Scientists in Singapore have found that most coral reefs across the Indo-Pacific may struggle to grow fast enough to keep pace with accelerating sea-level rise.

The team from Nanyang Technological University, Singapore (NTU Singapore) found that 76% of the reef sites surveyed do not have the capacity to keep pace with projected sea-level rise under a high greenhouse gas emissions scenario, threatening coastal protection and marine habitats.

The researchers also identified a critical threshold. When relative sea level rises by more than 5.3 millimetres a year, there is a greater than 90 per cent chance that reefs will be unable to maintain their upward growth.

This threshold is likely to be surpassed within 35 years unless global greenhouse gas emissions are substantially reduced.

The study was led by NTU’s Earth Observatory of Singapore (EOS) research fellow Dr Riovie Ramos (above) and Nanyang assistant professor Kyle Morgan, who is also the associate chair (Graduate Studies) at NTU’s Asian School of the Environment (ASE).

The team added for thousands of years, coral reefs have survived rising seas by building themselves upwards, and outwards, over time.

Corals and other organisms produce calcium carbonate, the hard material that forms the reef’s structure. This allows reefs to remain close to the sea surface, where corals receive sunlight and the reef can break the force of waves approaching the coast.

But reefs can grow only so quickly. If sea level rises faster than a reef can build upwards, the reef will sit under progressively deeper water, a process scientists call reef drowning.

The living corals may not disappear immediately, but the reef can lose the shallow, complex structure that protects coastlines and provides habitats for fish and other marine life.

Morgan said: “Coral reefs have kept pace with rising seas for thousands of years, but the rate of sea-level rise expected in the coming decades could exceed the limits of most reefs.”

“When that happens, reefs will become progressively submerged. They may still contain living corals, but they will no longer function in the same way, potentially allowing greater wave energy to reach the coastline and increasing risks to coastal communities,” he explained.

The analysis found that long-term reef growth is controlled by a combination of sea-level change, coral communities and local physical conditions.

Lead author Ramos, added: “An individual coral can grow relatively quickly, but that does not mean an entire reef will build upwards at the same rate.

“Reef cores can show us how much structure was preserved after accounting for growth, erosion and other natural processes, which then provides a more realistic measure of whether reefs can keep pace with future sea-level rise.”
The researchers found that the type of coral community present also affects how quickly reefs can build.

The researchers said reducing local pressures, including pollution, overfishing and physical damage, could help reefs maintain healthy coral communities and continue producing reef material.

However, local conservation and reef restoration cannot fully compensate if sea levels rise faster than reefs can grow.