By Lynsey Atkinson, July 2020
Blue Carbon is about mitigating climate change through coastal ecosystem management.
Blue carbon refers to the carbon captured by the world’s ocean and coastal systems. These environments utilize the natural carbon sinks within seagrass meadow, mangrove, kelp forest, and salt marsh ecosystems to sequester, store, and release carbon over thousands of years. Blue carbon habitats are even more efficient at carbon capture than terrestrial forests and due to this, these environments are becoming key areas in the search for natural and artificial carbon sinks to aid in the reduction of atmospheric carbon dioxide, and can be found around the world.
Figure 1. Known distribution of blue carbon habitats including kelp forests (green), mangroves (black), salt marsh (blue) and seagrass meadows (yellow). Graphic from Science Nordic.
Blue Carbon and the Carbon Cycle
The ocean and the coasts are important carbon sources and sinks in the global carbon cycle. While the ocean and the coastal carbon cycles are closely related, they each have their specific components that provide unique services to each habitat.
Ocean Carbon Cycle
The ocean carbon cycle is a foundational component of the global carbon cycle as it has the ability to capture massive amounts of carbon and store it for thousands of years; it is estimated that 70-80% of the CO2 expected to be released by human activities will eventually be absorbed by the ocean, with 30% already absorbed by the ocean to date.
The ocean carbon cycle has two complex components, a fast and a slow cycle. The fast carbon cycle involves daily carbon exchanges between the surface waters and the atmosphere where CO2 and other gases are absorbed into and emitted out of the ocean constantly. The slower component stores carbon and CO2 for thousands of years through the biological and physical carbon pumps of the ocean. The biological pump (referred to simply as the bio pump), transfers carbon deep into the ocean through the food web and by precipitating carbon-containing marine debris (or organic matter) to the sea floor. Blue carbon ecosystems, such as kelp forests and salt marshes, aid the bio pump by sequestering carbon in plants through photosynthesis and embedding carbon in soils and root systems. The physical pump is driven by ocean circulation; as carbon is dissolved deep into the ocean, it takes thousands of years for it to be recirculated back up to the surface. The slow cycle will return carbon to the atmosphere through volcanic activity, crustal plates, and surface ocean-atmosphere exchanges.
Coastal Carbon Cycle
The coast is also an important part of the global carbon cycle, particularly in connecting its terrestrial and oceanic components. The coastal carbon cycle is largely fed by river influxes, air-sea gas exchanges, ocean upwelling, primary production and respiration of oceanic organisms. Just like the oceanic carbon cycle, the coastal carbon cycle involves the physical pump which exchanges carbon and other gases between the atmosphere and surface waters and exports carbon into the seabed and into the open ocean. Figure # illustrates the physical and bio pumps of the coastal carbon cycle in a simplified diagram. Most blue carbon ecosystems are involved in the coastal carbon cycle either by acting as sites of primary production and respiration or embedding carbon into the seabed.
The ecosystems involved in the coastal carbon cycle are heavily impacted by human activities often due to their proximity to population centers; impacts include climate change, ocean acidification, pollution, nutrient enrichment, overfishing, and habitat destruction. While all blue carbon ecosystems are impacted by climate change, each habitat has experienced its own set of unique impacts. Keep reading to learn about the different types of blue carbon ecosystems, current threats from human activities and opportunities to protect and enhance their carbon sequestration benefits.
Figure 2. A simplified description of the coastal carbon cycle. Graphic sourced from NOAA.
Carbon Cycles and Sequestration
Within these carbon cycles, there are different methods of carbon sequestration and storage, different timescales on which carbon is sequestered, and different sizes of carbon sinks in which the carbon is stored. Temporary and short term sequestration frequently occurs within biological sequestration systems, such as in above ground plant matter which regularly absorbs and emits carbon through biological processes that occur on short timescales, like photosynthesis and respiration. Long term sequestration can occur in all three types of sequestration (which includes biological, geological, and technological sequestration) and can store carbon for thousands of years, whether through biological processes that store carbon in root systems and soils, geological processes that store carbon in rocks, or technologically which artificially converts carbon into graphene.
Additionally, these sequestration processes and timescales are dependent on the size of the carbon sinks in which they are being absorbed into. For example, some coastal plants are generally smaller carbon sinks as they can only store carbon within their structures or absorb CO2 through photosynthesis; but, once they reach the end of the lifecycle and die, they re-emit all absorbed carbon back into the system. The ocean, on the other hand, is a massive carbon sink that can absorb and re-emit carbon on short timescales, but also store carbon in the deep ocean for thousands of years. Coastal plants, such as mangroves, also have the ability to fix carbon into the soil where their roots reside, which stores carbon for long periods. The storage potential of each carbon sink is important as it allows for the possibility of each sink to be utilized in the intentional sequestration of carbon, which will in turn (ideally) limit the atmospheric warming that we are experiencing today.
To learn more about the processes associated with carbon sinks, sources, sequestration, and storage, check out Beachapedia’s Carbon Sequestration article.
Blue Carbon Ecosystems
Blue carbon ecosystems can be found along the world’s coasts and ocean in habitats such as mangrove forests, seagrass meadows, and salt marshes. With the increased focus on reducing atmospheric CO2 to quell rising global temperatures, there has been a shift of focus towards the restoration and preservation of blue carbon ecosystems to utilize them as carbon dioxide sinks, in addition to providing a wide array of co-benefits such as marine life habitat. Furthermore, each ecosystem has its own contribution in the global, ocean, and coastal carbon cycle and has been impacted differently by climate change.
Carbon Benefits of Mangrove Forests
Mangrove forests are essential ecosystems of coastal environments and are some of the most carbon-rich forests on Earth. They act as storm buffers, protecting shorelines from damaging storms and surges, prevent coastal erosion by stabilizing sediment with their root systems, and maintain water quality by filtering run-off pollutants and sediments from land. Mangrove forests also fix, release, and sequester more carbon by area than all other coastal habitats (see Figure #); due to their complex root structures, high sedimentation rates, waterlogged soils (which are free of fire risk), and anoxic soils (soils deprived of oxygen), mangrove carbon burial, turnover, and release rates are significantly slower than terrestrial forests, as well. As such, mangroves are important sites for those looking to preserve coastlines and harness blue carbon as a carbon mitigation strategy.
Figure 3. Summary of estimated carbon sequestration benefits (ton of carbon dioxide equivalent per hectare of habitat) stored in soil organic carbon (brown) and in living biomass above ground (green) in each identified ecosystem. Graphic sourced from IUCN Blue Carbon.
It is estimated that mangrove ecosystems sequester and store about 3,767 tons of CO2 equivalent per hectare of mangrove habitat. Figure 3 describes the different carbon sequestration benefits of each blue carbon ecosystem. These estimates were completed by the IPCC 2013 Supplement to the 2006 Guidelines for National Greenhouse Gas Inventories: Wetlands project through carbon sampling of soils and biomass to construct an understanding of how the wetlands are contributing to carbon sequestration. Through this report, it was found that coastal ecosystems sequester and store more carbon than terrestrial forests, thus proving that blue carbon ecosystems are likely to be highly important in global carbon sequestration efforts. It is also important to note that these blue carbon systems store more carbon in the soil rather than in their biomass (as opposed to terrestrial forests), which is important, as carbon stored in soils is stored on longer timescales than carbon stored in biomass, as mentioned above.
Mangrove Ecosystem Threats
Unfortunately, mangrove deforestation and deterioration has been increasing recently due to aquaculture, urbanization, coastal landfill development, and indirect effects of pollution and upstream uses. Additionally, mangroves are heavily susceptible to sea level rise and the El Nino Southern Oscillation (ENSO) intensification; increasing sea levels, more intense storms, and changing sea temperatures (which are associated with ENSO and a changing climate) disrupt and weaken mangrove forest root systems, causing them to become unstable and wash away. The destruction of mangrove forests has significant impacts, not only in terms of coastal degradation, but due to the carbon that is released during their destruction. Over the last 50 years, mangrove forests have declined by 30-50% and these stressors are causing mangroves to release carbon, rather than sequester it; [mangrove deforestation accounts for 10% of all CO2 released from global deforestation, even though they only account for 0.7% of global tropical forest area. Therefore, as more mangrove forests are destroyed, they are turning from powerful carbon sinks to carbon sources.
Carbon Benefits of Seagrass Meadows
Seagrass meadows are submerged flowering plants that serve as highly productive and efficient water column filtration and sediment stability ecosystems and carbon sink hot spots. These meadows can be found along the shore of every continent except Antarctica. They are essential parts of the coastal ecosystem due to their low decomposition rates and anaerobic (oxygen-free) sediments which allow them to capture and store large amounts of carbon and carbon dioxide. Through the IUCN report mentioned above, it is estimated that seagrass meadows sequester about 511 tons of CO2 equivalent per hectare of seagrass habitat by fixing carbon directly into the seabed and by exchanging it with the water itself. In addition to the substantial habitat and sequestration benefits, seagrasses are also highly efficient at mitigating ocean acidification, reducing local acidity by up to 30 percent.
Seagrass Ecosystem Threats
Just as mangrove ecosystems are declining, seagrass meadows have been highly impacted due to climate change and disturbances caused by local human activities. Many efforts on coastal restoration have focused on artificially replanting meadows to improve coastal stability and carbon sequestration. Thus far, studies have shown that after 12 years, artificial meadows are just as efficient at carbon capture as naturally-occurring meadows; because seagrass meadows have a wider global distribution and can inhabit more area (as compared to mangroves, for example, that are largely restricted to tropical zones), artificial meadow planting could potentially help restore seagrass meadows at a large scale and increase their efficiency as carbon sinks. However, extreme events, which are becoming more frequent and (typically) more destructive with climate change, are diminishing recovery progress of seagrass meadows.
Carbon Benefits of Salt Marshes
Salt marshes are some of the most abundant, fertile, and accessible coastal habitats which provide more ecological services than any other coastal environment; these ecological services include sediment stability, carbon and nutrient fixation, and water filtration. Additionally, salt marshes are incredibly important due to their role at the interface between terrestrial and oceanic ecosystems; salt marshes serve to regulate the temperature, biotic and abiotic processes of river inflows by altering water throughput, flow path, dissolution rates, and carbon stocks. Salt marshes are also important carbon sinks, particularly at high latitudes (closer to the poles), where carbon release in salt marshes is slower and more efficient than peatlands in the same area; it is estimated that salt marshes sequester about 949 tons of CO2 equivalent per hectare of salt marsh habitat.
Salt Marsh Ecosystem Threats
With more than 40% of the world’s population residing on the world’s coast -- which is only 4% of the world’s land surface -- salt marshes have been severely impacted by local human activities and climate change. Thus far, this destruction has resulted in the loss of marshes at a rate of 1-2% each year. Currently salt marshes cover 140 million hectares of the Earth’s surface, but this is 50% of their historical coverage. In the US alone, salt marshes are being lost at a rate of about 9% per year; at this rate, scientists have predicted that all salt marshes in the US will be destroyed in 350 years, and while this may seem like a long time to us, geologically it’s a blink of an eye.
Due to the high rates of coastal degradation, policymakers and coastal residents have begun to focus on marshland restoration, attempting to reverse historical damages. Programs such as these focus on restoring the marshlands to “pre-impact or reference conditions” or restoring to “maximize a single ecosystem service”, such as sediment filtration or fish habitat stabilization; while these programs are typically single-service programs focused on restoration, they also provide secondary services, such as carbon storage.
Carbon Benefits of Kelp Forests
Kelps are large brown macroalgae that live in forests close to shore and provide food and shelter for thousands of fish, invertebrates, and marine species. Kelps, along with other macroalgae species, are the dominant primary producers along the coasts, but their habitats are not considered to accumulate large amounts of carbon. However, recent studies suggest that macroalgae could play an important role in the processes that sequester carbon in marine sediments and in the deep ocean. It is estimated that 43% of the carbon sequestered through photosynthesis in the ocean is exported to the algal bed, which is then transported to the deep sea through animal or microbe grazing; due to kelp’s short life cycle, they can quickly move this carbon from the surface to the deep ocean. Not only could this sequestration assist in reducing atmospheric carbon levels, it can help moderate ocean acidification by reducing the amount of carbon that is in the water. The role that kelp forests play in blue carbon sequestration has yet to be quantified and fully understood, but restoration and conservation efforts are increasingly focusing on maintaining these habitats due to the biodiversity, shelters, and food they provide and their potential as powerful carbon sinks.
Kelp Forest Ecosystem Threats
Due to their quick growth rates, kelps are considered to be quite resilient to adverse conditions that can impact their ecosystem stability. However, with increased instances of disturbances, which include kelp harvesting, grazing by fishes, plant competition, El Nino events, and pollution, their ecosystem health and resiliency are declining. Current estimates put global kelp decline at a rate of 0.018 acres per year; while this rate of change seems small, any habitat loss, particularly for a very resilient organism such as kelp, is alarming. Recent studies on kelp ecosystems are focusing on these natural and anthropogenic impacts in order to gain a better understanding on ecosystem degradation and how these impacts can be mitigated.
Carbon Benefits of Coral Reefs
Coral reefs are among the most diverse ecosystems in the world. Corals themselves are calcified structures that are inhabited by photosynthetic algae, known as zooxanthellae, and the reefs they form are home to hundreds of fish, algae, and microbe species -- in fact, about 25% of the ocean’s fish depend on coral reefs. Coral reefs are incredibly efficient systems and largely have a net zero carbon impact on their environment -- any CO2 released during calcification is typically sequestered by processes performed by the other organisms on the reef. Due to this, coral reefs are not yet considered carbon sinks. However, recent research has been investigating if coral reefs may take in more CO2 and carbon than previously thought; therefore, it is possible that in the future, coral reefs may be considered blue carbon sinks.
Coral Reef Ecosystem Threats
Recently, coral reefs have been exposed to numerous stressors which are contributing to global reef declines. Warming sea temperatures and increasing carbon dioxide levels in the water (which causes the water to become more acidic) has been contributing to massive coral bleaching events, which causes the coral to expel the zooxanthellae that gives them critical nutrients. Other factors, such as extreme storm events, increased sedimentation, pollution, and fish trawling have been impacting coral reef structures, as well. While coral reefs typically have an impressive resilience to destructive events, as these events happen more frequently, they are less likely to survive on a long-term basis. It is estimated that 50% of the world’s coral reefs have died in the last 30 years, and 90% are expected to die in the next century. As coral reefs begin to die off, their calcified structures release carbon into the ocean, which could cause dead reefs to become carbon sources. However, with more conservation efforts and more focus on mitigating climate change, many are optimistic about the prognosis of coral reefs as long as meaningful, substantial changes are on the immediate horizon.
Blue Carbon Restoration and Offsets Efforts
With severe threats facing the health and longevity of these valuable ecosystems, including potential loss of their important carbon benefits, blue carbon ecosystems are increasingly being considered as potential sites for carbon offset programs to help enhance their protection and restoration. Additionally, these programs help drive funding and attention from individuals, businesses and nations aiming to mitigate the impacts of climate change and warming global temperatures. Under current recommendations from the Intergovernmental Panel on Climate Change (IPCC), carbon offsets and sequestering programs need to be implemented at large and efficient scales in order to limit global warming; the IPCC’s recommendation insists that the decarbonization of our technological and energy sectors (in other words, efforts to substantially reduce the emission of greenhouse gases) needs to be matched by physically taking carbon out of the air. As such, carbon offset programs have been designed and implemented to assist in the reduction of atmospheric greenhouse gases through either a reduction of GHG emissions or an increase in sequestered carbon storage capacity to compensate for emissions elsewhere.
There are numerous organizations that focus their efforts on the restoration of blue carbon habitats in the hopes of increasing their potential as carbon sinks for carbon offsetting. The Blue Carbon Initiative and the Nordic Blue Carbon Project are international projects that aim to educate and provide opportunities for blue carbon restoration and sequestration projects. In 2014 the Blue Carbon Initiative released a manual Coastal Blue Carbon: methods for assessing carbon stocks and emissions factors in mangroves, tidal salt marshes, and seagrass meadows. By restoring and building up coastal ecosystems, these habitats will not only be preserved but their ecosystem services, such as the fisheries they support and their role in carbon sequestration, can be preserved as well.
Additionally, the Restore America’s Estuaries conservation group has seen significant success in their tidal wetland habitat restoration project in the Snohomish Estuary in the Puget sound. Their 2014 study, Coastal Blue Carbon Opportunity Assessment for Snohomish Estuary: The Climate Benefits of Estuary Restoration, found that the planned and in-progress restoration projects in the estuary would result in at least 2.55 million tons of CO2 to be sequestered from the atmosphere over the next century, which is the equivalent of 1 year of emissions for 500,000 passenger cars. This study has been used as the foundational study for other estuary projects to use towards restoring and boosting the carbon benefits of wetland habitats.
Presently, there are no blue carbon programs that specifically sell carbon offset credits like they do for terrestrial offset programs, such as the United Nations REDD Program; this program offers the sale of carbon credits to fund restoration and preservation programs. While no such programs exist for blue carbon ecosystems, the Blue Carbon Initiative and Nordic Blue Carbon Project are both attempting to be the first to pioneer blue carbon credits which will essentially trade the development, enhancement, and/or protection of the blue carbon ecosystem, which will sequester carbon from the atmosphere, in exchange for carbon emissions elsewhere.
Below is a list of current restoration projects and resources by specific ecosystem. At the moment, all of these programs, including the Blue Carbon Initiative and Nordic Blue Carbon Project, are focused on restoring the ecosystems through citizen-based action.
- Sea Trees - International
- Mangrove Action Project - United States
- Conservancy Mangrove Projects - Florida
- Marco Island Mangrove Restoration Project - Florida
- Khaled bin Sultan Mangrove Education & Restoration Project - Bahamas and Jamaica
- Kaimana Coastal Conservation - Indonesia
- Wildcoast - Baja California
- Eelgrass Restoration Project - Virginia
- URI Seagrass Restoration Methods
- NOAA Guidelines for Seagrass Restoration
- Florida Fish & Wildlife Seagrass Restoration Techniques
Salt and Tidal Marsh
- Elkhorn Slough - California (Monterey Bay)
- APCC Salt Marsh Restoration - Cape Cod, Massachusetts
- Neponset River - Massachusetts (Boston)
- Tomago Wetland Restoration - Australia
- NYC Parks Salt Marsh Restoration Design Guidelines
- Gulf of Maine Salt Marsh Project Planning
- URI Salt Marsh Restoration Methods
- Kelp Forest Restoration Project - Santa Monica, California
- Farallones Kelp Recovery - Australia
- Bull Kelp Restoration - Puget Sound