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High sea levels thousands of years ago aided island formation

  • A recent study has found that high sea levels were critical to the formation of coral reef islands in the Indian Ocean thousands of years ago.
  • The findings suggest that rising sea levels driven by climate change might not destroy all coral reef islands.
  • However, the authors caution that the same higher-energy waves that help build these islands could also destroy the infrastructure on them that humans depend on.
  • They also say that, for coral reef island formation to occur, the reef must be healthy to begin with — something that risks being negated by rising water acidity and temperature, both the result of climate change.

Some types of coral reef islands could potentially grow as a result of climate change-driven sea level rise, according to a recent study, which suggests that all of these islands might not be doomed by current trends.

“Coral reef islands are typically believed to be highly vulnerable to rising sea levels,” Holly East, a geographer and lecturer at Northumbria University in the U.K., said in a statement. “However, we have found evidence that the Maldivian rim reef islands actually formed under higher sea levels than we have at present.”

Mainadhoo Island in the Maldives, one of the islands included in the study. Image by Holly East/Northumbria University.

Low-lying island countries like the Maldives appear destined to bear the brunt of climate change-driven sea level rise. Indeed, no country lies lower than the coral reef island nation that straddles the equator in the Indian Ocean: Its highest point sits just 2.4 meters (less than 8 feet) above sea level.

But when East and her colleagues pieced together the formation of several of these islands using core samples, collected with aluminum pipes hammered into the coral, they discovered that much of the reef construction happened between 1,600 and 4,200. At that time, sea levels were as much as 0.5 meters (about 1.5 feet) higher than they are currently today.

Rising global sea levels may actually be beneficial to the long-term future of coral reef islands, such as the Maldives. Video courtesy of Northumbria University.

As they do today, storms off the southern African coast touched off waves, which packed a lot of energy, in part because of the higher water levels. When these waves reached the spot where the Maldives now sits, they broke off pieces of the coral reefs and flung them onto “platforms” that eventually served as the basis for islands that eventually rose above the water.

“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,” East said, “it may actually create the perfect conditions to reactivate the processes that built the reef islands in the first place, rather than drowning them.”

The team published their work in October in the journal Geophysical Research Letters.

Higher sea levels may aid in coral reef island formation, but the energetic waves they produce could destroy infrastructure on islands. Image by John C. Cannon/Mongabay.

At the same time, however, the stronger waves that accompany sea level rise could destroy the infrastructure on the islands that humans depend on, the authors wrote. The 1,200 islands that make up the Maldives are home to around 436,000 people.

They also caution that this sort of reef-building will only happen if the coral reefs that provide the building substrate are in good shape.

“As these islands are mostly made from coral, a healthy coral reef is vital to provide the materials for island building,” East said.

The coral reef rim islands of Huvadhoo Atoll in the Maldives. Image by Paul Kench/Simon Fraser University.

Along with higher sea levels, climate change is spiking the acidity of the oceans, which can lead to the breakdown of the bony structure of corals. It’s also driving up the temperature of the water at the surface, leading to widespread bleaching. Those developments could negate the benefits of higher sea levels for island formation, East said.

“If the reef is unhealthy,” she added, “we could end up with the perfect building conditions but not the bricks.”

Banner image of Huvadhoo Atoll in the Maldives by Paul Kench/Simon Fraser University. 

Citation

East, H. K., Perry, C. T., Kench, P. S., Liang, Y., & Gulliver, P. (2018). Coral Reef Island Initiation and Development Under Higher Than Present Sea Levels. Geophysical Research Letters.

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