The ability of vegetated coastal ecosystems – such as mangroves, salt marshes, and seagrass meadows ( Figure 1 ) – to capture and store large amounts of carbon is due to their high productivity, as well as to the saturated and saline conditions of their soils that slow down organic matter decomposition and limit the production of potent GHGs such as methane (CH₄) that freshwater wetland ecosystems typically generate. Mangroves are significant carbon sinks, with a biomass productivity (mass per unit area and time) comparable to that of tropical forests (Alongi, 2009), in addition to typically high soil carbon (SOC) sequestration rates (103 g SOC/m²/y) (Alongi, 2020). Salt marshes are efficient in accumulating carbon in the soil (212 g SOC/m²/y) (Alongi, 2018), with a highly variable biomass productivity depending on climate and dominant species (Duarte and Chiscano, 1999). Seagrass carbon stocks and coverage are less well known than that of mangroves and salt marshes; estimated differences between seagrass productivity (Duarte and Chiscano, 1999) and total carbon stocks (Alongi, 2018), suggest significant export of the biomass produced, potentially into shelf sediments and the deep sea ( Table 3 ).

Due to their ability to sequester carbon in biomass and especially in soils, the preservation, conservation, and restoration of these ecosystems has been recognized as an action to contribute to climate change mitigation with many co-benefits for adaptation (IPCC, 2014; Howard et al., 2017). However, net additional emissions reductions can only be achieved through avoiding coastal ecosystem degradation and destruction – as the annual emissions from their loss were estimated as equivalent to 3–19% of annual GHG emissions from global deforestation (Pendleton et al., 2012) – and enhancing or restoring their carbon sequestration capacity, which varies across regions. Climate change is also impacting the ecosystems themselves, as climate hazards such as sea level rise, increasing temperatures, and extreme weather events can alter productivity and change species composition – thereby altering the extent of coastal ecosystem carbon stocks.

In addition to carbon sequestration, coastal ecosystems also provide numerous adaptation services (Barbier et al., 2011; Nordlund et al., 2016) such as buffering severe coastal storm impacts, mitigating floods, protecting against accelerated sea level rise, and supporting coastal water quality and food security. Effectively managing coastal ecosystems as carbon sinks while preserving their ecological integrity requires robust estimates of ecosystem extent and conditions (Friess et al., 2019). Mangrove mapping has improved in recent years (Hamilton and Casey, 2016), but the extent of seagrass bed coverage remains uncertain (McKenzie et al., 2020). Despite their ecological and climate regulating value, coastal ecosystems continue to disappear at a fast pace, releasing stored carbon into the atmosphere. Barriers to proper preservation, conservation, and restoration, such as habitat unsuitability and lack of community engagement (Lee et al., 2019; Lovelock and Brown, 2019; Ellison et al., 2020), must be addressed for these activities to succeed.

Macroalgae comprise the most extensive and productive benthic (seafloor) vegetated marine habitats globally; widely distributed across coastal latitudes including kelp forests in cold, coastal waters and sargassum in tropical and temperate coastal areas, they are identified as potentially important carbon sinks (Krause-Jensen and Duarte, 2016; Krause-Jensen et al., 2018). Compared to rooted coastal vegetation, highly productive yet rootless macroalgal communities generate less on-site carbon burial and export more of their carbon away from the site of origination (Queirós et al., 2019; Filbee-Dexter and Wernberg, 2020; Hurd et al., 2022). Kelp and other macroalgae are gaining attention due to their highly recalcitrant (i.e. difficult to decompose) organic matter, and their ability to grow quickly and capture significant amounts of atmospheric carbon in their biomass ( Table 3 ).

The global average stocks of macroalgae are only recently being assessed (Krause-Jensen and Duarte, 2016; Krause-Jensen et al., 2018) and the extent to which they currently exist compared to their potential area with ecosystem restoration and management is not well known. Recent estimates in Australia reveal that thanks to their high productivity, macroalgae account for 10-23 MtC or 30% of national blue carbon stocks (Filbee-Dexter and Wernberg, 2020), demonstrating how significant this carbon pool can be. These systems are not yet adequately managed or protected by current policies as only a portion of macroalgal-carbon is sequestered and stored in ocean sediments through on site burial, thus complicating management (Laffoley, 2020). It is estimated that 80-90% of macroalgal carbon production leaves the original site and is incorporated into the marine food web and eventually buried at depth (Krumhansl and Scheibling, 2012; Krause-Jensen and Duarte, 2016). Furthermore, Ortega et al. (2019) studied vertical macroalgal carbon export and estimated that 69% of the macroalgae carbon at surface will sink below 1,000 m, close to permanent sequestration and storage depths, and 24% will reach deep seafloor sediments (>4000 m). Similarly, Krause-Jensen and Duarte (2016) estimated that 19% of the macroalgae carbon production will stay in the coastal ocean as POC, of which 4.6% will be buried in the coastal and shelf sediments. Lateral carbon export of macroalgae can also be traced from polar to tropical regions (Krause-Jensen and Duarte, 2016). Understanding the linkages between macroalgae and coastal sediments is therefore key to assessing their potential as carbon sequestering systems (Queirós et al., 2019).

While these ecosystems do not yet fit well in current carbon accounting frameworks, as the amount of carbon carried away and buried into ocean sediments is not easily measured and monitored (Krause-Jensen et al., 2018; Queirós et al., 2019; Hurd et al., 2022), their high sequestration rates at depth are promising. Understanding of macroalgal carbon cycling and their potential role in ocean carbon dynamics is growing quickly as well as interest in seaweed farming for climate mitigation. Funding for research should be encouraged to better understand the suitability of macroalgae preservation efforts and farming options in carbon accounting and blue carbon strategies (Krause-Jensen and Duarte, 2016; Ortega et al., 2019).

Zooplankton graze on phytoplankton at the surface, then move down the water column and actively transport carbon via egestion, respiration, and mortality from the surface to depths of 1,000m or more as part of the biological pump, accounting for 16-18% of total carbon export flux (out of the top ~100 m) (Aumont et al., 2018; Archibald et al., 2019). Below 1,000 m, zooplankton active transport increases export by at least 0.44 Gt C/yr (though this includes contributions from migrating micronekton as well) (Hernández-León et al., 2020). Further quantification of the global contribution of total zooplankton active transport to the biological carbon pump is an active area of research.

Antarctic krill, a type of zooplankton and the keystone species of the Southern Ocean, are also an increasing focus of research for their role in the Antarctic biological carbon pump (Cavan et al., 2019). Although researchers theorize that krill carbon export may be significant, its quantification is still highly uncertain (Belcher et al., 2019; Cavan et al., 2019). Antarctic krill, while accounting for the largest krill population globally at 400 Mt, are being impacted by ocean acidification, warming water, and changes in sea ice dynamics (Kawaguchi et al., 2013; Klein et al., 2018; Meredith et al., 2019), which may be negatively impacting the population and shifting it southwards towards the continent (Atkinson et al., 2019; Meredith et al., 2019). Krill are further stressed by increasingly concentrated fishing in the region (Watters et al., 2020).

Fish also contribute to the biological carbon pump via the same vertical migration-induced active transport mechanisms as zooplankton, as well as passive transport via sinking fecal pellets formed at the surface, a relatively large flux compared to some of the other marine organisms discussed here ( Table 4 ). It is unknown the extent to which protection and rebuilding of fish stocks would change the biological pump. However, the reduction in fish carcass deadfall to the seafloor due to industrial fishing is somewhat better studied. Mariani et al. (2020) estimate that industrial fishing of large, dense fish (including tuna, mackerel, billfish, and sharks) has prevented the sequestration of 21.8 ± 4.4 Mt C through deadfall since 1950 and emitted 0.2 Gt C (primarily through fuel consumption) into the atmosphere. This study estimates that if fishing levels were reduced to zero (taking 2014 levels as a reference), an additional 1.09 MtC/yr would be stored in large fish biomass with ~0.6 MtC/yr sequestered via carcass deadfall (Mariani et al., 2020). In addition to fishing pressures, fish populations are also impacted by climate change, which alters habitable zones and food web structures. Fish biomass is projected to decrease by 10 to 25% by 2011 under the IPCC AR6 high greenhouse gas emissions scenario SSP58.5 (Tittensor et al., 2021). These climate and fishing-driven decreases in fish stocks have serious repercussions for biodiversity, ecosystem resilience, and human well-being for coastal communities (IPCC, 2022). The direct additional impact of restoring fish on blue carbon sequestration is relatively low and is therefore unlikely to be an effective climate mitigation tool. In contrast, a reduction in industrial trawling could preserve significant amounts of carbon stored in the sediment floor (Sala et al., 2021).

Whales contribute to the biological pump through fertilization of the surrounding ocean with iron-rich excrement, which also contributes to fast-sinking particulate organic carbon, and to a lesser extent whale fall, when the carcass of a dead whale sinks to the seafloor and an unknown fraction is buried in ocean sediments. Researchers have estimated the restoration of whale populations to pre-whaling levels could generate additional carbon sequestration of 0.16 - 6.7 MtC yr^-1 depending on the population (Lavery et al., 2010; Pershing et al., 2010; Durfort et al., 2020 preprint). However, whales are threatened by climate impacts as well as increased shipping traffic and pollution (Tulloch et al., 2019). Despite their potential importance in the biological carbon pump, whales’ role in marine carbon sequestration is minimal (Maldonado et al., 2016) ( Table 4 ), and the time scale and uncertainties for population recovery, threatened by climate change, further complicates the calculation of additional carbon sequestration that could be attributed to these species (Durfort et al., 2020 preprint).

Calcifying organisms such as oysters and coral reef systems are also discussed in the context of carbon sequestration. Oysters are filter feeders that excrete organic carbon onto the seabed (Lee et al., 2020), but they also release CO₂ through respiration and the shell formation (calcification) process. For this reason, oysters have been kept out of blue carbon frameworks for now, as the balance between calcification and respiration needs further investigation (Ullman et al., 2013).

Recent estimates indicate that coral reefs cover about 52.7 million hectares worldwide (Mora et al., 2006), potentially representing 31 Mt of organic carbon (Crossland et al., 1991). However, coral reefs are generally considered a small net source of CO₂ rather than a net sink despite supporting photosynthetic algae, and are excluded from blue carbon frameworks (Howard et al., 2017). Like oysters, reefs undergo continuous calcification and dissolution, and their role as a carbon sink or source depends on the balance between calcification and dissolution. Calcification releases CO₂ to the atmosphere, whereas dissolution represents an oceanic carbon sink. The relative rates of calcification and dissolution depend on ocean acidity and the concentration of dissolved inorganic carbon in the water. Under current conditions, the amount of CO₂ released to the atmosphere by the reefs is slightly greater than that being captured through dissolution (Howard et al., 2017). This balance may change in the future as ocean acidification alters the biochemistry of the ocean column in favor of dissolution.

Calcification is common in other coastal ecosystems as well, as a restored subtidal habitat (e.g., seagrasses) can favor the proliferation of shelled molluscs. For accurate carbon accounting in all of the above ecosystems, site-specific assessments are necessary to assess the balance between organic and inorganic carbon production and sequestration (Macreadie et al., 2017). While the climate mitigation benefits of these systems might be negligible, oysters and coral reefs provide multiple ecosystem services (e.g., water quality improvement, coastal resilience, and fish habitat, among others) and warrant protections.

Under the UNFCCC’s global objective of the Paris Agreement to mitigate anthropogenic GHG emissions, Parties can put forward nature-based solutions as part of their emission reduction and climate adaptation pledges known as nationally determined contributions (NDCs) (UNFCCC, 2015). The Agreement does not identify specific ocean-based interventions, but rather emphasizes the reduction of Parties’ GHG emissions as the central tenet to be implemented for mitigating and adapting to climate change.

As of October 2021, 71 countries have mentioned coastal and marine nature-based solutions within their new or updated NDCs. Of these, 45 countries mention the protection, management, and restoration of coastal and marine ecosystems to meet both mitigation and adaptation climate objectives (Lecerf et al., 2021). Costa Rica for example has committed to protecting all their coastal wetlands as recorded in the country’s National Wetland Inventory, while Seychelles has committed to protecting all its coastal wetlands by 2030, and Belize has committed to both protecting and restoring mangroves along with developing a seagrass management plan.

Although most countries have at least one type of coastal wetland, for a handful of countries that have substantial amounts of these ecosystems (Taillardat et al., 2018), proper conservation, restoration, and management of these ecosystems may result in sequestration rates significant enough to reduce overall country emissions (Bindoff et al., 2019). While there is burgeoning interest to include marine protections within climate mitigation policy frameworks, several key criteria must be met to ensure that these protections advance climate objectives ( Table 5 ).

Recent papers have pointed to the potential monetary value of polar blue carbon (Armstrong et al., 2019; Bax et al., 2021), but without clear oversight, double-counting of emission reductions (when two entities claim the same carbon removal), could occur with unfixed blue carbon sources or sequestration that takes place on the high seas (e.g. two nations taking part in the multilateral governing body could both claim reductions from the same source). While deep-sea sediments were not discussed in this manuscript, this risk of double counting also extends to sediments located beyond countries’ exclusive economic zones; while additional data is needed to quantify these stocks, their disturbance could result in the remineralization of this carbon (Atwood et al., 2020). Rules to operationalize Article 6 of the Paris Agreement, the UNFCCC framework to establish a global mechanism for carbon markets to reduce emissions, were established at COP26 in November 2021, but action on the high seas is not included within the UNFCCC (Gallo et al., 2017). Article 6 does however specify that when parties transfer international mitigation outcomes to NDCs, they must avoid double counting (UNFCCC, 2021 see Article 6 paragraph 4). Furthermore, the risk of green-washing (or in this case blue-washing), where entities label environmentally deleterious actions as sustainable, is elevated in the climate space due to the monetization of carbon credits (In and Schumacher, 2021).

While many marine ecosystems may not be suitable for climate mitigation policies at this time, they can be important to building climate adaptation and resilience, protecting biodiversity, and fostering societal well-being (Bindoff et al., 2019; IPBES-IPCC; Sala et al., 2021). The latest IPCC WGII report (IPCC, 2022) underlines the role of resilient biodiversity and ecosystems and their services in mitigation and adaptation and achieving climate resilient development. The effectiveness of ecosystem-based adaptation and approaches to climate change mitigation based on ecosystems are reduced as warming increases and limits to natural adaptation capacity are reached. This underscores that scientists, advocates, and policymakers should not have a singular focus on climate mitigation policies in the blue carbon context, but utilize, or create new, frameworks more appropriate to their conservation, adaptation, and well-being goals, while seeking connections between the frameworks that may strengthen them collectively.

In instances where carbon accounting towards inventories is not suitable, there are a suite of policy frameworks that may be more appropriate to achieve these broader goals. The Convention on Biological Diversity (CBD), which came into force in 1993 and includes 193 Parties to the agreement, sets up an overarching framework to address the underlying causes of biodiversity loss, reduce the direct pressures on biodiversity, and safeguard ecosystems. The CBD also acknowledges the interlinkages between biodiversity and climate change and has adopted voluntary guidelines for the design and implementation of ecosystem-based approaches to climate change adaptation and disaster risk reduction (CBD, 2019).

Other frameworks support the creation of marine protected areas, which can protect carbon capturing marine ecosystems and species, and help build resilience to climate change (Roberts et al., 2017). For example, a recent assessment found that marine World Heritage sites and their immediate surrounding areas comprise at least 21% of the global area of blue carbon ecosystems (UNESCO, 2020). The Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR) has agreed to designate a representative network of marine protected areas (MPAs) in the Southern Ocean (Brooks et al., 2020), though progress in recent years towards this goal has stalled. The potential U.N. agreement for the conservation and sustainable use of marine biological diversity of areas beyond national jurisdiction (BBNJ) could lead to widespread protections from shipping, fishing, and deep-sea mining on the little understood high seas (Gjerde and Rulska-Domino, 2012; Visalli et al., 2020), an area covering half of the planet (UN 2017) and managed by over 20 varied governance organizations. Despite political challenges, the discussions within CCAMLR and the U.N. are a testament to the international scientific and diplomatic success that can be found through multilateral frameworks.

In addition to these frameworks, tools to identify and protect vital marine ecosystems for carbon storage have been developed by a number of scholars (see McLeod and Salm, 2006; Di Nitto et al., 2014; Laffoley, 2020). Institutions can further support the restoration and conservation of blue carbon ecosystems through creative carbon financing. Additional funding is needed to better understand the sequestration potential in other marine ecosystems, as noted above. The authors underscore that funding for ocean research remains inadequate (IOC-UNESCO, 2020), even as the percentage for ocean and climate funding grows (Our Shared Seas, 2022). Due to this shortfall, it is important to recognize that it is more cost-effective to preserve, rather than restore, blue carbon ecosystems (Jakovac et al., 2020; Su et al., 2021), and that the ecological integrity of a system supports its ability to store carbon and deliver other co-benefits. With the rapidly closing window for significant climate mitigation (IPCC, 2021) and adaptation action (IPCC, 2022), and continued shortfalls of climate financing, a near-term focus on enhancing current validated blue-carbon ecosystems may be justified (see Macreadie et al., 2019, for a set of blue carbon research priorities).

Lastly, to better enable policymakers to operationalize scientific research, results should be communicated in both the metrics used in the field, as well as put into the greater context of emissions reduction. As described previously, in this article we standardized units of measurements to enable comparisons between potential blue carbon gains and current emissions ( Figure 2 ). Scientists can easily communicate their findings by similarly standardizing measurements, as well as utilizing emissions calculators (such as the one provided by the U.S. Environmental Protection Agency, see EPA, 2022), and including clear messages for policymakers.