Mangroves could store less carbon – and even begin releasing it – as sea levels rise, new research suggests.
Mangroves are made up of salt-tolerant plants that grow in coastal areas. They cover less than 1% of Earth's surface but store about 15% of all ocean carbon, most of it in their soils. This ability to store carbon makes them important in efforts to limit climate change.
Previous research has suggested rising seas could increase carbon storage in mangroves, but the new study challenges this.
The research team, led by the University of Exeter with partners in Colombia and the United States, developed a new model to assess how sea-level rise will affect carbon storage across entire mangrove forests.
The findings show that, while carbon storage might increase in localised spots as sea levels rise, storage at the scale of whole forests is likely to decline over the next 100 years.
"Mangrove forests are efficient carbon sinks and are therefore crucial for slowing climate change," said Dr Arya Iwantoro, who carried out the research at the University of Exeter and is now based at the University of Plymouth.
"Research about carbon storage in mangroves is usually based on field observations, and such studies have found that carbon storage can increase as sea levels rise.
"But this may not reveal the wider picture of what is happening across the forest as a whole.
"To investigate this, we developed a new model that links water flow and sediment transport, mangrove growth and dieback, and carbon storage while keeping track of changes in the composition of muddy beds where mangroves grow.
"In effect, we created three models in one to assess the way these complex ecosystems may respond to rising seas."
The results suggest that sea-level rise will initially trigger more carbon accumulation in some locations, but will reduce carbon storage in the forest as a whole.
"Mangrove plants are highly specialised, and they require a certain duration of flooding with each tide," said Luisa Fernanda Gómez Vargas, also from the University of Exeter.
"If this period is exceeded, a location will no longer be suitable – the plants will 'drown' and mangroves will die back.
"Mortality and erosion of carbon-rich soils can turn mangroves from a carbon sink (storing carbon) into a source (releasing it)."
The study assessed the impact of different sea-level rise scenarios from the Intergovernmental Panel on Climate Change (IPCC), and found that greater sea-level rise leads to stronger negative impacts on mangrove carbon storage.
Dr Barend van Maanen, who leads the mangrove and carbon project at Exeter, said: "Mangroves face an uncertain future due to climate change and other human impacts on rivers and coasts.
"As well as being vital carbon stores, mangroves protect coasts from storms, provide livelihoods to coastal communities and habitats for a wide range of species.
"Our findings emphasise that understanding the coastal landscape as a whole is crucial when predicting how mangroves might respond to climate change, and how we can protect them."
The study was funded by the Natural Environment Research Council.
The paper, published in the journal Earth's Future, is entitled: "The importance of scale in the future of mangrove blue carbon under sea-level rise."
