Which Sources of Blue Carbon Measure Up to the Mitigation Challenge?

New research shows that coastal habitats - mangrove forests, tidal marshes, and seagrass meadows - have the most potential amongst marine systems to act as long-term carbon sequestration solutions.

Natural climate mitigation: using nature to help buffer rising atmospheric carbon dioxide levels. It’s a hot topic amongst conservationists, and there’s no doubt that natural systems, like the oceans, will play a big role in helping the world’s nations avert more than 2 degrees Celsius of warming. But exactly which ocean systems have the greatest potential? Could phytoplankton be the key to climate mitigation? Or coral reefs?

According to new research published by Nature Conservancy scientists in Frontiers in Ecology & the Environment, countries and conservationists looking to implement carbon mitigation strategies using natural ecosystems should prioritize three coastal habitats — mangrove forests, tidal marshes, and seagrass meadows.

The Potential of Blue Carbon

The ocean represents the largest active carbon sink on Earth, absorbing 20 to 35 percent of all anthropogenic carbon-dioxide emissions. Coastal wetlands are well-recognized as important reservoirs of “blue carbon,” with some habitats sequestering up to four times as much carbon per equivalent area as terrestrial forests.

But recent discussions among scientists and policymakers have asked if other marine systems — like coral reefs, kelp forests, and even fish and other marine life — could also be key carbon sinks.

“Many people assume that all marine habitats sequester carbon equally, and that’s just not the case,” says Dr. Lizzie McLeod, a climate adaptation scientist with The Nature Conservancy and co-author on the research. McLeod and a team of international scientists synthesized the existing research to identify exactly which blue carbon solutions should be prioritized as natural climate solutions.

Their results, published recently in Frontiers in Ecology & the Environment, indicate that coastal wetlands — mangrove forests, marshes, and seagrass meadows — hold the greatest potential for sequestering blue carbon. Other marine systems either have limited sequestration potential, are actually low-level carbon sources, or present significant management challenges.

Red Mangrove (Rhizophora mangle) grows along the edge of Baie Liberte. Photo © The Nature Conservancy (Tim Calver)
Red Mangrove (Rhizophora mangle) grows along the edge of Baie Liberte. Photo © The Nature Conservancy (Tim Calver)

Coastal Habitats Hold the Key

Often underappreciated for the critical services that they provide, coastal wetlands have significant carbon sequestration potential. Mangroves cover an estimated 13.8 to 15.2 million hectares globally, and conservative estimates suggest they can store between 5.9 and 6.1 gigatons of carbon per year.

Most of that carbon dioxide is locked up in in the soil, and not in the plant’s biomass. McLeod explains that mangrove roots trap and store carbon in thick coastal sediments — which continue building over time if the forests have access to sufficient sediment — allowing mangroves and salt marshes to keep pace with moderate sea-level rise while continuing to store carbon. Seagrass meadows also sequester a majority of their carbon in sediments, protected from erosion by their dense fibrous root systems. These sediments are also anoxic, meaning that few bacteria are present to break them down and release trapped carbon dioxide.

“They’re really efficient at storing carbon,” says McLeod, “because of their high sediment accumulation rates, low oxygen, and slower microbial decomposition rates.” While terrestrial forests can sequester carbon for decades to centuries, carbon in coastal sediments can be sequestered for thousands of years.

For example, recent research on seagrasses in the Mediterranean revealed that one meadow of Posidonia oceanica may be up to 200,000 years old. Sediments from another meadow of the same species, which had accumulated over more than 6,000 years, had carbon deposits greater than 10 meters thick that contained massive carbon stores ranging from 40 to 770 kg C/m2.

Scientists estimate that mangrove forests, salt marshes, and seagrass meadows currently store 10.4 to 25.1 billion megagrams of carbon. But carbon mitigation is not the only benefit of these coastal habitats — they’re also critical for climate adaptation, too.

“Coastal wetlands buffer storm surges and save millions of dollars each year,” says Emily Landis, the Conservancy’s blue carbon strategy lead and co-author on the research. In addition to coastal protection, they also provide many benefits to people, including improving coastal water quality, providing food security, building materials, and tourism revenue.

But not every marine system has potential as a natural climate solution. According to the synthesis, systems like kelp forests, phytoplankton, coral reefs, and fish carbon have limited potential. Though kelp forests are dense stands of biomass and critical habitat for marine life, they cannot act as long-term carbon sinks. Kelp plants have a short lifecycle, limiting their storage abilities, and do not have extensive root systems to store carbon in marine soils.

Coral reefs actually release small amounts of carbon dioxide as they calcify, meaning that they’re small sources of atmospheric carbon dioxide. But reef conservation can still bolster blue carbon efforts by providing critical storm surge protection for coastal habitats.

Marine fauna — including teleost fish, bivalves, and krill — either don’t remove carbon directly or have such limited potential that they’re not suitable as large-scale natural climate solutions. And these systems are not bound within exclusive economic zones (EEZs), adding significant management challenges.

Phytoplankton provide about 70 percent of atmospheric oxygen, but their short lifecycle — often mere days — means that any carbon stored in their biomass for just hours to weeks. And any attempts to increase their sequestration potential require geoengineering that could alter entire food webs.

 Mad Island Marsh Preserve, Texas. Photo © The Nature Conservancy (Ian Shive)
Mad Island Marsh Preserve, Texas. Photo © The Nature Conservancy (Ian Shive)

Elevating the Value of Blue Carbon

Unfortunately, all three types of coastal habitats with high sequestration potential are also highly threatened. Both mangrove forests and saltmarshes are declining due to human conversion and degradation related to coastal development, agriculture, and aquaculture. Seagrass meadows are also threatened, typically by reduced water quality as a result of sediment and nutrient runoff from anthropogenic sources. And as these habitats are drained, bulldozed and burned, the carbon currently stored in them is released into the atmosphere … both destroying mitigation potential and adding to current emissions.

“This research is a key step in highlighting blue carbon’s role in mitigation,” says McLeod. “Now we will be working with countries with large tracts of these habits to develop supportive policies that take into account their sequestration value in addition to their other ecosystem benefits.”

Much of McLeod’s work with the Conservancy focuses on ensuring that benefits from coastal protection and restoration, especially climate adaptation benefits, are equitably distributed to local communities in the Pacific.

Led by Landis, the Conservancy is also incorporating blue carbon into its global coastal risk and resilience strategy, which combines research, policy reform and proof of concept in countries such as Indonesia, the United States, and Mexico.

Landis hopes that this new research will help elevate the role of coastal systems in international mitigation efforts, including under the Paris Agreement, and sharpen governments’ focus to the areas with the most potential. She says that, to date, only 29 out of 123 countries with mangrove forests have included blue carbon in their Nationally Determined contributions under the Paris Agreement.

“There was a bit of a marine free-for-all when climate mitigation and blue carbon became hit buzzwords,” says Landis. “But we need to use our resources to pursue the most scientifically sound and politically viable marine climate mitigation opportunities.”

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  1. Delon Marthinus says:

    Thanks a lot Justine, you put a lot of resources for my research..

  2. Wade Harrison says:

    Is there any new thinking on how best to enable the migration of these coastal systems inland as sea level rises?

    1. Nick Hinkell says:

      As a matter of fact, yes! This is a primary focus of an NGO in the San Francisco Bay Area called Save the Bay. They are in the process of restoring and investigating the best methods for tidal marsh transition zones between dry land and marsh land. They act as habitat for animals when storm surges bring the water higher and will eventually become tidal marsh as they become more consistently inundated by higher water levels. You can read all about it at SaveSFBay.org, but I imagine this science is being pursued by similar non-profits throughout the country/ world.