To curb climate change, governments across the globe have set goals to achieve “net zero emissions.” This means that for every unit of greenhouse gases put into the atmosphere, the same amount is removed through a nature-based solution—like forest protection—or artificial ones like carbon capture technology.
Blue carbon is a nickname for the carbon dioxide absorbed from the atmosphere and stored in the ocean and coastal ecosystems. It’s a focus of the administration’s Ocean Climate Action Plan, announced in March.
It’s a type of carbon sink—natural or artificial reservoirs that absorb and store CO2, the heat-trapping gas primarily responsible for warming the planet. But scientists are still figuring out how people can support this process and warn that it’s not the solution to the climate crisis.
What is blue carbon?
First, you should know that blue carbon isn’t really blue.
“We just call it blue because we’re associating it with the ocean,” says Matthew Costa, a postdoctoral scholar researching blue carbon at the Scripps Institution of Oceanography in California.
Carbon dioxide is like food for plants, which suck the gas out of the atmosphere and use photosynthesis to convert it into plant matter. Plants in the ocean and on the coast do the same thing. Some of that plant matter, which stores carbon, gets trapped in sediment and can stay there for hundreds or even thousands of years. This process results in a carbon sink.
It’s an example of an ecosystem service, which is an aspect of a natural environment that benefits people. Other examples include forests that filter our air, wetlands that buffer against storms, and the plants we eat. “We put it in the service context in economics terms because it’s basically a service that the system is doing, but we don’t have to pay for it,” Costa says.
Salt marshes, mangroves, seagrass beds, and kelp forests are the ecosystems people generally refer to when discussing blue carbon in the United States. Mangroves are in southern Florida and some parts of Texas and Louisiana, while most algal beds are on the West Coast. Salt marshes are found on coastlines, while seagrasses are wherever there’s ocean water.
The top meter of sediment in the open ocean stores about double the amount of carbon stored on land, according to a 2020 study published in the journal Frontiers in Marine Science. Dead animals and plants, which hold carbon, sink and become buried in the seafloor. Phytoplankton—tiny single-cell organisms found throughout the ocean—play a significant role in this carbon burial.
But since the ocean is so vast, tracking how much carbon is stored is difficult. And more importantly, strategies for increasing carbon storage in the open ocean, like increasing phytoplankton growth, are less established and feasible than strategies for managing coastal ecosystems, according to Costa.
Why is blue carbon important?
While researchers stress that blue carbon won’t “solve” the climate crisis, it is one of many approaches governments can take to chip away at their net zero goals.
Coastal ecosystems around the globe make up only a few hundred thousand square kilometers, which is relatively small compared to the ocean. But they are particularly good at absorbing carbon. Carbon accumulates in mangroves, salt marshes, and seagrasses at a rate ten times faster than in terrestrial ecosystems. These areas also store about four times more carbon than terrestrial forests, according to Trisha Atwood, an associate professor of watershed sciences at Utah State University.
Costa says there are two reasons why these coastal ecosystems are more potent at storing carbon than forests, another major carbon sink. First, carbon builds up in the sediment, not just in the plants. Second, coastal ecosystems also import carbon from other environments. For example, when the tide rolls in and out in a tidal marsh, it carries particles of organic matter, which contain carbon. That organic matter also gets trapped in the sediment, storing even more carbon.
“When a giant tree falls to the forest floor, that trunk is sitting on the forest floor within a couple of years,” Costa says. Fungi, insects, and microorganisms quickly break down the wood and roots. Subsequently, the carbon transforms back into CO2.
“Those organisms are eating that material and breathing it out, just like when we eat food and breathe out CO2,” Costa adds. “So that carbon has a lower residence time, we’ll say it doesn’t get to spend as much time trapped in that ecosystem.”
Meanwhile, carbon tends to stay in coastal sediment once absorbed. The exception to this is when it’s disturbed by people.
“If you bulldoze that salt marsh or mangrove, or you disturb and dredge the sediment or something like that, you can then release a lot of that carbon,” Costa explains.
How much can blue carbon help?
Atwood stresses that restoring blue carbon ecosystems is different from replanting a forest, and we have to be careful not to over-promise what blue carbon can achieve.
“These systems are often in difficult-to-reach places, and seagrasses are submerged so they are not really visible,” she explained over email. “As a result, they can be hard to monitor, and we need a good way to ensure that restoration and protection efforts remain effective through time.”
However, if these coastal ecosystems are restored, they can do more than store carbon. Atwood says these natural spaces also reduce the impact of storms on coastal communities, act as nursery habitats for economically important fisheries species, and bring in tourism.
Ultimately, investing in blue carbon is just one of the many actions we must take to mitigate climate change, says Costa.
“This is not a sort of a silver bullet,” he says. “If we’re protecting these ecosystems and not reducing our emissions, we’re not going in a good direction.”