Mangroves ‘drowning’ in Maldives due to sea level rise

An extreme rise in sea levels driven by the Indian Ocean Dipole have caused some Maldivian islands to lose more than half of their mangrove cover since 2020, a new Scientific Reports study has found.

Lead author Lucy Carruthers of East Carolina University in the US says the findings are a warning for coastal areas worldwide.

“Our findings reveal the vulnerability of mangrove ecosystems to rapid sea-level rise and highlight the need for urgent adaptive conservation strategies in small island developing states,” says Carruthers.

“As climate change and extreme events intensify, some mangrove forests may struggle to keep up with sea level rise. The Maldives, as the world’s lowest-lying nation, can therefore potentially be the canary in the coal mine.”

Coastal regions around the world rely on mangrove forests to act as natural barriers against storms, erosion and flooding, as sources of biodiversity supporting food security and livelihoods, and as sources of energy and construction materials.

In the Maldives, a 900 km long archipelago in the central Indian Ocean, mangrove forests are found on at least 150 of its coral reef islands.

In 2020, about a quarter of these mangrove-containing islands in the Maldives experienced dieback, in which mangrove trees experienced gradual deterioration before dying.

Analysis of mangrove wood revealed that the dead trees showed greater signs of salinity stress compared to living trees. This indicated that the roots of the trees were struggling to cope with increased salt levels, which was a key factor in their eventual death.  

They found that sea levels around the Maldives rose at an accelerated rate of more than 30mm per year between 2017 and 2020. Towards the end of this period an unusually intense climate phenomenon known as the Indian Ocean Dipole (IOD) caused warmer sea surface temperatures and an increase in sea level in the Western Indian Ocean.

Mangroves growing on coral reef islands can naturally build up their own sediment to stay above water, allowing them to adapt to gradually adapt to rising seas. However, the rate of sea level rise vastly outpaced this process, leading to the mangroves losing their resilience and drowning.

Dr Vasile Ersek of Northumbria University in the UK says: “Dieback was first observed in the centre of low-lying basin areas before gradually spreading outwards.

“As these basin areas have something we call limited tidal flushing we saw evidence of the rising sea level inundating the forests with seawater. This prolonged exposure created higher concentrations of salt.

“As the mangroves’ build-up of sediment slowed down due to the pace of the rising sea level, the soil salinity increased beyond what even these salt-tolerant trees could handle. Essentially, the mangroves were drowning.

“The extreme magnitude of dieback seen in the Maldives is a vivid illustration of how climate change may push natural systems past their limits, with cascading consequences for both nature and people.”

Critically, they also absorb massive amounts of carbon dioxide from the atmosphere, and can store it for millennia. The researchers are concerned that the loss of mangrove forests could also release large amounts of stored carbon, further accelerating climate change.