Rising seas pose serious threats for low-lying coral reef island nations. Widespread assumptions hold that encroaching waters will affect all reef islands in the same way, with many disappearing beneath the waves. However, emerging evidence, including new research by East et al., suggests that reef islands form differently in different settings, and some might actually grow as waters rise.
The new paper builds on work performed in the Maldives, a 1,200-island nation that is home to about 436,000 people. There the researchers had previously studied islands that formed on small, ring-shaped reef structures known as faros. But 90% of Maldivians live on the islands that form on reef structures around atoll perimeters; how and when these rim-type reef islands originally formed were unclear.
To reconstruct rim reef island formation, the scientists selected two contrasting field sites in Huvadhoo Atoll, the second-largest atoll in the Maldives. One site included three islands on the southwestern, windward side of the atoll. The other site consisted of two islands on the northeastern, leeward side of the atoll.
At each site, the researchers used a variety of tools to peer into the past. These included topographic surveys using laser levels, examination of subsurface layers using ground-penetrating radar and coring, and radiocarbon dating of materials found in the core samples.
This analysis revealed that islands at both sites formed during the postglacial period when sea levels were higher than present levels. Before the islands formed, existing coral reefs grew vertically toward the sea surface. Then, intense wave action associated with the elevated waters deposited pieces of broken-off coral and other materials, which accumulated on the reef platform to initiate island formation.
Despite being in the same atoll, however, the two different settings gave rise to islands on different timescales. Islands on the leeward rim began to form about 4,200 years ago, whereas those on the windward rim initiated formation about 2,800 years ago. Accumulation of wave-deposited material followed different patterns between the two sites, suggesting that reef islands in different settings may respond differently to climate change.
This research suggests that in some settings, future sea level rise and associated wave action could reactivate island-building processes, spurring further growth of reef islands, which could enhance their resilience to climate change.
Still, the authors note that rising temperatures and associated coral bleaching events could stymie coral growth, limiting the supply of coral rubble and sand materials required for island building. Also, they studied island growth at a millennial scale, whereas shorter time periods are more important for existing island nations. And even with reactivated growth, the intense wave events required, as well as changes in island shape, could challenge island infrastructure and habitability.
Nonetheless, the findings could help inform vulnerability assessments and planning for the future of island nations facing the specter of rising seas. (Geophysical Research Letters, https://doi.org/10.1029/2018GL079589, 2018)
—Sarah Stanley, Freelance Writer