Quirks & Quarks

Will coral reef islands rise or fall? It's a greenhouse-gas paradox

Coral reef islands like those in Kiribati or the Maldives may actually benefit from rising sea levels, provided the coral is healthy

Coral reef islands could actually benefit from rising sea levels, provided the coral is healthy

Coral reef rim islands, Huvadhoo Atoll, Maldives (Paul Kench)
Listen8:22

Fears that sea-level rise associated with climate change could drown low-lying coral islands, may not come to pass. Research is suggesting that these islands could grow as sea level rises, as long as there's something to make them out of.

Will the Maldives keep their heads above water?

400,000 people live in The Maldives, a south Asian country made up of a chain of 26 coral atolls, or ring-shaped coral reefs islands. The Maldives is also the lowest country in the world, with the highest point in the nation at just 2.4 metres above sea-level. So there has been a great deal of concern that with climate-change driven sea level rise, these islands would sink beneath the waves.

But a new study led by Holly East, a lecturer in Physical Geography at Northumbria University in England, has found that rising sea levels could actually help build, rather than destroy coral reef islands like The Maldives.

Mainadhoo Island, one of the reef islands studied is only about one metre above sea level. (Holly East)

Climate change may lead to coral island formation

Coral reef islands form in different ways depending on whether sea levels are rising or falling. Rising sea levels bring large waves that are powerful enough to break off pieces of coral from the atoll's reef. Over time, these broken pieces, along with sand from the reef, build up to form islands.  

By drilling down to the very foundation — a point below live coral in the surrounding ocean — scientists were able to figure out how and when the islands that make up The Maldives formed.

Radiocarbon dating of sediments in core samples suggest a key phase of island building occurred between 4,200 and 1,600 years ago. Those samples also indicate that sea levels were actually 0.5 metres higher than present-day levels. In other words, the islands formed at a time of higher sea level.

This suggests that rising sea levels associated with climate change might not doom these coral islands. It might just jump-start the same process that formed them, so that they grow higher.

Holly East takes cores sample from a reef island. (Yiqing Liang)

If it sounds too good to be true...

The optimism that comes with the fact that rising sea levels may actually create the perfect conditions for reef-island building, comes with at least one caveat. In order for this process to occur, the coral needs to be healthy to provide 'building materials' for the islands. 

Corals today face a range of threats thanks to greenhouse gases and climate change. Increasing water temperature and ocean acidity are destroying reefs around the world. Holly East says "if the reef is unhealthy, we could end up with the perfect building conditions but not the bricks."   

Using a laser level to survey island topography. (Holly East)

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