Rising Sea Levels May Build, Rather Than Destroy, Coral Reef Islands

  • Date: 16/11/18
  • Northumbria University Newcastle & Geophysical Research Letters

Rising global sea levels may actually be beneficial to the long-term future of coral reef islands, such as the Maldives, according to new research published in Geophysical Research Letters.

Rising global sea levels may actually be beneficial to the long-term future of coral reef islands, such as the Maldives, according to new research published in Geophysical Research Letters.

Low-lying coral reef islands are typically less than three metres above sea level, making them highly vulnerable to rising sea levels associated with climate change. However, research has found new evidence that the Maldives – the world’s lowest country – formed when sea levels were higher than they are today.

The evidence was discovered by researchers who studied the formation of five islands in the southern Maldives. Using a coring technique, they were able to reconstruct how and when the islands formed.

They found that large waves caused by distant storms off the coast of South Africa led to the formation of the islands approximately three to four thousand years ago. These large waves – known as high-energy wave events – broke coral rubble off the reef and transported it onto reef platforms creating the foundations for the reef islands.

At the time, sea levels were up to 0.5 metres higher than they are today, which gave the waves more energy. This means that higher sea levels and large wave events were critical to the construction of the islands.

The researchers say that under climate change, projected increases in sea level and the magnitude of large wave events could actually lead to the growth of reef islands, but only if the coral reef remains healthy to provide the building material.

The research was led by Dr Holly East of the Department of Geography and Environmental Sciences at Northumbria University, Newcastle. She explained: “Coral reef islands are typically believed to be highly vulnerable to rising sea levels. This is a major concern for coral reef island nations, in which reef islands provide the only habitable land.

“However, we have found evidence that the Maldivian rim reef islands actually formed under higher sea levels than we have at present. This gives us some optimism that if climate change causes rising sea levels and increases in the magnitude of high-energy wave events in the region, it may actually create the perfect conditions to reactivate the processes that built the reef islands in the first place, rather than drowning them.”

However, Dr East stressed that this could only occur if healthy live coral was available in the region’s reef communities.

“As these islands are mostly made from coral, a healthy coral reef is vital to provide the materials for island building,” she said. “However, this could be problematic as corals face a range of threats under climate change, including increasing sea surface temperatures and ocean acidity. If the reef is unhealthy, we could end up with the perfect building conditions but not the bricks.”

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Their paper, Coral Reef Island Initiation and Development Under Higher Than Present Sea Levels is now available in Geophysical Research Letters.

Coral Reef Island Initiation and Development Under Higher Than Present Sea Levels

Geophysical Research Letters — https://doi.org/10.1029/2018GL079589

Abstract

Coral reef islands are considered to be among the most vulnerable environments to future sea level rise. However, emerging data suggest that different island types, in contrasting locations, have formed under different conditions in relation to past sea level. Uniform assumptions about reef island futures under sea level rise may thus be inappropriate. Using chronostratigraphic analysis from atoll rim islands (sand‐ and gravel‐based) in the southern Maldives, we show that while island building initiated at different times around the atoll (~2,800 and ~4,200 calibrated years before present at windward and leeward rim sites, respectively), higher than present sea levels and associated high‐energy wave events were actually critical to island initiation. Findings thus suggest that projected sea level rise and increases in the magnitude of distal high‐energy wave events could reactivate this process regime, which, if there is an appropriate sediment supply, may facilitate further vertical reef island building.

Plain Language Summary

The habitability of reef island nations under climate change is a debated and controversial subject. Improving understanding of reef island responses to past environmental change provides important insights into how islands may respond to future environmental change. It is typically assumed that all reef islands will respond to environmental change in the same manner, but such assumptions fail to acknowledge that reef islands are diverse landforms that have formed under different sea level histories and across a range of settings. Here we reconstruct reef island evolution in two contrasting settings (in terms of exposure to open ocean swell) in the southern Maldives. Important differences in island development are evident between these settings in the timings, sedimentology, and modes of island building, even at local scales. This implies that island responses to climate change may be equally diverse and site‐specific. We present evidence that island initiation was associated with higher than present sea levels and high‐energy wave events. Projected increases in sea level and the magnitude of such high‐energy wave events could therefore recreate the environmental conditions under which island formation occurred. If there is a suitable sediment supply, this could result in vertical island‐building, which may enhance reef island future resilience.

Full paper

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