Field and lab methods (MRV)

The term ‘blue’ signifies that these habitats, their ecosystems and the nature of their carbon storage occur near and below the sea. By comparison, ‘green’ forests are typically referred to as terrestrial vegetation; thus, green carbon is the carbon contained in living vegetation and soil in forest ecosystems of the terrestrial realm. Blue carbon, however, is the carbon stored in coastal and marine ecosystems.

Blue forests, sometimes called blue carbon ecosystems, are unique and paramount in stabilising our planet's climate. While only covering about 4% of the total land area and 11% of oceans, they are among the planet’s most productive ecosystems.¹ They also protect our coastlines from flooding, erosion, storms, and winds. They provide shelter and food to wildlife and provide livelihoods for communities.

Mangroves, seagrasses, and saltmarshes capture and store staggering amounts of carbon dioxide in natural sinks – i.e., blue carbon. Protecting and restoring these blue carbon habitats, a nature-based climate solution is an affordable way to help avert dangerous climate breakdown while providing other benefits to people and the planet.

To go deep…

See more on green and blue carbon: Green and Blue Carbon: The role of habitats in mitigating carbon emissions.²

Mangroves are one of Earth's most carbon-dense habitats, capable of being ten times more effective at sequestering carbon dioxide on a per area basis per year than boreal, temperate, or tropical forests.³,⁴ This blue forest will be vital to achieving the large-scale carbon drawdown essential if we can limit global warming to 1.5 °C, as reported by the Intergovernmental Panel on Climate Change (IPCC). Despite mangrove forests' limited world extent, confined to the 20 °C isotherm with approximately 0.7 % of tropical forests,⁵ they are globally important carbon sinks because of their efficiency in carbon assimilation, including their below-ground storage. Additionally, they provide essential habitats for many species.⁶

Characteristics of mangroves

Mangroves are estimated to cover 147 000 km² globally. They grow in the coastal zone, between the high and low tide lines, and along the intertidal zone washed by rivers, also called estuaries. They form a plant community adapted to a volatile environment, evolving particular adaptations that enable them to live in salty, oxygen-poor soil. Mangroves are halophytes, meaning they are salt-tolerant plants that thrive in saline waters that are typically inhospitable for other woody plants. Mangroves have evolved to form a barrier that allows them to secrete salt through their pores and glands of leaves. Mangroves are also viviparous, meaning their seeds germinate while attached to the parent tree and can float.  Pneumatophores, a breathing root system, allowed the mangrove to adapt to anoxia or low oxygen levels of their surrounding soil, giving mangroves their unique and fascinating aerial roots. Evolutionarily, there are three independent regions of diversification of mangrove ecosystems: Southeast Asia, the Caribbean and Eastern Pacific, and the Indian Ocean region. Rhizophora and Avicennia are the dominant mangrove genera. As of 2020, there was an estimated 147,359 km² of mangrove forest globally – of which 51% occurred in the Asia Pacific region, with 29% in the Americas and 20% in Africa. ⁷

To go deep…

See more on the state of the world's mangroves ⁸

Loss of mangroves

Healthy mangroves are natural storm barriers that save lives and protect infrastructure against more extreme storms and rising seas, including sea winds of cyclones. They also support essential fisheries, including gastropods (snails and slugs) and crustaceans (crabs), sustaining the daily lives of tens of millions of coastal people. Yet we continue to deforest mangroves, now reduced to a rate between 0.2 to 0.4% a year due to mainly human-driven loss and naturally driven events such as erosion, flooding and cyclone exacerbated by climate change. ⁹ Carbon productivity is also a delicate balance with mangrove forests – when mangroves are degraded or deforested, their carbon sequestering abilities are inhibited, and this carbon is instead emitted into the atmosphere. Mangrove deforestation and other blue forest loss currently account for 3-19% of global emissions from deforestation.¹⁰ Global mangrove CO₂ emission is estimated to reach 34.1 Tg C per year (TgC: teragrams of carbon or 1012 grams of carbon)¹¹. Thus, mangrove ecosystems can go from a carbon sink into a carbon source.

To go deep…

See more on future carbon emissions from global mangrove forest loss¹²

See more on global mangrove deforestation and its interacting social-ecological drivers¹³

Seagrasses are marine flowering plants found in shallow waters from the tropics to the Arctic Circle, typically occurring in soft-bottomed marine coastal areas and estuaries. Seagrasses are one of the most widespread blue forests on earth, found in 159 countries on six continents and covering over 300,000 km². ¹⁴ Often forming extensive underwater meadows, seagrasses provide highly productive and biologically rich habitats for marine life. Seagrass harbours charismatic fauna such as dugongs, manatees, seahorses, and marine turtles. They can also protect coasts from erosion, curb pathogens in water, sequester and store carbon and contribute to food security by helping support healthy fish stocks. This also supports coastal livelihoods associated with fisheries and shell gleaners in the tropics.

Characteristics of seagrasses

For seagrasses to live underwater, they successfully evolved in the Tethys Sea of the late Cretaceous (70 to 100 million years ago) from a lineage of freshwater plants (Alismatales).¹⁵ To thrive in marine waters, seagrass had to acquire several adaptations. They developed an anchorage structure composed of rhizomes and roots, which connects individual shoots through a network of nodes to transport nutrients and encourage the formation of meadows that can withstand wave energy. They also developed a buoyancy mechanism, so seagrass leaves could stand vertically in the water column. Seagrasses also pollinate in water with viviparous fruits, absorbing nutrients from roots and leaves. Around 70 species of seagrasses occur globally, belonging to four plant families (Zosteraceae, Posidoniaceae, Hydrocharitaceae and Cymodoceaceae).¹⁶

To go deep…

See more on the global distribution of seagrass meadows¹⁷

Loss of seagrass

Seagrasses are robust nature-based solutions to climate change, and though they cover only 0.1% of the ocean floor,¹⁸ seagrasses store around 18% of oceanic carbon.¹⁹ Since the late nineteenth century, almost 30% of known seagrass area across the globe has been lost,²⁰ with degradation potentially reaching 7% per year.²¹ Seagrasses are impacted by various anthropogenic and climate stressors such as light reduction, nutrient and other types of pollution, siltation, physical impact, and erosion. The primary drivers of decline include urban, industrial and agricultural run-off, coastal development, dredging, unregulated fishing and boating activities, and climate change.

Saltmarshes are tidal wetlands of salt-tolerant grasses, herbs, and shrubs that flourish between land and open sea. They are found in bays and estuaries along tidal coastlines in parts of the world with low-lying land and a temperate climate. They occur in 99 countries worldwide,²² and their extent is estimated to be 90,800 km². ²³ Sediment bottom is often a prerequisite for saltmarshes settlement and growth, as they evolve from young marshes to old marshes, with nutrients also being carried by tidal currents through tidal channels. Like mangroves, saltmarsh species are halotolerant and have evolved to adapt to saline waters.

Characteristics of saltmarshes

When the marsh surface builds above the water level, high marsh species invade, outcompete and replace the low marsh plants. The most stress-tolerant plant species occupy the lower reaches of the marshes, while less specialised and stressful competitive species occupy the upper elevations. Fine sand and mud deposition raise the marsh to the highest tidal water levels. The marsh may then become dry land, nearly disconnected from the ocean. Saltmarshes are marshy because the soil often contains deep mud and peat. Peat is made of decomposing plant matter that is often several feet thick. Peat is waterlogged, root-filled, and very spongy. Because tides frequently submerge saltmarshes and contain large quantities of decomposing plant material, oxygen levels in the peat can be extremely low – a condition called hypoxia.²⁴ Hypoxia is caused by the growth of bacteria that produce the sulphurous rotten-egg smell often associated with marshes and mud flats.

Loss of saltmarshes

Historically, saltmarshes have faced significant threats through humans repurposing the nutrient-rich soil for agriculture or draining them for coastal development. While some saltmarsh species are highly effective at filtering nutrients from sewage, urban run-off, and agricultural and industrial wastes, not all are equally equipped to filter such high nutrient loads, thus leading to species competition and restructuring of marshes for where only the most nutrient-tolerant plants can thrive. Sea-level rise is another stressor for marshes as they will become more exposed to flooding from open water zones, causing a landward shift. One of the primary drivers of landward shift is rising sea levels. As sea levels increase, saltmarshes migrate towards higher elevations to maintain tidal flooding and ebbing regimes. This movement allows them to keep pace with the rising water levels and avoid being inundated. Around 50% of saltmarshes, have been lost over the last 20–50 years.²⁵

Despite saltmarshes' manifold ecosystem benefits, these ecosystems are often overlooked and understudied. Knowledge of their global spatial variation still needs to be completed. In Norway, for example, there was no official terminology for saltmarshes until 2020, which has implications for monitoring and protecting these ecosystems.

Kelp, or giant macroalgae /seaweeds, are often left out of blue carbon discussions because accounting for the long-term fate of the carbon they store can be very challenging. Also, kelps often grow in rocky coastal areas with minimal carbon-rich soil build-up. Kelp are one of the world's most extensive marine vegetated ecosystems,²⁶ found in approximately one-quarter of the world’s coastlines from polar to temperate regions. They form thick, three-dimensional forests that harbour diverse marine species, including invertebrates, fish, and marine mammals like elephant seals, sea otters, and sea lions. Kelp forests can also be considered blue forests due to their carbon storage capacity. However, the process in which kelp store carbon is very different from their blue forests counterparts since kelp do not have roots and sediment beneath them – instead, they have holdfasts that are used to attach to rocks and other hard substrates. While kelp can store carbon in standing biomass, most of the carbon that kelp sequesters is exported to other places as pieces of kelp break off and drift away to different vegetated coastal ecosystems, nearby sediments on the coastal shelf, and the deep ocean. Buried in deep seafloor sediments, kelp carbon can remain for thousands of years.

Characteristics of kelp

It is estimated that kelp likely originated roughly 100 million years ago.²⁷ Needing light from the sun to photosynthesise, most kelp forests are found from the water’s edge to 25 meters deep.²⁸ In addition to sunlight and carbon dioxide, kelp also require inorganic nutrients such as nitrate and phosphate. Growing with astonishing speed, up to an average of 5% per day,²⁹ kelp consumes plenty of carbon dioxide, revealing the hidden potential for blue carbon sequestration. Kelp evolved through convergent evolution, meaning that different groups have independently developed similar structures such as blades, stipes, and sporangia – a type of reproductive structure.

Loss of kelp

Kelp are facing a global decline in abundance of 1.8% per year ³⁰ Human activities like overfishing, pollution, and climate change are significant stressors for kelp forests. Other major stressors stem from reduced water quality through eutrophication, pollution, sedimentation and ocean darkening, partly due to freshwater runoff from agriculture and industry sources. Typically, kelp forests only survive in cold water, and rising water temperatures caused by climate change can prove impossible for kelp and other marine species to adapt. Kelp deforestation can also be attributed to the overfishing of top predators who eat sea urchins. When this food chain is disrupted, sea urchins can graze kelp forests until they are barren. Despite global trends of decline, there is a high regional variation in the status of kelp forests. In some temperate regions with less intense climate-related stressors, kelp populations have not shown a decline. In some cases, these stressors have even been favourable to certain species.

Test your knowledge:

1. Where and when did mangroves AND seagrasses evolve?

Blue carbon is the carbon captured by the world's ocean and coastal ecosystems. Blue carbon is a term coined in 2009 to draw attention to the degradation of marine and coastal ecosystems and the need to conserve and restore them to mitigate climate change and the other ecosystem services they provide.³¹ Mangroves, saltmarshes and seagrass are established Blue carbon ecosystems as they often have high carbon stocks, support long-term carbon storage, offer the potential to manage greenhouse gas emissions and support other adaptation policies across the globe. Some marine ecosystems do not meet critical criteria for inclusion within the Blue carbon framework (e.g., fish, bivalves and coral reefs). Others have gaps in scientific understanding of carbon stocks or greenhouse gas fluxes. There is currently limited potential for management or accounting for carbon sequestration (macroalgae and phytoplankton), but they may be considered blue carbon ecosystems in the future once these gaps are addressed

To go deep…

See more on the future of blue carbon science³²

Carbon sequestration is the capturing, removing and storing of carbon dioxide (CO₂) from the earth's atmosphere and its accumulation in both terrestrial and marine ecosystems. Carbon is the foundation of all life on Earth. Carbon helps regulate the Earth’s temperature, makes all life possible, is a crucial ingredient in the food that sustains us, and provides a significant energy source to fuel our global economy.

On carbon sequestration, oceans   are highly efficient absorbers of CO₂. They absorb an estimated 30% of emitted CO2 from the earth’s atmosphere.³³. This carbon is primarily held in the upper layers of the oceans. Carbon sequestration can prevent further emissions from contributing to the planet's heating. It can happen in two primary forms: biologically – in oceans, forests and soil – or geologically. Blue forests have an essential role in climate change mitigation by capturing and sequestering carbon from the atmosphere.

Blue forests are a small (in terms of extent) but powerful sink (or source if degraded or lost) of carbon as the soils and vegetation in these shallow coastal ecosystems collectively store between 10 and 24 billion metric tons of carbon.³⁴ A significant amount of the carbon buried in coastal sediments outside vegetated areas also comes from blue-carbon ecosystems, as currents carry organic matter farther out on the coastal shelf. So even without counting kelp and other macroalgae, blue carbon ecosystems are responsible for about half of the total carbon buried in coastal ocean areas yearly.

<strong>To go deep…</strong>

See more on carbon sequestration ³⁵

Blue carbon most commonly refers to the role that tidal marshes, mangroves and seagrasses can play in carbon sequestration. However, blue carbon also includes the carbon stored in the deep ocean waters where the vast majority of ocean carbon is held. Such ecosystems can contribute to climate change mitigation and also to ecosystem-based adaptation. When blue carbon ecosystems are degraded or lost, they release carbon back into the atmosphere.

Blue forests capture CO₂ from the atmosphere by sequestering the carbon in their underlying sediments, above- and below-ground biomass, and dead biomass. Although vegetated coastal ecosystems cover less area and have less aboveground biomass than terrestrial plants, they can potentially impact long-term carbon sequestration, particularly in sediment sinks. One of the main concerns with blue carbon is that the loss rate of these critical marine ecosystems is much higher than any other ecosystem on the planet, even compared to rainforests. Current estimates suggest a loss of 2-7% per year, which is not only lost carbon sequestration and emission of the stored carbon back to the atmosphere but also lost habitat that is important for managing climate, coastal protection, human health and food security.³⁶

Test your knowledge:

1. How much blue carbon is stored in shallow water ecosystems?
2. What are blue forests?
3. What is blue carbon?

While carbon sequestration is an important ecosystem benefit, there is now increased acknowledgement within the scientific community of blue forests' other benefits. Learn more about these blue forests ‘superpowers’ in Module 3.