Blue carbon is a fairly recent term. It means carbon associated with coastal and marine ecosystems.  It has long had implications for climate change. Unfortunately, we’re rapidly losing these ecosystems.

Achieving net zero emissions of greenhouse gases does not require that we stop emitting carbon entirely. We can offset it by removing an equal amount of carbon from the atmosphere. Technology offers carbon capture and storage.

Nature also removes carbon from the atmosphere. Forests, grasslands, and bodies of water that remove and store carbon are known as carbon sinks. The UN Intergovernmental Panel on Climate Change recognized the importance of blue carbon only in 2013. Since then, it has become an important part of environmental protection and mitigation of climate change. The nations of the world with ocean coasts have started to devise action plans to promote understanding of blue carbon. In the U.S., both federal and state governments have taken steps.

The Waquoit Bay Research Reserve in Massachusetts developed a tool for marketing blue carbon projects as carbon offsets in coastal wetlands. Corporations can finance blue carbon projects through purchasing offsets. It’s better for them to reduce emissions, but offsetting can be a useful strategy while they do so.

The Reserve also found that restoring tidal flows to damaged marshes reduces methane emissions. Restoring tidal flows from a single river, the Herring River, can prevent 300,000 tons carbon dioxide emissions over 40 years.

The U.S. was the first nation to include blue carbon in its inventory of greenhouse gas emissions. It is based on 30 years of measurement data by the National Oceanic and Atmospheric Administration.

Carbon sinks vs carbon sources

Plants make sugars for their nourishment through photosynthesis, which requires sunlight, water, and carbon dioxide. Plants extract carbon dioxide from the atmosphere and return oxygen as a waste product. On the other hand, animals extract oxygen from the atmosphere and return carbon dioxide as a waste product.

Of course, the natural balance of carbon dioxide and oxygen depends on more than simply breathing. Leaving aside human activity for a while, some natural processes release more carbon dioxide than they absorb. They are called carbon sources and include fires and volcanic eruptions. Carbon sinks, on the other hand, absorb more carbon than they release.

On land, forests and grasslands serve as carbon sinks. Forests store carbon in the trees and other plants. Grasslands store carbon mainly in the grass roots and soil. When a tree dies and crashes to the ground, the process of rotting adds some of its carbon back into the atmosphere. Insects consume it, for example. As animals, they inhale oxygen and exhale carbon dioxide.

A forest fire turns the forest from a carbon sink to a carbon source by releasing all the carbon that had been stored in the trees. Likewise, an earthquake, tornado, or anything that disturbs the soil releases the carbon stored there.

Of course, human activity from burning anything to plowing the ground for agriculture or building projects turns carbon sinks into carbon sources more drastically than any natural processes. We must restore forests and grasslands and reduce emissions from plowing as part of any successful program to restore the natural balance.

What is blue carbon and why does it matter?

In fact, coastal and marine ecosystems store more carbon and store it more efficiently than land-based ecosystems. That means that disturbing them releases more carbon into the atmosphere than disturbing land ecosystems.

The open ocean has vast potential to store carbon. Anything that dies in the ocean falls to the bottom and decomposes. The top few feet of sediment at the bottom of the sea stores vast amounts of carbon It will stay there forever unless something such as dredging or drilling for oil releases it.

So far, however, we have fewer established strategies for managing the open ocean than coastal ecosystems. Coastal ecosystems comprise only a few hundred thousand square miles scattered all over the world, but they remove carbon ten times faster than terrestrial forests and grasslands and store four times as much.

Coastal ecosystems store carbon both in the plants and the sediment. Not only that, but the tide also captures organic matter from land the drags it back into the ocean, where it sinks into the sediment.

Although coastal wetlands occupy less than 2% of the ocean, their sediments contain about half of all the ocean’s store of carbon. Besides serving as a carbon sink, coastal wetlands protect shorelines from the worst ravages of severe storms. They slow the action of waves, absorb excess floodwaters, and reduce the acidification that threatens marine life. They also provide recreational opportunities for humans and support local economies.

Coastal wetlands are among the most threatened of ecosystems. Over the past 50 years or so, their total area has decreased by more than a third. Destruction of these wetlands’ releases about 450 million metric tons of the carbon dioxide that they would otherwise store indefinitely. That figure amounts to the carbon emissions of more than 97 million cars.

Blue carbon refers to several different coastal and marine ecosystems: mangrove forests, tidal marshes (also known as salt marshes), kelp forests, and seagrass meadows.

Mangrove forests

A mangrove is a tree or shrub that grows where the land and ocean meet. It thrives where other trees would die: in loose, wet soil where the water is salty and they are submerged by high tides.

Mangrove forests are a type of tropical forest and therefore not found in all coastal waters. Their rate of carbon sequestration is about two to four times greater than strictly land-based tropical forests.

They serve as buffers to protect against coastal flooding. They serve as habitat for numerous species of marine life and sequester more than a ton of carbon per acre every year.

Between 30-50% of the mangrove forests that existed 50 years ago have been lost, and losses continue at a rate of about 1-2% every year. Unsustainable coastal development, including cutting down mangrove forests to build aquaculture ponds, accounts for much of this deforestation.

Salt marshes

Unlike mangrove forests, salt marshes exist along every continent except Antarctica and grow from the tropics to sub-arctic areas—most extensively in temperate waters.

But like mangrove forests, they occupy habitats in zones between sea and land. They are underwater or exposed to the atmosphere depending on the tides.

Salt marshes can sequester almost a ton of carbon per acre at a rate of two to four times greater than tropical forests on land. They can absorb as much as a million and a half gallons of floodwater. More than 75% of the species important to U.S. fisheries depend on salt marshes for food, shelter, and raising their young.

The wet soil in tidal marshes is several meters deep. The soil, rather than the plants, holds almost all of their carbon.

These marshes maintain coastal water quality by filtering pollution that runs off from land. Excess pollution can kill them, however. Tidal marshes absorb some of the energy from hurricanes and other storms. Their loss, therefore, intensifies storm damage. Both tidal and freshwater marshes disappear at a rate of 1-2% annually.

Kelp forests and seagrass meadows

Kelp comprises three kinds of algae: green, red, and brown. In kelp forests, the green algae usually grow closer to the surface, with red algae below them and brown algae even lower. Sea grasses are plants rooted in the ocean sediments. Kelp forests occur only in temperate areas, but sea grasses grow anywhere except polar regions.

Both kelp forests and seagrass meadows occupy open water, farther from shore than mangrove forests or salt marshes.

That is, they are completely submerged, but like salt marshes, the sea grasses grow in soil several meters deep. They account for only 0.2% of worldwide ocean territory but sequester about 10% of oceanic carbon. Being the smallest of the coastal ecosystems, they are among the world’s most threatened. Major threats include dredging and other poor land use practices.

Sea grasses and kelp provide food, shelter, and nursery facilities for larger species of fish. Each acre of sea grasses sequesters more than half a ton of carbon per acre every year. While that figure doesn’t match the carbon sequestration of the salt marshes or mangrove forests, sea grasses store about 10% of all everything the oceans store.

Restoring coastal and marine ecosystems, like restoring forests and grasslands, improves nature’s ability to capture and store carbon. We still have to reduce greenhouse gas emissions, though. Blue carbon is not some kind of magic that will let us continue our wasteful ways. But it is one of a number of important tools for combatting climate change.

Sources:

About blue carbon / The Blue Carbon Initiative
Blue carbon fast facts / National Ocean Service
Blue carbon is a natural climate solution with big potential / Zayna Syed, Popular Science. April 17, 2023
Coastal ‘blue carbon’: an important tool for combating climate change / PEW Charitable Trusts. October 1, 2021