4. Solving for Legacy CO2: Blue Carbon

Mangrove Forest

Coastal and near shore ocean ecosystems are some of the richest habitats on the planet, with a multitude of plant and animal species thriving at the intersection of shallow waters for aquatic plants to grow, the accumulation of nutrient effluent from rivers and influxes from the abundant life of the ocean.

By restoring and expanding these habitats, whether mangroves, salt marshes or sea grasses, or farming seaweed and shellfish, these sources of “Blue Carbon” sequestration are another effective way to drawdown atmospheric CO2.

Intertidal ecosystems make up only a small share of total land area globally, yet have high carbon storage density, in many cases greater than tropical forests. They also store most of their carbon in underwater soil sediments, which are more secure than as living biomass, as the chart below illustrates:

Credit: IUCN

Or did, until construction, agriculture, aquaculture, water pollution, dams and climate impacts began to harm them, causing significant destruction or degradation:

• More than 35% of the world’s mangroves have been lost

• Sea grasses have declined by at least 30% globally

• U.S. alone has lost 70% of of its salt marshes along the East Coast

The Blue Carbon Initiative estimates 1 billion tons of CO2 are emitted annually as a consequence of harm to intertidal ecosystems, equivalent to about 20% of total CO2 emissions from tropical deforestation, or all of the United Kingdom’s emissions from fossil fuels.

Credit: The Blue Carbon Initiative

A report by UN agencies found that up to 7% of the CO2 reduction necessary to keep our greenhouse gas levels below 450 ppm could be achieved by protecting and restoring these three key Blue Carbon habitats. 

While the aim of this series is to reach 350 ppm CO2, the 7% figure above illustrates the valuable, albeit supplemental, role that intertidal ecosystems can play as part of a broader, diversified drawdown portfolio. Once we understand the multitude of beneficial services provided by intertidal systems besides CO2 (which are often great enough, purely in economic terms, to justify protection and restoration), the case for including intertidal Blue Carbon becomes clear.

Like planting trees and restoring soils, blue carbon methods also serve a number of other beneficial functions:

• Providing protective habitat for young sea life

• Mitigating the impact of storms, waves and floods

• Cleaning and filtering to improve water quality

To realize their carbon sequestration and ecosystem services benefits, we need to both protect existing habitat from further degradation, and work to restore them where they have been lost or damaged.

Marsh Habitat

Protection requires laws, zoning and enforcement, along with community buy-in, to prevent harmful development and resource extraction. Though restoration differs somewhat for each of these habitats, the basic approach is to:

First, make environmental conditions more favorable to that habitat, by improving water quality through reduced pollution, and by returning natural levels of water flow from freshwater sources upstream and coming in from the sea. Better management of runoff from agriculture, urban stormwater and septic systems is key to water quality, while removing manmade barriers such as dams, dikes and tide gates replenishes nutrient, sediment and salinity to favorable levels for intertidal species.

Second, improving environmental conditions can begin the process of restoration on its own in some cases, but working to manually reestablish the chosen habitat will greatly improve the odds of long term success. This can involve scattering seeds or physically planting young plants of appropriate species, as well as removing invasive species to make way.

Seagrass

For governing bodies and local communities, taking these steps can actually be about the bottom line: the combined financial value of flood and storm protection, enhanced wild fish stocks, opportunities for ecotourism and carbon sequestration credits will often be greater than short term gains from development or resource extraction. For example, a landmark 1997 study estimated that wetlands globally provide $14 trillion annually in total economic value, equivalent to $23 trillion in current dollar value.

Out of three blue carbon intertidal systems covered here, mangroves hold the most promise, since they capture the most CO2, provide similar ecological functions and their restoration appears to be the most scalable.

As with planting trees, mangrove restoration can be also be accomplished using drones to seed mudflats from the air. In the Irrawaddy Delta of Burma, drones have already been employed to plant mangroves in mass, enhancing the manual efforts of the Burmese, who have already planted 2.7 million new mangrove trees.

Young mangroves growing after manual planting. Credit: Sustainability Times

Salt marshes and seagrasses, on the other hand, are slower and more labor intensive to restore:

Seagrasses can be also be seeded, but they grow deeper underwater, so they must either be manually planted or seeded from boats, with weights attached to ensure they settle at the bottom. Salt marsh restoration is more involved than simply shooting out seeds, requiring a mixture of fresh and salt water flows with minimal nitrogen content.

Both still warrant protection and restoration for both CO2 sequestration and ecosystem services, yet should be counted to a lesser degree than mangroves.

Intertidal habitat protection and restoration provides the floor for CO2 sequestration from Blue Carbon sources, yet there is a much higher ceiling of sequestration potential, if we begin farming seaweed on a massive scale.

Seaweed farmers going to work in China. Credit: National Geographic

Seaweed grows up to 30x faster than terrestrial species, requires far less inputs like fertilizer, don’t compete with conventional food crops, and support marine life by reducing ocean acidification and creating shelter habitat.

A 2012 study by the University of South Pacific explored the vast potential of seaweeds for CO2 sequestration: growing seaweed on 9% of the global ocean’s surface would sequester nearly 21 gigatons CO2, or about 56% of humanity’s global emissions in 2018 (37 gigatons CO2). Covering almost 1/10th of global oceans with seaweed would be a monumental effort, but this illustrates the seaweed’s huge potential to eat up atmospheric carbon.

Of all the seaweeds, giant kelp is king for total biomass production.

Found in the northeast Pacific from Baja Mexico up to Alaska, and in the sub-Antarctic oceans along Chile, New Zealand and Australia, Giant Kelp is one of the fastest growing sources of biomass on Earth, growing up to 2 feet per day, doubling its weight every 6 months and reaching a maximum height of 175 feet.

Giant Kelp forest. Credit: Oceana

One acre of Giant Kelp sequesters 31.6 tons of CO2 from the atmosphere. In nature, kelp already contributes significantly to carbon sequestration, as dead biomass falls to the ocean floor: about 7.75 gigatons get deposited in the deep ocean of Banks of the Bahamas.

If Giant Kelp were farmed on a large scale, it could be cut loose at full growth to sink under its own weight for deepwater storage, where it could stay for at least 1,000 years. This is precisely how Running Tide Technologies, a startup based out of Maine, is aiming to provide long term, scalable CO2 sequestration for corporations, who are increasingly looking to offset their climate impacts.

Yet simply growing and then dumping Giant Kelp and other seaweeds into the bottom of the sea may be shortchanging their full potential:

Seaweeds are nutrient rich, offering a valuable source of Omega-3 fatty acids, calcium, iron and other vitamins and minerals. Large seaweed plantings would de-acidify surrounding oceans, creating the optimum environment to cultivate shellfish like oysters and clams, the type of vertical ocean farm that groups like Green Wave are creating. Seaweed could also significantly reduce methane emissions from cattle - a potent greenhouse gas - by adding a small portion to their feed.

Alternatively, seaweeds can be utilized for carbon-neutral fuel or power. Paired with carbon capture, however, seaweed-as-fuel would become carbon negative while providing clean energy - a dual benefit that could play a pivotal role in returning Earth’s climate to 350 ppm. The potential for open ocean, carbon-negative energy production will be the focus of the 5th part of this series, Bioenergy Capture and Storage.