A Systematic Review and Global Trends on Blue Carbon and Sustainable Development: A Bibliometric Study from 2012 to 2023

Inextricably linked to climate, the oceans play a key role in climate change mitigation and adaptation [135,136,137,138,139,140], and blue carbon plays a potential contributing or even the predominant role [11,55,140,141,142,143,144,145,146,147,148,149,150,151]. As part of the ongoing discussions regarding the ocean–climate nexus and blue carbon, more attention is being paid to carbon cycling and storage processes in the open ocean as a potential solution to climate change [107]. Maintaining and increasing blue carbon such as Caulerpa farming is one of the most basic strategies for combating global warming [152,153,154]. Due to regional warming, the northern Antarctic Peninsula is likely to experience macroalgal expansion and blue carbon gains as a result of glacial retreat [155]. Meanwhile, a natural methane (CH₄) emission from blue carbon ecosystems may counteract atmospheric CO₂ uptake [156]. A seagrass–colonized coastline is a net source of CH₄ to the atmosphere; CH₄ production is sustained by methylated compounds created by the plant, as opposed to the fermentation of buried organic carbon [157]. In contrast to the short lifetime of CH₄ in the atmosphere, undisturbed coastal wetlands produce limited quantities of CH₄ emissions [153,158,159]. As natural carbon storage hotspots, blue carbon ecosystems are also at risk from global change [159,160,161,162,163,164,165]. As an example, the typical blue carbon ecosystem exhibits a high level of heavy metal accumulation capacity; however, extreme rainfall can lead to a change in sediment particle size, thereby causing heavier metal concentrations to increase towards the sea [166].

In many international organizations and countries such as Fiji, blue carbon as a carbon mitigation strategy is incorporated into National Development Contributions (NDCs), national management plans, etc. [11,107,167,168]. Community participation is essential in the implementation process as the blue carbon ecosystem is deeply woven into the need for NDCs [169]. In fact, key issues involved the reliance of blue carbon measures on slowing sea level rise as well as restoration efforts [11]. In particular, coastal wetlands are dependent on the interaction between human impacts and sea–level rise to survive [158]. In addition to providing climate change mitigation benefits, MPAs in different places can also contribute to the preservation and enhancement of blue carbon pools [170].

Seagrasses (dead seagrass, seagrass beds, seagrass meadows), mangroves, coral reefs, kelps, saltmarshes, macroalgae (or seaweeds), benthic microalgae, etc., constitute the blue carbon ecosystem, whose comprehensive benefit is also determined by the size, quality, and extent of the ecosystem [69,71,76,104,110,124,135,136,161,164,165,168,170,171,172,173,174,175,176,177,178,179,180,181,182,183,184]. Due to the structural complexity of coastal vegetation ecosystems (root systems, dense vegetation, and leafy canopy in seagrass systems), salt marshes, mangroves, and seagrass beds are capable of efficiently capturing sediment and associated organic carbon from both riverine and oceanic sources [164]. Among these, coastal wetlands (mangrove, tidal marsh and seagrass) sustain the highest rates of carbon sequestration per unit area of all natural systems, primarily because of their comparatively high productivity and preservation of organic carbon within sedimentary substrates [99,158,159]. However, blue carbon ecosystems have been severely depleted in the last 50 years, primarily as a result of human activities [72,78,101,162]. Of course, human activities like human–made structures can also enhance the biogeochemical sink capacity [185]. Consequently, it is necessary to conduct qualitative and quantitative assessments of the blue carbon ecosystem’s components, carbon storage, carbon cycling, carbon sequestration, monitoring, and potential risks under different conditions [10,72,100,110,112,114,116,117,118,119,123,141,145,146,149,151,160,164,167,172,174,175,186,187,188,189,190,191,192,193,194,195,196,197,198,199,200,201,202,203,204,205,206,207,208,209,210,211,212,213,214,215,216,217,218,219,220,221,222,223,224,225,226,227,228].

Generally, the capacity of coastal ecosystems to sequester blue carbon per unit area is greater than that of terrestrial and ocean ecosystems [13]. A mangrove ecosystem with a low freshwater demand, for example, is recognized for providing valuable ecosystem services as well as having the highest carbon content of any forest ecosystem [74,229]. The ecosystem functions of restored mangroves are higher than unrestored degraded mangroves, but are lower than that of natural mangrove groves [113,188,230].

Carbon sinks can be increased through the efficient carbon sequestration of blue carbon ecosystems [231]. A main benefit of microalgae is their ability to sequester carbon and produce biomass without the need for arable land [152]. It may be possible to increase blue carbon by reducing the use of chemical fertilizers on land in order to promote microbial carbon sequestration in marine ecosystems [232]. Although invasive species (Phragmites australis, Sporobolus alterniflora) are probably harmful, soils provide an effective carbon sink [233,234,235]. Furthermore, the Abu Dhabi Blue Carbon Demonstration Project indicates that coastal ecosystems provide numerous additional environmental benefits, including habitat for sea turtles and dugongs, stabilization of shorelines, fish production, and water quality maintenance [236].

As a result of technological advancements, we discovered that blue carbon consists not only of shellfish and macroalgae but also the carbon transformed by microbes, dissolved organic carbon, and sedimentary particulate carbon [164,237]. Habitat (distribution) maps are increasingly being created to account for blue carbon using remote sensing data, acoustic methods, species distribution models (SDMs), and machine learning algorithms [74,104,142,151,164,175,179,187,238,239,240,241,242,243,244]. By using satellite images and biological data, we are mapping the percent seagrass cover (SPC), the above–ground biomass (AGB) and the below-ground biomass (BGB) on islands to monitor temporal changes in the distribution of seagrass meadows [245]. It is possible to trace the carbon flows in blue carbon ecosystems using ecological network analysis (ENA) as total coastal carbon flows were many times greater than terrestrial ones [164,246,247]. In order to support coastal and small island zonation planning, conservation prioritization, and marine fisheries enhancement, multi-source spatial datasets can be used to map the climatic and human pressures on blue carbon ecosystems [205]. In order to improve soil health in blue carbon ecosystems, biochar–based technologies must be developed [242].

In order to protect the blue carbon pool and formulate a sustainable management plan, incorporating REDD+ (Reducing Emissions from Deforestation and Forest Degradation Plus) will play an important role and is the ultimate objective of developing blue carbon policies [71,140,248]. The ability to quantify carbon accumulation in sediments is a useful tool for estimating the amount of carbon stored in mangrove ecosystems, which is a precondition for the implementation of REDD+ programs [117,164,228].

In Vietnam, the benefits of mangrove-aquaculture systems (MAS) are a possible triple–win approach for communities towards sustainable development [120]. The Blue Carbon Strategy Framework (including coordination, policy, and funding) is imperative for Indonesia [249,250]. Local cases in the Philippines indicate that the concept of “blue carbon” has not yet been fully integrated into management plans [251]. Despite the fact that the drivers were not ranked based on the assessment, key respondents cited ‘institutional capacities’ as a major factor hindering the management of blue carbon ecosystems [252]. A crucial component of the improvement of blue carbon sinks in China’s reclamation history districts was coastal management practices (the size of industry and population control, balanced fertilization techniques in reclamation areas, and maintaining adequate vegetation cover in reserves) [147]. The fundamental drivers for reducing the total blue carbon stock of the Sundarban, the world’s largest contiguous mangrove forest, are recurrent tropical cyclones, soil erosion, anthropogenic pollution, and so on [253]. Plantations of iteroparous mangrove species may provide an effective solution to these challenges [254]. Furthermore, it is proposed that living shorelines that incorporate blue carbon ecosystems into their design could sustain and/or increase carbon stocks and carbon sequestration capacity in Australia [150].

Increasing the blue carbon potential of marine protected areas (MPAs) may be a key contributor to carbon emissions reduction [139]. It will be possible to achieve substantial gains with a small amount of coverage with MPAs on specific carbon pools [170]. The design, location, and management of MPAs could be utilized to protect and enhance carbon sequestration, and to ensure the integrity of carbon storage through conservation and restoration practices [78,138]. The overall positive result may be diminished if MPAs were established solely based on biodiversity considerations such as coral reefs [170].

Blue carbon as a new funding mechanism can be applied and developed to the sustained funding for marine protected areas (MPAs); implementing many of these potential solutions (blue bonds, debt–for–nature swaps) have some capacity requirements [111].

Based on an uncertainty propagation approach, 0.15–1.02 billion tons of carbon dioxide are released globally annually, causing economic damages of US $6–42 billion [73]. Furthermore, large marine ecosystems have the potential to contribute to the harnessing and growth of the blue economy [145]. A carbon finance program can help to protect 20% of the world’s mangrove forests (2.6 million hectares) [255]. By utilizing and investing blue carbon as a source of climate finance, we are able to fill the finance gap associated with ocean sustainability [102,140]. The sequestration of blue carbon must be quantitatively evaluated and exchanged in order to become an economically viable product [256].

In terms of evaluation, mangrove restoration has positive benefit–cost ratios ranging from 10.50 to 6.83 under variable discount rates [188]. The plural valuation of mangroves may therefore be applied to sustainability initiatives [257]. The benefit transfer method is one of the most common valuation methods, but it risks recycling old estimates without advancing our understanding. In spite of continued use of replacement costs and improper use of carbon prices, estimating the economic value of carbon storage and sequestration remains a challenge [184]. In order to fund large-scale blue carbon restoration needs, tools such as payment for ecosystem service (PES) schemes and common asset trusts (CATs) can be used together [258]. First, we must estimate the carbon price, and then we must address economic, social, and governance issues [142,259]. Despite increasing promotion of PES to protect blue carbon ecosystems, biophysical stressors external to the PES site (pollution, etc.) will affect the potential contribution of PES sites [75,260]. One of the few positive stories for ocean acidification is that if ocean acidification results in a significant increase in above– and below–ground biomass, this increase in sequestration capacity will be worth between £500 and 600 billion between 2010 and 2100 [70].

At the implementation level, increasing economic sustainability can also be achieved by applying the ecosystem services concept and framework to aquaculture, etc. [261]. The potential for return (ROI) is a key factor in attracting more investment in rehabilitation-oriented blue carbon [262]. Voluntary carbon markets (VCMs) are more attractive to smaller projects due to their lower transaction costs using the blockchain technology, etc. [69,76,79,263]. Local communities may benefit from coastal carbon offset projects, and carbon credits can be traded on carbon markets [236]. Using the blue carbon economic model, fuel and food may be produced from marine ecosystems by sequestering, storing, and harvesting carbon [264]. The farming and industry of macroalgae, seaweed, etc., can provide future energy, economic growth, and sustainable livelihoods if the interactions between these operations and the surrounding marine ecosystems are taken into consideration [264,265,266]. Phytomariculture of seagrass offers the advantage of producing a seedbed and nursery for the development of blue carbon projects, such as the restoration of seagrass habitats [267]. In order to achieve the green and sustainable development of the carbon industry system, the maximum removal of black carbon impact, the maximum increase of gray carbon scale, and the maximum development of the blue carbon economy must be the main goals [268].

However, there are a number of challenges to overcome. In spite of the fact that ecosystems are excellent carbon reservoirs, blue carbon is marginalized on global markets for several reasons [80]. Very few operational blue carbon sites have been identified [121]. In Vietnam, financial incentives have contributed to the planting of more mangroves, but their effectiveness has been limited by conflicting national policies (such as the expansion of aquaculture in mangrove areas) [269]. Few blue carbon credit projects are operational due to low credit–buyer incentives, uncertainty regarding the amount of emissions reductions that can be credited, and high project costs [262]. Local residents have not combined their perceptions of tourism and blue carbon ecosystems [270].

Blue carbon policies are continuously evolving in related countries. Blue carbon policies in China have shifted from protecting ecosystems to increasing stocks, and the policy approach has evolved from simple protection to a comprehensive approach [271]. Due to the government’s revocation of the mangrove protection act, Brazil’s mangroves are no longer protected permanently, which will likely result in increased loss rates in the future [272]. It is possible for policy subsidies from the government to encourage the carbon trading platform to cooperate and improve their carbon sequestration capacity; however, if the subsidies are too high, the system will not have an evolutionarily stable strategy [273]. The balance is constantly shifting in policy. Coastal ecosystems are managed by policies and decision–makers that integrate physical, ecological, and social factors; natural threats and lack of law enforcement were the primary factors contributing to mangrove forest degradation in the Philippines, posing a fundamental disadvantage for local people [274,275,276]. It is essential to investigate stakeholder preferences, especially those related to livelihoods, in order to ensure the sustainable development and conservation of blue carbon ecosystems [79,277]. As a result of various environmental and social constraints, the effective implementation of regulations and guidelines regarding sustainable aquaculture practices in Indonesia remains a challenge [122]. For the fisheries policy to be a more effective one, the maximum carbon sequestration must be incorporated into fisheries management, rather than only focusing on Maximum Sustainable Yield (MSY) [109]. By conserving large fish species preferentially, fisheries management can increase overall carbon storage in the fish community while balancing several SDGs [278].

To determine the best eco–site, a spatially explicit, integrative, and culturally relevant site selection process is necessary, with blue carbon storage value being assigned the greatest weight [108]. Natural–based solutions and ecosystem–based approaches are good policy options [279,280]. In detail, blue carbon can be integrated into national Marine Spatial Planning (MSP) as a conservation management tool in the proposed spatial planning laws [106,163]. MSPs that address ocean climate change (‘climate-smart MSPs’) may find that ocean climate change modelling is a key decision–support tool [281]. The primary proximate drivers of coastal aquaculture expansion were identified as aquaculture development and economic opportunities, whereas factors relating to institutional policies played a lesser role [125]. Government policy interventions should be prioritized to increase the expansion of sustainable coastal aquaculture and mangrove conservation [125].

As most places where blue carbon occurs are managed under common–property or open–access regimes, the impacts of blue carbon projects will be highly dependent on how they address property rights [76]. Furthermore, as part of the UNFCCC process, blue carbon sequestration may serve as a governance niche [139]. Conserving and restoring blue carbon ecosystems overlap with protected area management, which are overseen through the Convention on Biological Diversity and the Antarctic regime complex for the Southern Ocean [140]. However, there is a limited number of empirical values generated by these studies [183], especially in terms of policy perspectives.

In light of the evaluation of the blue carbon development index (BCDI), global cooperation could contribute to improving the global average BCDI score and sequestering carbon dioxide [282]. The establishment of a blue carbon co–operation and trading mechanism with other countries would enhance the implementation of global fishery resources and extend the industrial chain [283]. A source of future international support for blue carbon–rich countries is institutional recognition [140]. The Australian Government announced the establishment of an international partnership for blue carbon in 2015 [77]. Despite the lack of formal recognition within the climate process, communities of practitioners have served as networked constituencies (such as the Blue Carbon Partnership) [140]. An economic model of blue carbon international cooperation proves the economic feasibility of blue carbon cooperation [256]. There is a high likelihood that not all countries will participate in blue carbon international cooperation, but sub–alliance groups of multiple countries should be considered [256].