Coral reef islands have the potential to adapt to sea level rise

Coral reef islands can naturally adapt to rising water levels, according to a new study led by the University of Plymouth. The research suggests that many reef islands are not doomed to disappear with future sea level rise.

Coral islands are formed by the accumulation of sandy and rocky organic materials. These low-lying islands are expected to become submerged and uninhabitable in the coming decades as a result of climate change. 

“In the face of climate change and sea level rise, coral reef islands are among the most vulnerable coastal environments on the planet,” said study lead author Professor Gerd Masselink.

“Previous research into the future habitability of these islands typically considers them inert structures unable to adjust to rising sea level. Invariably, these studies predict significantly increased risk of coastal flooding and island inundation, and the concept of ‘island loss’ has become entrenched in discourses regarding the future of coral reef island communities.” 

“In turn, this has led to attention being focused on either building structural coastal defenses or the exodus of island communities, with limited consideration of alternative adaptation strategies.”

The research is the first to combine numerical models of island morphology with physical model experiments to simulate how reef islands will respond to sea level rise.

The analysis revealed that islands composed of gravel material will evolve as overtopping waves transfer sediment from the beach face to the surface of the island. In other words, the island’s crest will rise along with sea levels.

The researchers explained that this natural method of adaptation, along with management strategies such as changes to infrastructure, could support near-term habitability. 

“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,” said Professor Masselink. 

“The height of their surface is actually determined by the most energetic wave conditions, therefore overtopping, flooding and island inundation are necessary, albeit inconvenient and sometimes hazardous, processes required for island maintenance.”

The scientists created a scale model of Fatato Island, which is part of the Funafuti Atoll in Tuvalu, and placed it in the Coastal Ocean and Sediment Transport (COAST) Lab at the University of Plymouth.

When the model was subjected to a series of experiments designed to simulate sea level rise, the island’s crest rose as intense waves deposited sediment on the island’s surface.

According to three numerical models, the island rose by just under 0.7m in response to a simulated sea level rise of 0.75m. This demonstrates that islands can keep pace with rising water levels, but the precise rate of sea-level rise will be the critical determining factor.

“The model provides a step-change in our ability to simulate future island responses to sea level rise and better resolve what the on-ground transformations will look like for island communities. Importantly, our results suggest that island drowning within the next few decades is not universally inevitable,” said study co-author  Professor Paul Kench.

“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.”

The study is published in the journal Science Advances.

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By Chrissy Sexton, Earth.com Staff Writer