Coastal wetlands will survive rising seas to act as a buffer and offset carbon dioxide emissions, shows analysis
In the US, coastal wetlands can offset yearly carbon dioxide emissions from 800,000 cars. And even as sea level rises with climate change, these coastal wetlands will prevail and continue to retain their power as carbon sinks, according to researchers from the Marine Biological Laboratory (MBL) in Massachusetts.
While coastal wetlands provide stunning views and are hosts to vast biodiversity, they provide another service to the warming Earth by capturing carbon from the atmosphere and sequestering it in their sediment at high rates. According to experts, coastal wetlands offer one of the best lines of defense against a hurricane's waves and surge. But scientists worry that these vital ecosystems could be at risk from sea-level rise. The question often asked is will coastal wetlands be overwhelmed by rising sea levels from climate change or will they remain carbon sink powerhouse?
"We found even though sea level will continue to rise at different rates [from location to location], sediment accrual rates in coastal wetlands will outpace sea-level rise," says MBL Ecosystems Center scientist Jianwu (Jim) Tang, senior author of the study.
The researchers found that more importantly, the carbon tucked away in coastal wetlands will stay locked away as new sediment deposits on top, allowing the marshland to act as a vault for excess carbon in the environment. This, says the team, reiterates the fact that wetlands are vital in the fight against climate change.
The researchers found that Lousiana had the most extensive tidal wetlands and the highest carbon accumulation amount among all the coastal states. “Our data indicated that annual carbon sequestrated by tidal wetlands in the conterminous US would remain consistent until at least to 2100 under Representative Concentration Pathway or RCP 2.6, RCP 4.5, and RCP 8.5 scenarios, even under the most-restricted accommodation space," says the team in their findings published in Nature Communications.
Coastal wetlands cover only about two percent of the ocean surface, but they are estimated to hide away more than 50% of the carbon captured by the ocean every year. According to experts, they fix carbon in their sediments at rates 10 to 100 times higher than forests. “Tidal wetlands, including tidal marshes and mangroves, contain long-term soil organic carbon of which continuously sequester atmospheric carbon dioxide at rates 10–100s times higher than terrestrial forests,” says the study.
The key to the study's findings, says the research team, is that salt marshes be allowed to accrue and build up sediment as they naturally would to remain effective as carbon sinks. However, humans have a track record of altering salt marshes in ways that reduce their effectiveness to offset climate change, explains the team.
“Excessive nitrogen pollution from agricultural runoff, coastal overdevelopment or the building dikes to block tidal flows, for example, hamper the ability of marshes to sequester carbon. And coastal overdevelopment also destroys these natural carbon sinks,” says the team.
The researchers explain what people and policymakers need to understand is that coastal wetlands are extremely valuable. Accordingly, conservation of wetlands is critical. "Yes, they are beautiful and attract fish and have a recreational purpose. But people don't realize that they have another, very important ecosystem value. Conservation and even the restoration of wetlands, in some cases, will help us to increase and protect their function as carbon sinks to the benefit of society and the warming planet,” says Tang.
Tang’s research is part of a global movement called the Blue Carbon Initiative. Scientists globally are working to mitigate climate change through the restoration and sustainable use of coastal and marine ecosystems.
A 2018 study had predicted that wetlands will survive rising seas to buffer the world's coastlines against future storms and provide their many other ecological and economic benefits, but only if humans preserve the room needed for the wetlands to migrate inland -- what scientists call "accommodation space."
The study, published in Nature, had suggested that large-scale coastal wetland loss could be avoided if sufficient additional space can be created by increasing the number of innovative 'nature-based adaptation' solutions to coastal management. The researchers had said whether coastal wetlands get bigger or smaller in the future depends on how much dry land is lost to sea-level rise, and how fast wetlands move into that submerged land.