Fatato Island in Tuvalu—an island nation in the South Pacific—and conducted a series of experiments simulating predicted sea level rise. They found that the island's crest—it's highest point—rose just enough to keep up with rising sea levels, while also retreating inland.

They then used a computer model to explore how the island physically responded to sea level rise of 0.75 meters [2.46 feet]—the global average increase predicted for 2100 by the Intergovernmental Panel on Climate Change. These numerical simulations also demonstrated that reef islands can keep up with rising sea levels, although, the researchers said that the rate of SLR is key in determining their future.

"It is important to realize that these coral reef islands have developed over hundreds to thousands of years as a result of energetic wave conditions removing material from the reef structure and depositing the material towards the back of reef platforms, thereby creating islands," Masselink said in a statement.

"The height of their surface is actually determined by the most energetic wave conditions, therefore flooding and island inundation are necessary, albeit inconvenient and sometime hazardous, processes required for island maintenance," he said.

Because every case is different, it is very difficult to predict how long any individual coral reef island will survive under current SLR trajectories based on these results, the researchers said. This will depend on many factors including the rate of sea-level rise, the supply of sediment from the reef system, coastal defence structures, type of sediment (sand or gravel,) and vegetation, among others.

"An island completely enclosed by a seawall is not able to adjust to rising sea level as flooding is prevented, and it is floods that bring in the sediment required for building the island up. Such islands will probably be uninhabitable in 20-50 years or so, unless the seawall is upgraded to deal with the elevated sea level," Masselink told Newsweek.

"On the other hand, there is no reason why an island with a natural shoreline and a healthy coral reef system that produces significant amounts of sediment for building the island cannot keep up with rising sea level as long as the rate of sea-level rise is not too fast—say less than 10 millimeters per year. Between these two end members of the island spectrum it is much more difficult to be confident about the future."

Nevertheless, the researchers say the results show that the future for coral reef islands, which often provide the only habitable land in atoll nations, is not necessarily one where they will all inevitably become uninhabitable. This finding could have implications for how coral reef island communities adapt to climate change and SLR, although natural adaptations may also need to be augmented by human interventions in order to ensure habitability in the near-term.

"[An uninhabitable] scenario only provides two potential adaptation strategies for island communities: relocation or coastal defence. An island that has the potential to naturally adapt to rising sea level by retreating and building up, offers alternative adaption strategies for island communities," Masselink said.

Islands could build more flood-resistant coastal infrastructure, for example, houses on stilts, or use nature-based solutions such as "island nourishment."

"For the island to naturally adjust, it must be regularly flooded, as this brings in the sediment. So any important infrastructure needs to be designed to deal with this flooding. There are large quantities of suitable sediment in the atoll lagoons. Some of this could be dredged and used to nourish the islands to help the natural adaption process," Masselink said.

Paul Kench, a co-author of the study from Simon Fraser University, Canada, also said that such human interventions could be important for the approximately one million people living on low-lying reef islands throughout the tropics.

"Future infrastructure should be designed to accommodate periodic flooding and sediment deposition and most structures should be designed to be relocatable, so the community can gradually adjust with the island," Kench told Newsweek.

"Understanding how islands will physically change due to sea level rise provides alternative options for island communities to deal with the consequences of climate change. It is important to stress there is no one-size-fits-all strategy that will be viable for all island communities—but neither are all islands doomed," he said in a statement.