Carbon that’s captured and stored by the world's ocean and coastal ecosystems is known as blue carbon. Marine environments have absorbed about a third of human carbon emissions since the start of the Industrial Revolution. But as they are degraded or destroyed, they lose their ability to trap carbon and can release it into the atmosphere, accelerating climate change. Scientists are studying these processes in an effort to build the case for preservation.
Mangrove forests, salt marshes and seagrass meadows sequester carbon 10 times faster than mature tropical forests, but they are disappearing four times faster. Fast-growing coastal wetland plants take in CO2 during photosynthesis and deposit carbon-containing leaves, branches and roots in underwater sediment.
In this oxygen-poor environment, plant materials decompose slowly and can persist for thousands of years.
But development, overfishing and other pressures can transform coastal environments from sinks to sources of CO2 as well as methane, another potent greenhouse gas.
Seagrass roots accumulate vertically beneath the seafloor, creating a vast repository of buried carbon.
Though these habitats occupy only a tiny fraction of the ocean’s floor space, they store more than 10% of its buried organic carbon.
Clustered along tropical and subtropical coastlines, mangroves are among the most efficient carbon capture and storage systems on the planet.
Since the late 20th century these forests have disappeared at an alarming rate, but conservation and restoration efforts are now slowing the loss.
The ocean collects and stores vast amounts of carbon in seabed sediment, vegetation, marine animals and the seawater itself.
But warming waters and unsustainable exploitation can disrupt deep-water carbon cycles and release long-trapped gas back into the atmosphere.
Marine forests of kelp and other seaweeds can rapidly trap large amounts of carbon.
But these underwater forests are disappearing due to warming waters and other stressors.
Seaweed aquaculture can sustainably produce food while drawing down atmospheric carbon and regenerating habitats.
Ocean waters store much more carbon than the atmosphere and all the world’s plants and animals. Most of this dissolved gas is found at depths below 3,300 feet, where it remains locked away from the atmosphere.
Unfortunately, the more water warms, the less CO2 it is able to capture. If temperatures climb too far, seawater will begin to discharge its CO2 back into the air.
Small, abundant fish living in the mesopelagic zone at depths of 650 to 3,300 feet eat and store the carbon in smaller organisms and help cycle it throughout the oceans. Deep-water fish then carry carbon into the deep ocean for long-term storage. The collapse of fish populations due to overfishing is disrupting this deep-water carbon cycle.
Via the food web and other mechanisms, some carbon eventually makes its way to the bottom of the ocean, where it can remain locked up for millennia. Bottom-trawled fishing gear disturbs and releases these ancient blue carbon stores.
To learn more about blue carbon, check out EDF’s interactive blue carbon explainer.