Greenhouse Gases, Carbon Sequestration and Carbon Storage in Mangroves

Project developers must understand the carbon cycle to quantify emissions reductions and properly design their project. A well-designed project has the potential to generate more and higher quality credits.

Greenhouse Gases and Carbon Flux

Greenhouse gases (GHGs) trap heat in the Earth's atmosphere and contribute to climate change. The most important GHGs are carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O). These gases are released into the atmosphere naturally and from human activities and can remain there for anywhere from several years to centuries.

GHGs are captured and stored in carbon pools, which include plants, soils, the atmosphere, and the ocean. GHGs move in and out of carbon pools naturally through the cycles of respiration, photosynthesis, growth, decay, burning, and more—this is called carbon exchange or carbon flux.

Carbon Sequestration vs Carbon Storage

Carbon sequestration is the process of removing carbon dioxide from the atmosphere. When carbon is sequestered and held in place for a long time, this results in long-term carbon storage. Generally, carbon becomes stored when it is converted into a form not easily released back into the atmosphere, such as when it is incorporated into a plant’s structure or becomes mineralised (turned into stone).

Plants and algae sequester carbon dioxide during photosynthesis; this captured carbon may then be turned into energy (used by the plant or algae) or stored in its biomass, which is the organic matter of a plant, animal, or algae.

In the short-term, biomass may be broken down by microorganisms and decay, releasing carbon back into the atmosphere. Alternatively, another organism may consume the biomass, turning it into energy or incorporating it into its tissues. Biomass may also become buried and trapped in soil, peat, or deep in the ocean. Trapped carbon will remain there until natural processes or human activities disturb it—this is carbon storage.

Figure 1. A Simplified View of the Coastal Carbon Cycle

Carbon dioxide or other GHGs (such as methane or nitrous oxide) are released into the atmosphere through carbon emissions, which occur with the decomposition of biomass, respiration of animals or microbes, fires, and soil disturbance. Emissions are natural in all ecosystems, but habitat type and site-specific conditions determine how much the ecosystem emits. However, highly disturbed, degraded, or exploited sites usually emit far more carbon than they sequester.

Mangroves as a Carbon Sink

At the same time, mangrove soils are mainly anoxic, meaning they lack oxygen. Anoxic soils harbour soil microorganisms called anaerobic microorganisms that naturally release GHGs like methane.

The carbon storage in soils and biomass balances the natural release of GHGs. Intact and well-functioning mangrove ecosystems are effective carbon sinks. In contrast, degraded or damaged mangrove ecosystems can become a carbon source, emitting more carbon than they capture. Human activities such as deforestation or coastal development are often responsible for mangrove loss and degradation.

Mangrove ecosystems contain several distinct carbon pools, including:

Aboveground non-woody biomass: The plant matter of living non-woody plants such as ferns, reeds, and grasses above the ground.
Aboveground woody biomass: The plant matter of living woody plants, including the trunks, branches and twigs of mangroves and other trees and shrubs.
Belowground biomass: The matter of the living plant’s roots.
Litter: The dead plant material that falls to the ground and collects on top of the soil, such as leaves.
Dead wood: The dead and decaying woody material of mangrove trees and other woody plants.
Soil organic carbon: The carbon stored in trapped organic matter and microorganisms in the soil.
Wood products: The use of mangrove wood for various purposes, such as fuelwood, charcoal, and construction materials.

Mangrove Greenhouse Gases

The GHGs in mangrove carbon pools mainly consist of the following:

Carbon dioxide (CO₂): A gas that naturally occurs in the atmosphere and is also produced through human activities such as burning fossil fuels and deforestation. In mangroves, the respiration of microorganisms and decomposition of organic materials naturally release CO2.
Methane (CH₄): A gas produced through natural processes, such as the decomposition of organic matter in anoxic soils and human activities, such as fossil fuel production or livestock farming.
Nitrous oxide (N₂O): A gas produced through natural processes, such as by bacteria in soils and oceans, and human activities, such as agricultural practices and fossil fuel use.

Each GHG has a different warming impact on the atmosphere, called its global warming potential (GWP). Carbon dioxide is the reference point to calculate the GWP of other gases. In other words, carbon dioxide has a warming potential of 1 over 100 years. Comparatively, nitrous oxide has a GWP of 310 over 100 years, meaning it is 310 times more powerful than carbon dioxide at trapping heat in the atmosphere. Methane has an average GWP of 21 over 100 years, but a GWP of 80-86 over the first twenty years after its release.

The interactions between these gases in mangroves are complex and influenced by ecosystem conditions and human activities. Disturbances to the natural exchange of gases can result in the ecosystem becoming a carbon source.

There are seven primary emissions sources in mangroves:

Nitrogen fertilisers (N₂O): Agricultural lands or aquaculture ponds often use nitrogen fertilisers that can leak into the mangroves, causing the microbes in the soil to increase their N₂O emissions.
Nitrogen-fixing species (N₂O): Some species of mangroves, such as the black mangrove (Avicennia germinans) and red mangrove (Rhizophora mangle), can fix atmospheric nitrogen. However, this process also leads to the production of N₂
Biomass burning (CH₄/CO₂): Fires from natural causes or set by humans release large amounts of CH₄ and CO₂ into the atmosphere.
Fossil fuel use (CO₂): Human activities in and around mangrove ecosystems, such as transportation, electricity generation, fishing, and industrial development, can burn fossil fuels, which release CO₂ into the atmosphere.
Enteric fermentation (CH₄): Enteric fermentation refers to the natural digestive process of livestock such as cattle, sheep, and goats that produces methane. Mangrove communities can support large populations of domestic livestock which produce CH₄ through enteric fermentation.
Manure deposition (CH₄, N₂O): If there are livestock in mangroves, they produce manure which emits both methane (CH4) and nitrous oxide (N₂O).
Soil methanogenesis (CH₄): Mangrove soils are often anoxic. The anaerobic microbes in the soil produce CH4 through a process called methanogenesis.

Carbon Flux, Sequestration and Storage Throughout a Mangrove Project

Blue carbon refers to carbon stored in coastal ecosystems, such as mangroves, saltmarshes and seagrass. Projects that protect or restore blue carbon ecosystems may earn funding for either avoiding the release of emissions or removing carbon from the atmosphere. This is referred to as a carbon project. Carbon projects will be discussed in the article Introduction to Carbon Offsetting.

Carbon sequestration and carbon storage rates can shift throughout a project's lifetime.

Carbon Sequestration Rates

Young mangrove seedlings and saplings grow slowly at the beginning of their life cycle. Therefore, young mangroves sequester carbon at a low rate because they build biomass slowly. As a mangrove forest matures, the mangroves grow faster, developing biomass and sequestering carbon more quickly. As mangroves age, their growth rate slows down, so old mangroves sequester carbon at a slower rate.

Carbon Storage Rates

Mangrove seedlings and saplings do not store much carbon in their biomass because they don’t have much biomass. As mangrove trees grow, they become larger and can store more carbon in their biomass and trapped sediments within their developing root systems. Old mangrove forests that have had time to grow large and trap many sediments in their roots store the most carbon. This is also true of tidal wetlands without mangroves or trees/shrubs, where older wetlands have had time to accrete (or collect) sediments and organic matter and therefore store more carbon than young wetlands.

The large amount of carbon stored in old mangroves and wetlands highlights the importance of protecting ecosystems before they are degraded. While re-growing a mangrove sequesters and stores carbon, young mangroves take many decades to mature and to hold as much carbon as old-growth forests.

Figure 2. Carbon Dynamics in Mangroves Over Time. Carbon stored in mangroves increases over time as the trees accumulate biomass, but older trees do not sequester carbon as quickly as younger trees.

Carbon Capture and Different Project Types

There are two main project types: conservation and restoration. Conservation projects ensure the long-term protection of an intact and healthy ecosystem. Conversely, restoration projects implement activities to return the degraded ecosystem to a healthy one. See our Introduction to Carbon Offsetting for information about different carbon project types.

Projects that sequester carbon (i.e., Restoration and Improved Management projects) are issued credits based on the net carbon sequestration during the project compared to the net carbon sequestration before project implementation.

The amount of carbon removed from a project site depends on the types of implemented activities, the location and site conditions. For example, mangrove restoration projects may remove more carbon per hectare than terrestrial forest restoration projects, as mangroves tend to have more carbon-rich soils. In addition, restoration projects that plant a greater variety of species sequester and store carbon more reliably than projects planting only one species, because diverse plant species increase soil and soil microbial health.

The project location also affects the carbon storage potential, as it determines the soil type and climate. Warmer, wetter climates promote more plant growth (also called primary productivity) and microbial activity, increasing the growth rate of the forest biomass and carbon storage. At the same time, warm and wet climates have a higher decomposition rate and rate of emissions. In contrast, drier, colder climates have a lower forest growth rate and a lower rate of decomposition.

Ideally, stored carbon remains where it is for a long time. Carbon markets refer to this concept as permanence. High-quality carbon projects store carbon for 100 years or longer. Projects must monitor emissions sources and carbon pools for the entire project lifetime to ensure permanent carbon removals.

The Movement of Carbon in Coastal Ecosystems

Carbon stock is the absolute quantity of carbon held in a carbon pool at any specified time. Carbon flux is the movement of carbon between the land, oceans, atmosphere, and living things.

Monitoring carbon flux and carbon stocks is more complex in blue carbon ecosystems than in terrestrial ecosystems. The science behind blue carbon flux is also less understood and established than forest carbon. The ocean tides, currents, and wetland hydrology constantly move sediments and organic matter in and out of a project area. These conditions make tracking carbon attributed to the project activities challenging.

For example, some blue carbon scientists study kelp forests as a significant global carbon sink and an opportunity for future carbon projects. A hypothetical kelp carbon project could grow kelp, which traps carbon dioxide in biomass through photosynthesis. If the kelp is left unmonitored and unmanaged, it could wash up on shore, decaying and releasing its carbon back into the atmosphere. Similarly, if an ocean organism consumes the kelp, that organism may either die, decompose and become ocean sediment or be eaten by another organism. In both cases, the carbon storage is not permanent. Some supporters of kelp carbon projects suggest harvesting kelp and sinking it deep into the ocean. The question remains whether the kelp will remain at the bottom of the ocean.

For this reason, many scientists consider kelp carbon an emerging solution, whereas mangrove and wetland carbon science is more established. We know that mangroves and wetlands readily trap sediment and organic matter (especially in peat) and, if managed well, will store carbon for a long time.

Register for our free mangrove modules for more information on carbon sequestration and GHGs in carbon projects.

Suggested Citation: Francis, E., Wilkman, A. "Greenhouse Gases, Carbon Sequestration and Carbon Storage in Mangroves." Geneva, Switzerland: Fair Carbon, 2023. https://faircarbon.org/content/fc/ghgsandcarbonstorage

Photo Credits:

Photo by David Clode on Unsplash