Over the past decade, scientists and policymakers have joined efforts to create a science-based framework under the auspices of the United Nations to protect our remaining tropical forests. These carbon-rich ecosystems help to moderate the climate and serve as a treasure trove of biodiversity and a resource for local and indigenous peoples. Governments across the tropics have begun to incorporate forest conservation into their climate and development plans. Now it is time to do the same with coastal wetlands.

Some 2.4–4.6% of the world’s carbon emissions are captured and sequestered by living organisms in the oceans, and the UN estimates that at least half of that sequestration takes place in ‘blue-carbon’ wetlands. Often occupied by seagrass and mangroves, these saltwater ecosystems promote healthy fisheries and sequester carbon in their soils. Mangroves also stave off erosion and serve as the first line of defence against powerful storms as well as saltwater intrusion into local groundwater resources. The world has lost more than one-third of its mangroves over the past several decades, and more succumb each year to shrimp farms, rice paddies and palm plantations, as well as to tourism and real-estate development. There’s money to be made, but it’s the environment that pays.

Nascent efforts are under way to halt this degradation, and a few pioneering projects have already shown success. Senegal is home to the world’s largest mangrove restoration project, which began in 2008. Villagers have planted around 79 million mangrove trees across more than 7,900 hectares. The project has been registered and certified under the Kyoto Protocol’s Clean Development Mechanism (CDM), and is benefiting from the sale of carbon credits.

In 2010, the United Nations Environment Programme launched the Blue Carbon Initiative, which seeks to reverse current trends and increase the area of coastal wetlands under effective management by 2025. The global climate agreement signed in Paris last December opens the door to advance such efforts, for example by enabling carbon trading and a programme similar to the CDM that allows countries and companies to pay to reduce emissions or build carbon stocks in projects such as the one in Senegal. It will be up to governments to incorporate coastal management into their climate plans, and to begin creating what some have called the ‘blue-green economy’.

The available evidence justifies the pursuit of these efforts. Mangrove ecosystems alone could store as much as 20 billion tonnes of carbon — equivalent to more than 2 years of global carbon emissions — in their soils, much of which would be released into the atmosphere if the trees were destroyed. A 2012 study suggested that mangrove conservation could be effective at a cost of just US$4–10 per tonne of carbon dioxide, which is within the current range of prices on the European carbon trading system (J. Siikamäki et al. Proc. Natl Acad. Sci. USA 109, 14369–14374; 2012).

“The planet that humanity calls home already knows how to sequester carbon.”

In some cases, mangrove protection and restoration could even benefit from the existing forest-carbon-trading framework, which enables developed countries to invest in efforts to reduce deforestation in the developing world. But more science is needed, both to document the extent and causes of the problem and to provide the data that will be needed if countries are to incorporate coastal wetlands into their carbon inventories and climate planning. We know too little about what happens to the carbon locked up in plants and soils when they are converted for other uses.

Just as occurred with remedying tropical deforestation, science and policy can move forward in parallel. As countries establish coastal management policies, they will help to drive the development of both science and policies. One opportunity is in the Dominican Republic, which has devised a comprehensive plan to reduce emissions by conserving and restoring mangrove forests. That project is registered with the UN, and it incorporates scientific objectives, including quantification of the carbon sequestration and storage capacity of these ecosystems. This will inform the policy framework and provide the scientific basis for any economic returns that the initiative may reap years and decades into the future.

Meeting the objectives of the Paris agreement — to contain global warming over the course of the twenty-first century — will require urgent action on all fronts. Countries must work to reduce industrial carbon emissions, but ensuring that natural ecosystems continue to function is equally vital — and relatively simple. The planet that humanity calls home already knows how to sequester carbon. Let’s make our forests and coastal wetlands work for us.