Carbon stored in mangroves and wetlands (known as blue carbon) is playing an increasingly prominent role in discussions about the world’s emissions budget. Yet many questions remain about how coastal environments store and release CO₂ and behave under climate change. A new study published in Nature Communications looks at the future of blue carbon, stressing the need for a better understanding of how coastal ecosystems can contribute to climate adaptation and mitigation.

Blue carbon (BC) refers to organic carbon that mangroves, tidal marshes, seagrass, seaweed and other coastal and marine ecosystems capture and store. Scientists and the international community are exploring its potential to tackle climate change. The merits of these ecosystems go beyond carbon sequestration, protecting coastlines and securing livelihoods for local communities. But for BC to have a say in shaping climate action, blue carbon science needs a more solid footing, the study points out.

“There have been moments where the science has been responding to management agendas, rather than playing a role in setting them,” says Peter Macreadie, the paper’s lead author and head of the Blue Carbon Lab at Deakin University. Aiming to map key gaps and controversies still clouding BC science, the researchers asked ten “fundamental” questions to the scientific community.

Across the globe, BC ecosystems vary greatly in their exposure to climate change, as responses collected from 50 experts show. Sea level rise has one of the biggest influences on coastal carbon storage: the more sea levels rise, the more wetlands adapt by building up more soil from mineral and organic matter. This allows it to capture more carbon.

So far there are mostly global models looking at BC vulnerability to sea level rise, while studies of local areas – where extreme weather can affect carbon stocks and coastal ecosystems’ health – are lagging behind.

Researchers need investigate how human activity disturbs carbon production and storage locally. Globally, land-use change on coasts – from agriculture, dams, expansion of cities and other human activities – produces an estimated 450 million tonnes of CO2. At the same time, oil spills, aquaculture or an overgrowth of algae can lower BC ecosystems’ ability to store carbon. However, scientists need to take a closer look at what’s happening at local levels. By protecting local BC ecosystems from disturbances, countries can avoid damaging emissions.

“Developing countries with BC resources have the opportunity to use them towards their Nationally Determined Contributions (NDCs),” says Daniel Murdiyarso, principal scientist at the Center for International Forestry Research (CIFOR) and co-author of the study. NDCs refer to countries’ pledges to cut their emissions and adapt to climate change. “In Indonesia, for example, BC could help reduce emissions by as much as 200 million tonnes of CO2 annually, the equivalent of 30 percent of its emissions from land,” Murdiyarso says.

While scientists know the rough distribution of mangroves globally, 75 percent of which is concentrated in just 15 countries, tidal marshes and seagrasses remain insufficiently documented. Researchers point out that they first have to find out how many BC ecosystems the world actually has. Also of critical importance is how fast they can degrade. For example, the rate at which seagrass is being lost has jumped seven-fold since the 1990s. Surveys, therefore, are critical to better understand these changes to ecosystem health.

Promoting the conservation of coastal ecosystems for climate mitigation still requires more evidence on how blue carbon helps reduce emissions. Studies still fall short of showing how CO₂ exchanges between water and air affect carbon sequestration. Or how waves might impact the build-up of organic carbon, triggering disagreements among scientists.

Mangroves absorb as much as 700 million tonnes of carbon per year and return into the atmosphere 525 million tonnes annually, estimates show. But there’s still a lack of data on other BC ecosystems and, equally important, how emissions of methane – with more warming potential than CO₂ – and nitrous oxides might tip the scale in the carbon budget.

Ultimately, putting a number on blue carbon’s ability to offset emissions will give coastal ecosystems a place on the international mitigation and adaptation agenda. Some countries are already developing climate change mitigation schemes focusing on blue carbon, providing economic incentives for their conservation. Avoiding degradation of mangroves, tidal marshes and seagrasses could help avoid emissions of about 1 million tonnes of CO₂ per year; but international schemes for reducing emissions from deforestation and forest degradation don’t currently reward protecting these ecosystems. The study claims that they have been left off the agenda as their carbon storage remains uncertain.

“Findings in blue carbon so far are crucial, but we need to narrow down uncertainties and we should not wait,” Murdiyarso says. “Countries that want to include blue carbon in their mitigation and adaptation plans under official commitments need to know fast how much blue carbon their coastal ecosystems can store.”