Mangrove forests have been protecting coastlines around the world against erosion and storm surges for millennia. But in 2020, the residents on many islands in the Maldives noticed that many of their mangrove forests were starting to die off. Where once these forests had been lush, now they were turning brown and lifeless.
Our team of scientists has worked closely with coastal communities in the Maldives to investigate that 2020 phenomenon. In our new study, we highlight how mangrove die-off events like this have big implications not just for the Maldives, but also for other island nations and coastal ecosystems around the world.
Mangroves are remarkable trees and shrubs that grow along tropical and subtropical coastlines of more than 120 countries and territories.
Mangroves are carbon powerhouses, storing three to five times more carbon per equivalent area than tropical rainforests. They’re biodiversity hotspots and crucial fish nurseries. For many coastal communities, especially in developing nations, mangroves are essential for food security and livelihoods by providing important protein sources like prawns, crabs and fish.
Mangroves act as natural storm barriers, protecting coastlines from erosion and flooding. For island nations like the Maldives, mangroves are a vital defence against rising seas and storms. So, when over 25% of the Maldives’ mangrove-containing islands died off in 2020, local residents became seriously concerned. Using satellite imagery, we found that some islands lost more than half their mangrove cover.
To investigate potential causes, we examined the wood from affected mangroves. The chemical makeup of mangrove wood can reveal if the trees were struggling with too much salt. Our tests showed that the dead mangroves had been under significant salt stress, meaning they were drowning in saltwater.
Mangroves can typically keep pace with gradually rising seas by building up their own sediment. But when sea levels rise too quickly, and mangroves experience salt stress, this defence mechanism can’t keep up.
Sure enough, our research points to the culprit as being rapidly rising sea levels, supercharged by climate change. The Maldives is the world’s lowest-lying country, with an average elevation of just 1.5m above sea level. This makes it acutely vulnerable to rising sea levels.
We found that from 2017 to 2020, sea levels around the Maldives rose at a rate of over 30mm per year – much faster than the mangroves could build up sediment to stay above water. This rapid rise was linked to a climate phenomenon called the Indian Ocean dipole, which was unusually intense between 2019 and 2020.
Discovered in the late 1990s, the Indian Ocean dipole is a climate pattern which causes changes in wind, sea surface temperature and rainfall across the Indian Ocean basin.
Read more: Dipole: the 'Indian Niño' that has brought devastating drought to East Africa
During a positive phase, like between 2019 and 2020, countries in the western Indian Ocean, including the Maldives, experience warmer sea surface temperatures and an increase in sea level as the ocean expands. Meanwhile, the eastern Indian Ocean experiences cooler waters and a drop in sea level.
During a negative phase, this pattern is reversed. Worryingly, some studies suggest that climate change could increase the frequency and severity of future Indian Ocean dipole events, although more research is needed to reduce the uncertainties of these projections.
Signalling a global threat
This is a warning for coastal areas worldwide. As climate change and extreme events intensify, some mangrove forests around the world may struggle to keep up with rising sea levels.
The consequences could be serious. Losing mangroves doesn’t just remove a vital coastal defence – it could release large amounts of stored carbon, further accelerating climate change.
Our study of these Maldives’ mangroves illustrates how climate change can push natural systems past their limits, with cascading negative consequences for both nature and people. Mangroves do go through phases of natural die-off, but our evidence suggests that this event was unusual.
What happened in the Maldives is a reminder that the impacts of climate change are not evenly distributed. Small island developing states, which have contributed less than 1% to global greenhouse gas emissions, are on the front lines of climate impacts.
Read more: Save our seabed – the bottom of the ocean needs to become a top priority, and the UN agrees
Natural recovery after a die-back is possible. We saw signs of this in the Maldives as seedlings and new trees were beginning to appear. Since 2020, a much more salt-tolerant mangrove species has become dominant here because it was able to handle the saltier conditions. While this regrowth is encouraging, there are some trade-offs as the shifts in species affect habitat structure, plant productivity and food security.
For example, people often eat the fruits of low salt-tolerant mangrove Bruguiera cylindrica, so the loss of this species has a negative impact on a staple food source. Future episodes of rapid sea level rise could also push even these more salt-tolerant species beyond their limit, potentially leading to more widespread and permanent mangrove loss.
As our planet continues to warm, events like the Maldives mangrove die-off may become more common. The fate of mangroves in the Maldives and other low-lying coastal areas will be a key indicator of how well countries are managing the climate crisis. Mangrove forests have thrived at the interface of land and sea for centuries. Whether they can survive the rapid changes of the coming decades will depend largely on our actions today.
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Vasile Ersek receives funding from Natural Environment Research Council.
Lucy Carruthers receives funding from Northumbria University PhD studentship and National Environmental Isotope Facility.
This article was originally published on The Conversation. Read the original article.
