Rising seas could drown mangroves and release vast stores of carbon

These salt-tolerant coastal forests occupy less than 1% of Earth's surface, yet they hold roughly 15% of all carbon stored in ocean ecosystems. Most of that carbon is trapped in the soils beneath the trees, making mangroves an important ally in efforts to slow climate change.

While some previous studies have indicated that rising sea levels could boost carbon storage in mangrove ecosystems, a new study paints a more complicated picture.

Rising Seas May Reduce Mangrove Carbon Storage

Researchers led by the University of Exeter, working with partners in Colombia and the United States, created a new computer model to examine how sea-level rise could affect carbon storage across entire mangrove forests.

Their findings suggest that although certain areas may temporarily accumulate more carbon as water levels increase, the overall capacity of mangrove forests to store carbon is likely to decline over the next century.

"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."

Why Mangroves Can "Drown"

The model showed that rising sea levels may initially increase carbon accumulation in some locations. However, as conditions continue to change, the broader forest could lose its ability to store carbon effectively.

"Mangrove plants are highly specialized, 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)."

As mangroves die and carbon-rich soils erode, carbon that had been locked away can be released back into the environment.

Greater Sea-Level Rise Means Greater Carbon Loss

The team evaluated several sea-level rise scenarios developed by the Intergovernmental Panel on Climate Change (IPCC). The analysis showed that higher levels of sea-level rise produced increasingly negative effects 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 emphasize 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."

Beyond storing carbon, mangroves play a critical role in shielding shorelines from storms, supporting wildlife, and helping sustain coastal communities. The researchers say their results highlight the importance of considering entire coastal systems when planning for climate change and protecting these valuable ecosystems.

The study was funded by the Natural Environment Research Council.

The findings were published in the journal Earth's Future in a paper titled "The importance of scale in the future of mangrove blue carbon under sea-level rise."