Arctic Oceanic Carbon Cycle: A Comprehensive Review of Mechanisms, Regulations, and Models

Abstract

Understanding the oceanic carbon cycle, particularly in the Arctic regions, is crucial for addressing climate change. However, significant research gaps persist, especially regarding climate effects on the oceanic carbon cycle in these regions. This review systematically explores Arctic-related research, focusing on mechanisms, regulatory frameworks, and modelling approaches in the oceanic carbon cycle, carbon sink, climate change impact, and maritime shipping. The findings highlight the Arctic’s limited observer presence and high operational costs, hindering the data availability and studies on carbon-cycle changes. This underscores the need to integrate real-time Arctic Ocean monitoring data. Carbon sink research urgently requires direct methods to measure anthropogenic carbon uptake and address uncertainties in air–ocean carbon fluxes due to sea ice melting. Unlike terrestrial carbon cycling research, carbon-cycle studies in the oceans, which are essential for absorbing anthropogenic emissions, receive insufficient attention, especially in the Arctic regions. Numerous policies often fall short in achieving effective mitigation, frequently depending on voluntary or market-based approaches. Analyzing carbon-cycle and sink models has uncovered limitations, primarily due to their global perspective, hampering in-depth assessments of climate change effects on the Arctic regions. To pave the way for future research, enhancing Arctic Ocean climate data availability is recommended, as well as fostering international cooperation in carbon-cycle research, enforcing carbon policies, and improving regional modelling in the Arctic Ocean.

1. Introduction

The ocean, serving as a major carbon sink, has a capacity to absorb atmospheric CO₂ that is far greater than that of the atmosphere itself, playing a key role in regulating global climate [1]. The oceanic carbon cycle is a fundamental component of global carbon dynamics. It mediates carbon exchanges (Figure 1) between the oceans, atmosphere, earth, and seafloor and significantly regulates atmospheric CO₂ levels, impacting the climate and ecosystems [2]. Marine plants and algae intertwine carbon with oxygen cycles through photosynthesis and intersect with nitrogen and phosphorus cycles, collectively affecting the overall oceanic carbon dynamics [3]. Due to its high sensitivity to climate changes and significant environmental shifts, such as extensive ice melting and temperature fluctuations, the Arctic Ocean (Figure 2) is critical in the global carbon cycle. These unique conditions enhance the Arctic’s role in the dynamics of the global carbon balance and climate change [4]. In particular, in the Barents Sea, cross-shelf transport of dense Barents Sea water significantly contributes to carbon sequestration, with large volumes of deep water from the Barents Sea being injected into the Nansen Basin, underscoring the Arctic Ocean’s importance in global climate mitigation efforts [5].

The warming of the Arctic Ocean is primarily due to the absorption of human-emitted greenhouse gases, with the ocean having absorbed 90% of the heat from increased emissions, leading to significant temperature increases [6]. Anthropogenic activities, notably the burning of fossil fuels, have disrupted the natural carbon balance by increasing CO₂ emissions, further affecting the ocean’s capacity to sequester carbon dioxide. Climate change significantly impacts the Arctic Ocean, accelerating its warming rate to 2–3 times the global rate, leading to rising sea levels, changing climate patterns, and intensifying extreme weather events [7]. Additionally, the loss of sea ice has also harmed fish stocks, birds, and marine mammals, affecting the livelihoods of Arctic residents [8].

As a major carbon sink, the ocean absorbs approximately 90% of the excess heat in the Earth’s system, facilitating heat transfer from the equator to the poles [9]. Colder sea surface waters, such as those in the Southern Ocean, act as effective carbon sinks due to their higher capacity for CO₂ absorption and dissolution [10,11]. Additionally, nutrient-rich colder regions exhibit greater carbon absorption capabilities than warmer areas, making polar regions crucial carbon reservoirs that store substantial amounts of carbon [12]. Maritime shipping, essential for transporting over 80% of traded goods, is critical in the global economy and significantly contributes to environmental pollution. It accounts for 24% of the global CO₂ emissions from fossil fuel combustion and emits substantial amounts of nitrogen oxide and sulphur dioxide, key air quality pollutants [13]. Additionally, international maritime transport releases about 940 million tonnes of CO₂ annually, roughly 3% of global greenhouse gas emissions, with projections indicating an increase to 5% by 2050 [14]. Maritime shipping’s significant environmental impact necessitates the development of robust regulations to protect polar ecosystems and mitigate climate change effects. Such regulations must be underpinned by reliable data and accurate predictive models, especially crucial in the Arctic, where harsh conditions make data collection efforts challenging. However, most existing carbon-cycle models lack specific features to accurately represent the Arctic’s unique conditions. As a result, critically evaluating and refining these models is vital for a better understanding of the Arctic regions’ oceanic carbon cycles.

This review aims to conduct a comprehensive gap analysis of current carbon-cycle research, focusing on advancing the state of the art in the context of the Arctic Ocean. The gap analysis comprised three primary dimensions: challenges in research on mechanisms, policy frameworks, and modelling methodologies. Within this framework, three thematic areas of study are reviewed in detail, including oceanic carbon cycling, ocean carbon sink, and the impact of shipping emissions on marine carbon dynamics and Arctic pollution. Through this integrated assessment, the review highlights the current knowledge gaps in the Arctic Ocean’s carbon cycle, providing a consolidated perspective on the evolving research landscape.

To ensure comprehensive coverage of the topic, a systematic approach was employed for this review. Extensive searches were conducted in multiple databases, including PubMed, Web of Science, Scopus, and Google Scholar, selected for their broad scientific literature repositories and relevance to environmental science and marine research. Additionally, recent news and regulations related to the Arctic carbon cycle from official websites were specifically considered. The search strategy utilized combinations of keywords such as “Arctic”, “carbon cycle”, “carbon sink”, “climate change”, “ocean acidification”, “maritime transport”, “carbon sequestration”, “polar”, “environmental impact”, “oceanic carbon cycle”, “arctic ocean”, “mechanism”, “regulation”, “modelling”, and “policy”, using Boolean operators to ensure both a wide breadth and specificity in the search.

This review covers information published mainly from January 2010 to July 2023 to capture the latest advancements and necessary historical data. The inclusion criteria encompassed peer-reviewed articles and reviews focusing on the Arctic carbon cycle and related mechanisms, studies exploring the impacts of climate change on the Arctic Ocean, and documents involving regulatory frameworks, policy impacts, and modelling methods related to carbon cycles. The exclusion criteria comprised non-English articles, studies not directly related to carbon cycles, and research that solely focusing on terrestrial carbon dynamics without a connection to marine processes. Relevant data, including methodologies, key findings, limitations, and future research recommendations, were extracted and synthesized from the selected articles. Each study’s methodological rigor, relevance, and contribution to the field were critically evaluated to ensure the validity and reliability of the conclusions.

2. Carbon Cycle in the Arctic Ocean

Recognizing the pivotal role of oceans in absorbing atmospheric CO₂ and its importance for meeting global goals like the United Nations Framework Convention on Climate Change (UNFCCC) Paris Agreement and the UN (United Nations) 2030 Agenda also aligns with the UN Decade of Ocean Science for Sustainable Development (2021–2030) objectives [15,16]. Nonetheless, human emissions remain the primary driver behind rising oceanic CO₂ levels, significantly affecting ecosystems’ health and carbon-cycling capacity. The research indicates that most human-induced carbon is stored within the top 2000 m of the global oceans, except in the North Atlantic, where it accumulates at the ocean’s bottom [17]. This phenomenon can be linked to effective ocean water circulation, well-defined boundary currents, and the distinctive characteristics of the Atlantic Meridional Overturning Circulation (AMOC). Changes in boundary currents result in fluctuations in carbon absorption [18].

Arctic and Antarctic polar regions require ocean forecasts due to melting ice caps, maritime traffic, and resource extraction. Similarly, Arctic forecasts aid in understanding sea ice and ice shelf stability for activities and ecosystem maintenance [19,20]. Limited observations in the polar ocean hinder accurate predictions for ocean and sea ice. This gap impacts various applications, like emergency response, aquaculture, reanalysis, meteorological predictions, and search and rescue, leading to potential negative effects [21]. The Arctic Ocean, spanning the region north of 65° N latitude, demonstrates prominent impacts of recent climate change. These include elevated surface temperatures, reduced sea coverage, and thinner ice, particularly during the summer months [22]. Sea ice cover in the Arctic Ocean also influences the quantity of solar energy available for primary phytoplankton production and the area of open water available for air–sea gas exchange, which is essential for pelagic marine ecosystems and biogeochemical processes [23].

2.1. Challenges and Gaps

Despite its global significance, the Arctic Ocean carbon cycle lacks precise measurements, with localized data often constrained by spatial and temporal variability, hindering a broader applicability [24]. An enhanced understanding of polynyas in CO₂ uptake is due to comprehensive sampling strategy, year-round measurements in ice-covered coastal areas, and the use of satellites to study phytoplankton bloom dynamics and primary production carbon. Diverse observation platforms, both in situ and using remote sensing, serve as crucial data sources for environmental prediction in the Arctic [25]. Observing System Experiments (OSEs) underscore the valuable synergy of existing observations [26]. Yet, conducting observations in the Arctic region presents unique challenges due to its isolation and harsh conditions, complicating in situ measurements. The limited availability of observation stations and high operational costs significantly hinder data collection, impacting the applications [27]. Additionally, the sea ice cover in the polar areas prevents real-time Argo profiling, leaving a sizable gap. Additional mistakes in satellite data, such as difficulties discerning between snow/ice and clouds in polar regions, further widen this difference [22,28,29]. Accurate data input into models is critical to avoid misleading outcomes and ensuring the precision of oceanic carbon-cycle research conclusions. Establishing clear performance expectations, allocating resources effectively, and utilizing diverse model types are essential for the accurate and effective use of these data.

In polar regions, several methods gather ocean and sea ice data via the Global Telecommunications System (GTS) or other transmission networks [30]. In ice-free zones, various tools, including ships, Argo floats, surface drifters, gliders, and other apparatuses are employed for data collection [31]. In ice-covered areas, the only sensor system mounted on ice operates as a year-round, near-real-time data source. Global and regional environmental prediction systems have been established under the “CONCEPTS” program (Canadian Operational Network of Coupled Environmental Prediction Systems). However, to enhance water mass analysis in Canadian seasonal ice-infested seas, it is recognized that additional real-time data are required to supplement the existing observing system [32]. A project is underway to evaluate various observation technologies for their ability to gather cost-effective, accurate ocean temperature and salinity data. The initial deployment involved an Argo float in the Gulf of St. Lawrence with shallow parking and profiling depths to operate during ice-free months. Without an ice detection capability, this approach reduced costs and proved effective alongside existing moorings in the Gulf [29]. Another effort involved placing an Argo float on the Labrador shelf at a shallow 200 m depth for profiling, effectively countering lateral drift. Despite winter ice, the float intermittently surfaced every ten days to transmit oceanographic data. This filled data gaps, benefiting various applications. The real-time availability enabled anomaly detection and information dissemination among linked water masses, including the Labrador Shelf [29].

Ocean water does not absorb carbon consistently. Natural circulation patterns elevate carbon-rich deep waters, contributing to atmospheric carbon levels. Furthermore, the escape of carbon intensifies upwelling during the cold phase of the oceanic oscillations [33]. These complex and variable carbon dynamics make monitoring human-induced changes in the ocean challenging, as understanding these fluctuations is key to addressing the global carbon balance [34]. Forecasting the Arctic’s future impact on the global carbon cycle is challenged by limited correlations between observations and emission trends, along with difficulties in defining variability. Enhanced and better-integrated observational efforts are essential to understanding the Arctic’s response to emission changes and effectively monitoring its evolving environmental dynamics. The study highlighted several key gaps in understanding the North Atlantic carbon pump. These include the need for long-term monitoring of carbon dynamics, unclear interactions between organic and inorganic carbon forms, challenges related to alkalinity, and inconsistencies in salt conservation within physical ocean models [35]. Further, the gaps in carbon pump interactions encompass land–sea dynamics, coastal and open ocean connectivity, variations in mixed layer depth, surface–atmosphere exchanges, deep water mixing, ocean circulation patterns, impacts of the overturning frontal systems, and freshwater/heat transport. Similar issues are likely present in the Arctic Ocean, indicating that with intensified research in this region, addressing and resolving these issues will become increasingly important.

The current models primarily develop a mechanistic understanding rather than offering precise carbon accounting or measurements. Regional-scale models aid in comprehending Arctic carbon-cycle/sink conditions. Models are categorized by spatial coverage, resolution, coupled vs. hindcast, and forward vs. data assimilation, and there are four kinds of models. The commonly used models include Earth System Models (ESMs), global hindcast models, regional models, and coastal models. The key considerations in developing ocean models encompass grid design, grid points, model forcing, and the focus, whether global or regional. Unlike observation-driven hindcast models lacking predictive capability, ESMs, integrated with the atmosphere, strive for statistical consistency with historical observations without replicating specific years [36]. Refining models for accurate alignment with observations is essential for understanding and diagnosing environmental changes [37]. Comparing results with salinity, temperature, chlorophyll, and nitrate measurements, regional models outperform ESMs, as per studies evaluating their performance [38]. Regional models rely on boundary conditions from EMSs or other models. Their performance compared to observations varies depending on the specific factors studied. While existing models offer valuable insights, Earth System Models (ESMs) are particularly commendable for their comprehensive atmospheric connections and potential to enhance regional model accuracy. However, ensuring the relevance and applicability of these models requires a focused and coordinated approach, which is both crucial and challenging. The challenges include ensuring high-quality and consistent data from various sources, managing complex and variable data in fluctuating environmental conditions, and adjusting and validating models for accuracy. Additionally, coordinating international research efforts and achieving effective collaboration in technology and resource allocation also present significant hurdles in effectively utilizing these models.

2.2. Comparison with Terrestrial Carbon Cycle

Land, oceans, and the atmosphere form the main carbon pools, and their exchange governs the carbon-cycle duration field [39]. Although terrestrial and oceanic cycles significantly affect Earth’s carbon balance, they differ in their mechanisms and global impacts. Comprehending the links and transfers between oceanic and terrestrial carbon cycles is crucial [40]. For example, rivers carry carbon from land to sea, and oceanic processes influence atmospheric CO₂ levels, affecting terrestrial ecosystems. Oceanic and terrestrial carbon cycles can be compared regarding CO₂ exchange with the atmosphere, carbon uptake and release mechanisms, and their roles in the global carbon cycle. The process of carbon release is balanced by carbon uptake. In terrestrial ecosystems, carbon is released into the atmosphere through plant respiration, microbial decomposition, and combustion events like wildfires. Interestingly, carbon uptake in these systems far exceeds the amount released from burning fossil fuels [41]. Similarly, the ocean releases CO₂ into the atmosphere via processes like outgassing due to changes in water carbonate chemistry, which have led to more CO₂ leaking into the atmosphere through the oceans as it accumulates [42]. Biological processes, microbial degradation, and marine organism respiration also contribute to oceanic CO₂ emissions. A study spanning 240 years (1780–2020) using a data-assimilated ocean model indicated that 45% of the atmospheric carbon increase was due to emissions, with the remaining 55% resulting from natural carbon outgassing [43]. Due to different processes and timelines, comparing carbon release between terrestrial ecosystems and oceans is challenging.

The carbon-cycle processes (Figure 1) can be categorized into slow and fast phases. The slow carbon cycle involves carbon sequestration from ocean organisms like plankton into rock. Decaying organisms form sediment and new rock layers. Volcano eruptions release rock-stored carbon into the atmosphere. Carbonic acid from rainwater returns atmospheric carbon to rivers, breaking down rocks and releasing minerals like calcium. Marine calcifying organisms use these ions, starting a new cycle [44]. The fast carbon cycle is the carbon movement across the biosphere. Plants and phytoplankton absorb CO₂ via photosynthesis, cycling through food chains, and returning carbon to the atmosphere through decomposition. The fast cycle processes approximately 100,000 million metric tons of CO₂ annually [45]. The slow carbon cycle spans millions of years, in contrast to the fast cycle, occurring within the lifespan of organisms. These cycles maintain the temperature balance by distributing carbon between the Earth and atmosphere, preventing singular storage and resulting in natural warming and cooling cycles over time. Recognizing the significance of the global carbon cycle has become crucial for adapting to and mitigating climate change. The increase in anthropogenic activities, including fossil fuel consumption and deforestation, has elevated atmospheric CO₂ levels, which threaten to destabilize the natural cycle [46].

Climate significantly affects the terrestrial biosphere, thereby impacting the global carbon cycle. The impact of climate on the terrestrial carbon cycle is evaluated through various measures, such as vegetation indices, ecosystem respiration, productivity, and net biosphere–atmosphere fluxes [47]. Forests exemplify terrestrial ecosystems that act as carbon sinks, with the potential to mitigate climate change through significant carbon storage [48]. The feedback from the terrestrial carbon cycle will significantly impact future climate change. One significant feedback is the uncertain dependency of global terrestrial carbon storage on CO₂ concentration. Although increased CO₂ levels are theoretically expected to enhance carbon storage, especially in tropical regions, contrasting evidence from atmospheric CO₂ studies casts doubt on the magnitude of terrestrial ecosystems’ role as substantial carbon sinks. Schimel et al. [49] presented that significant tropical uptake and combining tropical and extratropical fluxes indicated that up to 60% of the current terrestrial sink is due to rising atmospheric CO₂. Validated atmospheric analyses support this finding, yet some ambiguity remains, prompting efforts to refine current models.

The ocean’s crucial climate role includes absorbing a large portion of the CO₂ from industrial activity. However, human disruption of the natural carbon cycle, slow ocean overturning, and ongoing acidification hinder the ocean’s CO₂ absorption [2]. Warming reduces CO₂ solubility in oceans, affecting the absorption [50]. Rising global temperatures increase vertical stratification in oceans, affecting biological productivity, decreasing carbon transport to the deep ocean, and reducing CO₂ release from upwelling regions [51]. Arctic temperatures respond more intensely to radiative climate forcing, causing threefold fluctuations compared to the global average (Arctic amplification). This region is highly vulnerable to climate change, leading to sudden physical and ecological changes [52]. Slight shifts in ocean circulation can significantly affect marine ecosystems, sea level rise, and the global climate. The ongoing debate over the carbon-cycle balance adds to the uncertainty in identifying the key drivers of climate change. Current research efforts are focused on identifying these variables to accurately establish a global carbon mass balance field [53].

2.3. Relevant Policies of the Arctic Carbon Cycle

Initiatives targeting the reduction of greenhouse gas (GHG) emissions in the marine industry are being implemented in tandem with a global endeavour to mitigate greenhouse gas (GHG) emissions across diverse sectors. The UNFCCC is the leading international platform for governments to negotiate and address climate change, aiming to maintain safe greenhouse gas levels [54]. The Kyoto Protocol, a significant outcome, established emission reduction targets, particularly for developed countries. It also prompted collaborative efforts with the International Maritime Organization (IMO) to limit GHG emissions from marine bunker fuels [55]. In July 2023, the International Maritime Organization (IMO) adopted more ambitious goals in the first scheduled revision. The new targets aim to achieve net-zero GHG emissions by or around 2050. Additionally, “indicative checkpoints” were introduced, calling for a 20% reduction in total GHG emissions by 2030 and striving for 30%, as well as aiming for 70% by 2040 and striving for 80%, all relative to 2008 levels. This represents a significant improvement over the IMO’s initial 2018 GHG strategy, which aimed to cut emissions by only 50% by 2050 and lacked absolute emissions reduction targets for the intervening years [56]. In 2010, the IMO enforced stricter air pollution regulations for ships under the revised International Convention for the Prevention of Pollution from Ships (MARPOL, Annex VI). This aimed to control the emissions of sulphur oxide, nitrogen oxide, ozone-depleting substances, particulate matter, and volatile organic compounds [57]. Additionally, a chapter adopted in 2011 introduced mandatory energy efficiency measures to reduce GHG emissions from ships. The convention also established emission control areas (ECAs) with stringent pollution limits. Two ECAs were established in European waters, covering the Baltic and North Seas [58]. The IMO Strategy’s implementation includes various actions and initiatives: (1) the development of mandatory energy efficiency measures, such as the Energy Efficiency Design Index (EEDI) and the Energy Efficiency Operational Index (EEOI) outlined by the IMO, which is aimed at enhancing ship energy efficiency and reducing GHG emissions [58,59]; (2) the promotion of alternative fuels and technologies, including low-carbon options like liquefied natural gas (LNG), biofuels, and hydrogen fuel cells, which was advocated by the IMO to mitigate GHG emissions from ships [60]; (3) the implementation of the IMO Data Collection System, which involves mandatory data collection by ships, including fuel consumption and distance, enabling the assessment of energy efficiency, GHG emissions, and informed decision-making for policy development [61]; (4) consideration of market-based measures by the IMO, which involves exploring incentives for GHG emission reductions in shipping through mechanisms like global carbon pricing or emissions trading, potentially including a carbon tax on international shipping, contingent upon significant government and industry support [62]; and (5) collaborations between government and industry stakeholders, which are essential for successfully implementing these initiatives.

The IMO’s role in ocean protection extends beyond ship oversight to include waste dumping prevention and climate change measures like carbon capture and storage, which is accomplished through the London Convention and Protocol [63]. The IMO’s Initial Strategy to decarbonize shipping has garnered mixed views. Praised for setting long-term emission reduction goals, it is criticized for lacking binding short-term measures and relying on voluntary and market-based approaches. Critics highlight the absence of clear targets and deadlines for adopting alternative fuels and technologies [64]. Nevertheless, the Initial Strategy initiated discussions, raised awareness, and spurred research and innovation for decarbonizing shipping. It lays a foundation for ongoing improvements. Implemented on 1 January 2017, the IMO Polar Code safeguards polar waters for ships in the Arctic and Antarctic regions. This code enforces regulations for safety, environmental protection, and pollution prevention. By imposing strict rules on discharging pollutants such as oil, chemicals, sewage, and garbage, the Polar Code safeguards fragile marine ecosystems and preserves the pristine environment [65]. In 2021, the Marine Environment Protection Committee (MEPC) resolution urged member states to combat black carbon emissions from shipping to the Arctic and report progress. Amendments to MARPOL Annex I, effective November 2022, prohibit heavy fuel oil (HFO) use in Arctic waters starting from July 2024 [65]. The IMO’s Maritime Safety Committee (MSC) has implemented various measures like the Polar Code and fuel regulation to reduce Arctic black carbon emissions, collaborating with international organizations, such as the Arctic Council and the United Nations Environment Programme (UNEP) [66]. The Arctic Council and Arctic states have agreed on a marine pollution response, with working groups monitoring and mitigating ecosystem impacts [67]. The Protection of the Arctic Marine Environment Working Group (PAME), established in 1991, focuses on policy, pollution prevention, and Arctic marine environment protection. It collaborates with the Arctic Council and IMO to promote the Polar Code [68]. The Arctic Council endorsed the “Arctic Council Framework for Action on Enhanced Black Carbon and Methane Emissions Reductions” in 2015. This led to the formation of the Expert Group on Black Carbon and Methane (EGBCM), tasked with conducting the necessary study. The Arctic Council Ministers approved the goal to reduce BC emissions by 25–33% below 2013 levels by 2025 in 2017. At the 2023 Ministerial conference, the EGBCM provided an update on this target and proposed a collective goal for methane [69]. The Arctic Monitoring and Assessment Programme (AMAP), established in 1991, tracks Arctic pollution and informs policy decisions. Its analysis significantly contributed to international agreements, such as the Convention on Long-Range Transboundary Air Pollution (LRTAP) Protocol of the United Nations Economic Commission for Europe (UN ECE), which has been vital in reducing pollutants across Europe and North America since 1983 [70].

Domestic efforts are vital for Arctic environmental management. We will use Canada as an example to introduce the efforts of individual countries. The Arctic Council, established in Canada in 1996, focuses on environmental protection, including reducing black carbon and methane emissions. Canada pledged to reduce greenhouse gas emissions by 40–45% below 2005 levels by 2030 [71]. The Canadian government has implemented new measures to surpass the 2030 emission reduction target and achieve net-zero emissions by 2050 through the Canadian Net-Zero Emissions Accountability Act (Bill C-12) proposed in November 2020 [72]. Canada’s endorsement of the Clydebank Declaration in November 2021 underscores its commitment to establishing emission-free green shipping corridors and connecting ports for sustainable maritime transportation [73]. In 2022, Canada and the US announced a collaboration for the Great Lakes, the St. Lawrence Seaway System Green Shipping Corridor Network Initiative, at UNFCCC COP27. This initiative aims to establish eco-friendly corridors in the region through stakeholder engagement, assessments, and exploring alternative fuels and power options within the system. It builds on previous efforts outlined in the “Joint Statement by the U.S. Department of Transportation and Transport Canada on the Nexus between Transportation and Climate Change” [74]. The findings on decarbonization and carbon-cycle management in the North Atlantic/Arctic Oceans suggest several recommendations: prioritize comprehensive research to understand carbon-cycle dynamics; fund research for zero-emission maritime technologies through collaborations between academia, industry, and government; encourage carbon capture and storage adoption through policies and funding; safeguard coastal ecosystems as carbon sinks; and raise public awareness and engage communities to support policies for effective carbon-cycle management.

2.4. Carbon-Cycle Modelling in the Arctic

Carbon-cycle models’ origins date back to 1841, with Dumas presenting initial cycles based on photosynthesis, decay, geochemistry, and the water cycle. These models categorized reservoirs like the atmosphere, oceans, earth, and biosphere, focusing on carbon amounts and fluxes between them [75]. Modern carbon-cycle models aim to predict how anthropogenic CO₂ emissions are distributed among reservoirs using fast/slow carbon-cycle fluxes. Accurate estimations are crucial for climate impact predictions and policymaking. The United Nations Intergovernmental Panel on Climate Change (IPCC) offers climate change assessments, future risk descriptions, and mitigation recommendations. Assessment reports are released every 6–7 years, featuring various Representative Concentration Pathway (RCP) scenarios predicting temperature increases and impacts tied to anthropogenic emissions [76]. The RCP8.5 scenario envisions a future without emission reductions, leading to a substantial 5 °C temperature increase by 2100. In contrast, the RCP2.6 scenario predicts a less than 2 °C temperature rise, achieved through significant carbon emission reductions [77]. Climate change scenarios are based on research and modelling coordinated by the Coupled Model Intercomparison Project (CMIP) under the direction of the World Climate Research Program. CMIP provides standards for objective model comparison and validation. C4MIP oversees carbon-cycle modelling within climate modelling areas. C4MIP’s objectives for inclusion in the IPCC assessment report, CMIP6, include quantifying carbon-cycle element responses to rising CO₂, validating models against historical data, and projecting future carbon-cycle changes [78].

Models included in C4MIP are Earth System Models (ESMs) that simulate global carbon-cycle interactions based on C4MIP criteria. These models are complex and utilized by centers with high computational power. While concentrating on global carbon-cycle impacts, the specific effects of the Arctic Ocean have received less attention. Notably, CMIP5 models highlighted CO₂’s role in ocean acidification, and CMIP6 plans to incorporate global sea-ice simulations [78]. Therefore, the review focuses on models developed for the Arctic carbon-cycle issues, which are adaptable to simulate these areas and simple code-based carbon-cycle models with a lower computational demand. Such research offers insights into typical and small-scale customized carbon-cycle models for the Arctic Ocean.

Table 1. Arctic Ocean carbon cycle models.

No.	Model	Description	Advantages	Disadvantages	Modelling Applicability	Reference
1	SCP-M (v1.0)	Box-type model of global carbon-cycle model with emphasized ocean characteristics	Accessible Python code Easy to change data Fast runtime CMIP5 model-aligned results	Reduced resolution Latitude-Partitioned Boxes Unable to independently study oceans	Yes, but combined in one box/area	[81]
2	BernSCM (v1.0)	Simplified box-type global carbon-cycle model for temperature and carbon uptake simulation	Open-source Linux code Effective evaluation on global emission reduction strategy Useful for carbon sink analysis	Only global results for carbon sink uptakes Only surface ocean considered for carbon storage	No, but pattern scaling can show Arctic temperature changes	[82]
3	Globe+	Box-type model to simulate global temperature based on atmospheric CO2 concentration	MATLAB-based code Comprehensive carbon-cycle modelling (ocean storage, sedimentation, etc.)	Model is only based on mass balances Only result is global temperature Excludes function for ice coverage	No	[83]
4	NorESM-OC (v1.2)	Oceanic carbon-cycle component of NorESM	A coupled model for CMIP5 Oceanic carbon-cycle effect integration (ocean temperature, AMOC, O2, DIC)	OEM software and usage license requirement Global focus	Yes, CICE sea-ice model is included	[84]
5	BCM-C	Regional carbon-cycle feedback assessment ESM	Carbon-cycle feedback integration	Intensive computational requirements Restricted public access	Yes, regions are fully represented	[85]
6	CSM1.4-carbon	Integrated ESM for assessing carbon-cycle effects of anthropogenic emissions	Sea-ice component incorporation Arctic carbon-cycle impact assessment, mainly acidification	Intensive computational requirements Restricted public access	Yes	[86]
7	MEDUSA	Oceanic biogeochemical carbon-cycle model for simulating marine productivity and climate change	Online availability of source code Good model agreement with CIMP5 Incorporation of carbon-cycle impacts (e.g., calcification changes)	Intermediate modelling complexity Extrapolation needed for Arctic data Open to inaccuracies	Yes, but initial Arctic data lacks observational basis	[87]
8	SURFER (v2.0)	SCM for global sea-level rise and ocean acidification from anthropogenic emissions	Minimal code Fast simulation time	Acidification modelled only in the surface ocean Only computes global mean sea-level rise	No	[88]
9	BRICK (v0.2)	SCM for global temperature rise and regional sea level increases due to anthropogenic emissions	Incorporation of socio-economic flood risk module	Omission of additional climate impacts	Yes, regional sea-level rise is viewable	[80]
10	BEAM	Box-type atmosphere–oceanic carbon-cycle model for estimating future atmospheric CO2 concentrations	Non-linear modelling of oceanic CO2 uptake	Omission of sea-ice loss and sea-level rise	No	[79]
11	CLIMBER-X (v1.0)	Intermediate ESM for global carbon-cycle simulation	Inclusion of carbon-cycle feedback Climate-induced oceanic impact modelling Air–sea carbon flux variations	HAMOCC integration with proprietary software (other code is open source)	Yes, includes sea-ice model SISIM and impacts in the North Atlantic Ocean	[89]

summarizes 11 carbon-cycle models, outlining the advantages, disadvantages, and applicability to Arctic oceanic carbon-cycle research. Detailed information on the selected 11 models can be found in Section S1 of the Supplementary Materials. Table S1 summarizes the impacts in each reviewed carbon-cycle model, including climate change feedback and socio-economic analyses. Carbon-cycle models vary in complexity from simple box-type overviews to sophisticated programs necessitating supercomputer centers. Box models offer quick simulations and accessibility, suitable for diverse researchers and students, while intermediate and complex models yield a higher accuracy and incorporate more variables. Carbon-cycle models primarily emphasize the global cycle, lacking regional focus. Complex models often cater to CIMP5/6 standards, limiting regional studies. Some ESMs use proprietary software inaccessible to the public, like NorESM. Data deficiencies in Arctic modelling impact predictions. Some observational data sets, such as GLODAP, used by SCP-M and MEDUSA, lack Arctic data, leading to inaccuracies in carbon-cycle predictions. Other sub-models, such as CSIM used by CSM1.4, overpredict sea-ice coverage in certain areas due to the initializing data. BEAM correctly included the effects of ocean acidification in future oceanic CO₂ uptake predictions [79], and BRICK included CO₂ flux changes due to the loss of sea-ice and glacier melt [80]. Oceanic climate change impact simulations reveal sea-level rise as the least modelled aspect, followed by ocean circulation. Notably, simple climate models (SCMs) focused on simulating sea-level rise, while ESMs primarily addressed ocean circulation changes. This highlights the need for specialized approaches in sea-level rise modelling and underscores the complexity of ocean circulation analysis, necessitating advanced models and supercomputers. A promising future research direction involves emulating oceanic circulation changes within SCMs due to carbon-cycle feedback. The BEAM model is the only one to incorporate socio-economic climate change impacts, albeit in a regional case study (Table S1). Although socio-economic aspects usually fall outside carbon-cycle studies’ scope, their significance grows with climate change-driven disasters like flooding. A noticeable gap exists in integrating socio-economic sub-modules into carbon-cycle models, presenting a substantial research prospect. SCMs are better suited for socio-economic analysis due to their capacity for extensive simulations. A regional focus is essential for socio-economic carbon-cycle studies due to diverse variables. Overall, there is a need for more publicly available data sets on Arctic oceanic parameters to enable accurate modelling and models of simple to intermediate complexity that focus solely on Arctic oceanic carbon-cycle issues.

3. Overview and Analysis of the Ocean Carbon Sink

The ocean carbon sink gathers new carbon emissions and modifies the ocean’s inherent carbon reservoir due to climate change-driven shifts in temperatures, winds, and the freshwater cycle [90]. This biogeochemical process heavily relies on the concentration of carbonate ions in the ocean, which combine with CO₂ and H₂O to yield bicarbonate ions [91]. The higher the reabsorption of CO₂ from the atmosphere, the greater the transformation into bicarbonate ions through this process. This contributes to the ocean acidification [92]. Friedlingstein et al. [93] indicated that the land biosphere and ocean have absorbed over 50% of the anthropogenic carbon emissions. The residual 5% points to a budget imbalance, signifying an unaddressed difference between emission and sink projections. Land biosphere and ocean sinks, capturing around 25% of the carbon emissions, mitigate global warming and climate change. CO₂, the most critical GHG, significantly impacts the equilibrium of atmospheric temperature and the climatic framework through human-generated emissions [11].

The ocean carbon sink absorbs CO₂ from the atmosphere. The research has highlighted physical and biological factors driving ocean carbon dynamics [94,95]. The ocean carbon sinks are essential to the global carbon cycle as they regulate the absorption, retention, and release of CO₂ and other chemical compounds, influencing the climate over different timescales [96]. The goal is to stabilize carbon by absorbing anthropogenic CO₂ emissions and reducing climate change effects; however, this increases ocean acidity, hindering future CO₂ absorption [97]. The ocean is a carbon storage facility with forms like dissolved organic carbon, dissolved inorganic carbon (DIC), and particulate organic carbon. CO₂ interacts with seawater, producing carbonic acid that then transitions into bicarbonate and carbonate ions within the oceanic carbonate equilibrium [51]. Carbon ocean storage is a potential climate change solution. However, careful consideration of the associated risks is required. Unexpected ecological effects like ocean acidification and impacts on marine life are among the concerns [98]. The sea is instrumental in maintaining the Earth’s climatic balance. Its ability to absorb radiation, interact with the atmosphere, and influence the climate system through exchanges shapes the dynamics of the planet’s climate [99]. The scientific community is concerned about potential harmful effects due to limited research on disequilibrium. The debatable concept of carbon sinks is complicated by interlinked cycles contributing to global warming [100]. Ocean acidification reduces natural pH levels due to increases in DIC [101].

The Arctic regions and adjacent seas are acidifying rapidly due to substantial carbon sink effects, particularly in the North Atlantic Ocean, which are exacerbated by the higher uptake of anthropogenic CO₂, leading to increased ocean acidity [102]. The North Atlantic experiences the largest pH decreases, which will worsen with rising anthropogenic CO₂ emissions [2]. The North Atlantic Ocean’s inherent buffering system, distinct from other oceans, historically assisted in slowing acidification. Mixing between acidic deep water and less acidic surface water due to oceanic circulation, different from the North Pacific, has maintained a lower acidity in the deep Atlantic Ocean [103]. Recent Arctic Ocean studies showed major alternations in its marine carbonate system, highlighting acidification. Yet, understanding its acidity is challenging due to the system variability influenced by factors like temperature, ice, and freshwater. Despite natural fluctuations, Arctic Ocean acidity is predicted to persist due to ongoing carbon emissions throughout the century [104]. McVeigh [105] indicated a significant increase in western Arctic Ocean acidity, up to four times higher than other ocean basins, over 43 years. This increase aligns with rapid Arctic warming, known as “Arctic amplification”, driven by sea ice reduction. Sea ice loss speeds up ocean acidification, forming a feedback loop in the Arctic. Glacier melt dilutes seawater alkalinity, weakening its buffering capacity against the acidification [106]. Deep ocean acidity surpasses surface levels. Surface-dissolved CO₂ sinks with organic matter, collecting in the abyss and causing gradual bottom-up ocean acidification [103].

3.1. Challenges and Gaps

Over the past two decades, ocean carbon observations have substantially increased, encompassing various oceanic regions and time frames, spanning temporal and geographical variations, and extending from the ocean’s surface to its depths [107]. Certain marine carbon-cycling aspects, such as the inorganic carbon buffering system, are well-established research disciplines [108,109]. Other parts, however, pose challenges due to theoretical complexities and technological limitations. Detecting elevated carbon levels from human activity in the ocean requires indirect methods. International initiatives like EU Framework Programmes, OCB, PICES, SOLAS, IMBeR, and IOCCP have advanced ocean carbon research. Further projects and extensions (GEO/GEOSS, GOOS, FOO, ICOS, etc.) are needed to continue the effort [11]. Oceanic mesoscale circulation exhibits extended timeframes and short geographical scales differing from atmospheric patterns. Mesoscale motion can distort specific oceanic data, possibly misleading long-term mean conditions [110]. Arctic Ocean acidification accelerates due to climatic changes. Limited sampling hampers accurate CO₂ sink estimates [111]. Diminishing the Arctic Sea ice extent and changes in its yearly formation patterns will significantly impact the organic and inorganic carbon cycles, and marine flow patterns and turbulence [11]. The impact of summer without Arctic Sea ice on the ocean’s carbon sink behaviour remains uncertain, particularly in terms of sea–ice physics and biogeochemistry. Additional investigations into the potential release of greenhouse gases like CH₄ from Arctic Ocean sources due to tectonic and climate-induced degassing are needed [112].

3.2. Relevant Policies for Arctic Carbon Sink and Protection

Preserving marine ecosystems, reducing CO₂ emissions, and promoting sustainable ocean management are critical for maintaining ocean carbon sink effectiveness in the face of persistent climate challenges. Most economic studies on carbon sinks focus on terrestrial carbon sequestration within the context of the 1997 Kyoto Protocol, including activities like afforestation, forest management, and more. However, marine sinks’ scientific complexity and understanding, along with their significant carbon flows, have received less attention in the comparison [113]. In 2022, the ‘Super Year of the Ocean’, significant strides were made in ocean-climate action. At the UN Ocean Conference, governments agreed to reinforce science-based measures. The Ocean and Climate Change Dialogue, held in June, aimed to enhance marine and climatic efforts under the UNFCCC. A key outcome was urging ocean inclusion in the Paris Agreement, promoting innovation and action, and addressing the funding gap for ocean-related climate change adaptation. Nature-based solutions and technology, like early warning systems and coastal vegetation restoration, were considered practical approaches [114]. The 2022 COP27 in Egypt emphasized the ocean’s climate role, adopting the “Sharm el-Sheikh Implementation Plan” that highlights marine ecosystems as carbon sinks within mitigation strategies [115]. COP27 marked progress, yet greater political commitment and comprehensive strategies are needed for the climate crisis. The UNFCCC was urged to integrate ocean-based efforts into its climate plans. UNFCCC tackles ocean-related climate action with cross-agency and stakeholder collaboration. Overall, COP27 was viewed as a step forward, but more political will and holistic approaches are required in the face of the climate emergency. States were encouraged to include ocean-based actions in their national climate goals and implementation. The UNFCCC tackles ocean-related climate action through diverse agendas, processes, and collaborations with UN agencies and stakeholders. The Scientific and Technological Advice (SBSTA) discusses science-based ocean research and observation, while the Intergovernmental Panel on Climate Change (IPCC) provides scientific assessments and guidance on climate change mitigation and adaptation. The IPCC Special Report on the Ocean and Cryosphere in a Changing Climate emphasized the ocean’s role in climate regulation and CO₂ removal strategies [116]. Adaptation efforts incorporate ocean and coastal action into Nationally Determined Contributions (NDCs) and National Adaptation Plans (NAPs). The Nairobi Work Programme (NWP) centers on ocean area-specific impact assessments and adaptation. The Warsaw International Mechanism for Loss and Damage and Reducing emissions from deforestation and forest degradation in developing countries (REDD+) addresses coastal zones. Mitigation actions are conveyed via NDCs and national GHG inventories. The global stocktake of the Paris Agreement assesses collective ocean-related action, while the Local Communities and Indigenous Peoples Platform promotes engagement in addressing climate change, including oceans. The UNFCCC secretariat collaborates with stakeholders via the Global Climate Action work program [117].

Countries in the Arctic regions have introduced their own protection measures and regulations on carbon sink and carbon acidification problems, with Canada serving as an example. Canada has led in marine protection since 1997, notably with the Oceans Act. Canada has consistently progressed in promoting ocean conservation and sustainable growth through strategies and ambitious marine conservation goals. The Minister of Fisheries and Oceans spearheaded integrated management plans for estuarine, coastal, and marine environments, paving the foundation for the Oceans Strategy of Canada in 2002. This strategy emphasized integrated management to enhance marine ecosystems [118]. The 2005 Oceans Action Plan, built upon the Oceans Act, was a comprehensive strategy prioritizing ocean understanding, sustainable development, and ecosystem-based conservation to preserve marine biodiversity and productivity in Canada’s oceans. The plan led to the establishment of Marine Protected Areas (MPAs) by Fisheries and Oceans Canada (DFO), reinforcing Canada’s dedication to ocean conservation [119]. In 2011, DFO launched the National Framework for MPAs, aiming to establish a comprehensive network for safeguarding marine biodiversity and promoting sustainable management. Canada’s 2021 Blue Economy Strategy envisions fostering sustainable growth in its ocean sectors. It focuses on utilizing blue carbon ecosystems, investing in carbon sequestration and marine science, and promoting nature-based climate solutions and coastal protection to identify new ocean conservation areas [120]. Collaborative efforts involving provinces, Indigenous Peoples, marine industries, and academia drive Canada’s conservation goals. The aim is to conserve 25% of marine and coastal regions by 2025 and increase to 30% by 2030. By January 2023, 14.66% of Canada’s ocean territories, covering 842,822 km², was safeguarded through measures like Marine Protected Areas, National Wildlife Areas, National Marine Conservation Areas, and other area-based conservation initiatives [121]. Canada actively endorses the UN Decade of Ocean Science (2021–2030), advancing ocean science, sharing knowledge, developing infrastructure, and building relationships [122]. Canada’s commitment to ocean conservation and sustainable development protects marine environments through legislation, strategies, and collaboration. The US is exploring marine carbon dioxide removal (mCDR) to combat climate change. A 2021 report by the National Academies of Sciences, Engineering, and Medicine suggested a research strategy for ocean-based carbon removal. The US Senate and House of Representatives further introduced several carbon removal bills, highlighting the ocean’s role as a carbon sink, and these bills would create an “ocean carbon removal mission” [123]. In 2008, the US National Oceanic and Atmospheric Administration (NOAA) and Environment Canada signed a Memorandum of Understanding (MOU) for collaborative ocean research and knowledge exchange. In 2016, Canada, the US, and Mexico committed to increasing cooperation in ocean management and climate change ocean research. DFO and NOAA further held a joint meeting on ocean acidification in September 2016 [124,125]. DFO also engages in international initiatives like the Arctic Monitoring and Assessment Programme (AMAP) and the Global Ocean Acidification Observing Network (GOA-ON) [125]. The “Vancouver Declaration” in 2016 endorsed the Pan-Canadian Framework on Clean Growth and Climate Change (PCF), aligning with DFO’s scientific and precautionary approach [126]. The US Congress passed the Federal Ocean Acidification Research and Monitoring Act (FOARAM) in 2009, leading to the establishment of NOAA’s Ocean Acidification Programme (OAP) in 2011. The OAP aims to coordinate research, monitoring, and activities to understand the impacts of ocean acidification, supporting local management and adaptation practices. DFO and NOAA are developing a collaboration framework to enhance coordination in monitoring, research, modelling, and data sharing, focusing on Marine Protected Areas and National Marine Sanctuaries for observing ocean acidification’s long-term changes [127]. These laws reflect the increasing acknowledgment of oceans’ vital role in carbon sinking and mitigating ocean acidification. By enabling governments to strategize ocean carbon absorption, they progress toward a sustainable, climate-resilient future. Oceans are gaining deserved recognition as a potent ally against global warming, with nations integrating ocean-based solutions into climate policies.

3.3. Carbon Sink Modelling in the Arctic

Unlike extensive carbon-cycle models, few models specifically target carbon sink effects. This review showcases the various models used to explore carbon sink aspects and identifies gaps in their representation. Gooya et al. [128] stated that concentrating efforts on regions with active carbon sinks like the northwest Atlantic, Arctic, and Southern Oceans can minimize uncertainties in oceanic carbon sink modelling and anthropogenic CO₂ uptake. Like the reviewed carbon-cycle models,

Table 2. Carbon sink models.

No.	Model	Description	Advantages	Disadvantages	Modelling Applicability	Reference(s)
1	OSCAR v3.1	Simple box-type earth system model for climate projection	Open source Calibration through complex ESMs	Highly land focused Assumption of unchanged biological pump Undivided oceanic carbon uptake by region	No	[130,131]
2	Pathfinder v1.0	Simple climate model for sea-level rise and ocean acidification prediction	Open source Emphasis on oceanic carbon sink effects	Global effects only Only surface ocean acidification considered Exclusion of biological pump consideration	Partially, sea-level rise accounts for glacial melt effects	[132]
3	FAIR v1.3	Impulse-response carbon-cycle model for climate change factors (CO2 concentration, temperature change)	Open source Temporal carbon sink efficiency consideration Capable of emulating diverse emission scenarios	Omission of natural emissions Global emphasis with unified ocean representation	No	[133,134]
4	NESM v3	CIMP6 Earth System model for global carbon-cycle simulation	Regional representation of Arctic/North Atlantic areas Strong representation of biological pumps (NPP, marine ecosystem)	Modelling center requirement Overestimated biological uptake Underestimated arctic NPP Underestimated North Atlantic DIC Constrained to Baltic Sea	Yes, incorporates CICE 4.1 sea-ice model and Arctic observational data	[135]
5	ERGOM v1.0	Biogeochemical model for nutrient and carbon cycles in the Baltic Sea	Capable of illustrating regional effects	Only considers biological sinks	No, but includes a coupled sea-ice model for the Baltic Sea	[136]
6	MPIOM	Biogeochemical model for the sea-ice carbon pump	Regional Arctic representation Useful for carbon sink effect studies (DIC, AMOC strength)	Modelling center support requirement Oceanic model only	Yes, includes sea-ice models and is used for Arctic studies	[137,138]

displays the models’ various complexities, goals, and shared problems. Section S2 provides a detailed introduction. SCMs have a global focus, lacking the capability to study regional Arctic carbon sinks. Only ERGOM, a regional model, was identified, but it does not encompass holistic carbon sink effects or Arctic regions. Additionally, these simple models overlook the oceanic biological pump’s impact on carbon sink modelling. Among the reviewed simple models, OSCAR and Pathfinder utilize the oceanic carbon-cycle sub-model developed by Joos et al. [129], which offers the advantage of consistency with other models. However, when models share sub-models, the outcomes tend to be similar, giving an illusion of a wide range of distinct models. This is a similar problem encountered with ESMs using the same sea-ice modules, such as CICE. While larger ESMs (Earth System Models) include carbon sink modelling in the Arctic Ocean, the focus often centers on terrestrial carbon sinks. A clear gap exists in simplified, regional oceanic carbon sink models that consider all aspects of an oceanic carbon sink. This is important for the Arctic regions due to the strength of carbon sink effects observed in these regions. Although ESMs cover these aspects, they lack efficient policy exploration across multiple scenarios.

4. Impacts of Marine Transportation Pollution on the Arctic Oceanic Carbon Cycle

Approximately 80–90% of global trade relies on marine transportation, transporting over 10 billion tonnes of cargo and containers each year [139]. At the start of 2018, the global fleet included around 94,171 commercial vessels [140]. These vessels predominantly run on carbon-rich fuels like heavy diesel, leading to environmental harm due to emissions [141]. Marine transportation remains vital for climate change mitigation due to resource demands, global reliance, and the need for widespread carbon reductions [142]. According to the IMO, marine transport contributed to 2.2% of human-made emissions in 2012, decreasing from a peak of 2.8% in 2007 before the global economic collapse [143]. Passenger travel, military operations, fishing, marine resource utilization, and maritime support services such as navigation and ship maintenance all constitute marine transportation. Trade primarily dominates the industry, exerting the most significant influence, while multiple uses amplify the maritime transport field’s environmental footprint [144]. The growing marine transport sector presents challenges and opportunities for decarbonization efforts driven by expanding global trade markets. The ongoing demand for transportation implies a need for low-carbon fuels and alternative methods [144]. The central aim is to make these alternatives practical and scalable for significant industries like global marine transportation, driving continuous research and development investment to achieve this goal.

In 2018, the IMO established goals to curtail greenhouse gas emissions from maritime shipping. These goals mandate a 50% reduction in industry emissions by 2050 (compared to 2008) and the control of CO₂ emissions from ships [145]. A comprehensive assessment of marine shipping’s environmental impact must consider these factors holistically. Marine vessels commonly use bunker fuel, particularly diesel Bunker C fuel oil. This fuel emits substantial greenhouse gases, sulphur, and pollutants that adversely affect the environment and human well-being [142]. Liquefied natural gas (LNG) emits 15–29% less CO₂ than conventional marine fuels. It also reduces nitrogen oxides, sulphur oxides, and particles. LNG is projected to power 10% of the global shipping fleet by 2030 [142]. Fuel type, engine efficiency, and design impact emissions, with historically used fuels including heavy fuel oil (HFO), marine diesel oil (MDO), and marine fuel oil (MFO) [146,147]. Bulk carriers, oil tankers, and container ships produce the majority of shipping-related GHGs [148]. Faster ships emit less CO₂ than slower ones. While LNG reduces CO₂ emissions by around 25%, it produces more CH₄. HFO and MDO fuels emit similar GHGs, but LNG is considered a growing marine fuel [142].

4.1. Challenges and Gaps

Maritime shipping operates in complex transnational, intermodal, and international environments, leading to unique regulatory challenges [149]. Unlike aviation, maritime transportation lacks significant tax exemptions, impacting the industry’s environmental behaviour due to the role of taxes in driving pro-environmental changes [142]. However, industry-wide acceptance and unity are essential for the system’s success [150]. Various measures like emission regulations, voluntary agreements, environmental indexing, taxation, and tradable licenses have been used to varying degrees of success to limit GHG emissions from marine transportation [139,151]. Supporting measures like emission goals and carbon valuation offer avenues to curtail GHG emissions [152]. Regulators enact laws to limit shipping emissions, but actual compliance through retrofitting or innovation is essential [153]. Taxation is a top-down regulation by governments, using economic principles to influence stakeholders. Eco-friendly shipping could involve a tax on net carbon emissions, encouraging emission reduction while allowing flexibility in the metrics [153]. In contrast, private industry often forms voluntary agreements to collectively commit to specific objectives or restrictions. These actions can be detailed or adaptable based on the stakeholders’ capabilities, such as fleet size and transportation methods. These agreements mitigate concerns about financial impacts and enhance corporate branding by showcasing efforts toward a positive purpose [142].

Predicting future climate change relies on comprehending oceanic carbon-cycle dynamics, encompassing historical and projected trajectories [154]. In the last four decades, the Arctic ice has undergone a substantial reduction, leading to the emergence of new shipping routes and raising significant environmental concerns [155]. Furthermore, the ice in Arctic regions continues to present navigation challenges. Thinner and fragmented ice moves unpredictably, with changing ice hazards caused by climate change [156]. This transformation enables new trade routes in some areas while heightening ice-related risks in other fields [157]. Over seven years, shipping moved 300 km north and east, closer to the North Pole. More Arctic shipping affects the local ecosystem and carbon cycle [158]. Oceans absorb CO₂, aiding ecosystems, but excess CO₂ causes acidification and harms marine life. Shipping significantly contributes to these issues [159]. Moreover, marine shipping emits black carbon by burning fossil fuels, accelerating ice melting with increased heat absorption. A quarter of global shipping emissions, primarily from HFO, are black carbon pollution [160]. The IMO is shifting shipping from HFO to cleaner fuels for environmental and oceanic preservation [161]. Global fuel markets are adapting to IMO 2020’s sulphur restrictions cut from 3.50% to 0.50%, which is impacting the refining, shipping, and bunker fuel demands [162]. Other types of pollution can also indirectly increase carbon emissions, affecting the oceanic carbon cycle. For example, ship ballast water, crucial for voyage stability, can introduce invasive species upon discharge, disrupting marine ecosystems [163]. Accidental oil spills from marine accidents can catastrophically impact aquatic environments, posing a significant threat to the marine ecosystem [164].

The limited data hinders our understanding of how shipping affects Arctic Ocean carbon cycles due to industry complexity and ecosystem dynamics. The impact of shipping on the ocean carbon cycle is still uncertain but is recognized to contribute to ocean acidification and lower oxygen levels. Collecting data in remote Arctic regions presents challenges [165]. Data accuracy issues, inadequate reporting, and variations in vessel operations lead to discrepancies between reported and actual emissions. Accurate carbon emission estimates require complex calculations involving vessel types, fuel types, engine efficiency, load factors, and routes [166]. The spatial and temporal coverage of marine carbon flux observations must be expanded [167], and new data inclusion is crucial to enhance ocean biogeochemical modelling and climate predictions. Utilizing traditional knowledge and resources, collaborative policies for climate adaptation and marine mitigation can be developed [168].

4.2. Relevant Policies for Arctic Shipping

The extensive international legal framework for oceans works with domestic laws to govern maritime zones and waters [169]. The United Nations Convention on the Law of the Sea (UNCLOS), in effect since 1994, is crucial for maritime concepts, including those relevant to the Arctic region and environmental preservation [170]. The IMO’s Convention on Oil Pollution Preparedness, Response, and Cooperation (OPRC) and its HNS (Hazardous and Noxious Substances) protocol, in particular, play a key role in managing the carbon footprint of Arctic maritime activities by addressing marine pollution incidents and threats. These efforts directly contribute to controlling carbon emissions in the Arctic, thereby impacting the regional carbon cycle. OPRC parties must handle incidents and collaborate. Arctic states have bilateral and all-Arctic agreements for oil spill preparedness and responses [171]. Additional details on IMO policies related to marine shipping are provided in Section 2. Canada is a signatory to several international conventions related to marine transport, like the International Convention for the Prevention of Pollution from Ships (MARPOL) and the International Convention for the Safety of Life at Sea (SOLAS), essential for pollution prevention and safety in maritime operations [172,173]. Canada’s Arctic shipping regulations are shaped by global agreements like UNCLOS and IMO’s maritime laws framework [174]. Nationally, Canada enforces shipping laws through acts such as the Canada Shipping Act 2001, the Marine Liability Act, the Navigable Waters Protection Act, and the Marine Transportation Security Act [173]. Canada signed the 2010 HNS Protocol, addressing 6500 potentially polluting substances in the maritime transport [175]. Various IMO Conventions are incorporated into Canadian laws and regulations. The PAME 2021 report highlights Canada’s domestic strategies to mitigate environmental impacts and enhance Arctic shipping safety, aligning with IMO area-based protection measures. It also promotes discussions for future collaborations among Arctic states and stakeholders [176]. The Arctic Waters Pollution Prevention Act (AWPPA) safeguards Canadian Arctic waters by enforcing a ‘zero discharge’ policy, prohibiting waste disposal. It defines offences and penalties and empowers Pollution Prevention Officers for enforcement. The Arctic Shipping Pollution Prevention Regulations (ASPPR) and Arctic Waters Pollution Prevention Regulations (AWPPR) support the AWPPA, providing guidelines and requirements for pollution prevention in Arctic shipping and related activities [177]. Integrating the Polar Code into Canadian law in 2017 brought about substantial changes to Arctic shipping regulations in Canada. The Arctic Shipping Safety and Pollution Prevention Regulations implement the Polar Code in Canada, outlining icy condition risk assessment, reporting, and ice navigator requirements. These measures uphold Canada’s reputation for secure and eco-friendly Arctic shipping, enhancing safety and pollution prevention standards [65]. Canada is diligently establishing a network of Arctic Marine Protected Areas (MPAs) to safeguard biodiversity, ecosystem function, and unique natural features, reflecting the government’s commitment to preserving Arctic ecosystems [178]. Canada’s 2016 Oceans Protection Plan (OPP), focusing on creating a top-tier marine safety system, prioritizes economic growth and coastline protection and indirectly supports efforts to manage carbon emissions in the Arctic region. This is vital considering the significant role of maritime shipping in the global carbon cycle, especially in sensitive regions like the Arctic [179]. The OPP involves Indigenous organizations managing Arctic navigation via Minimal-Impact Maritime Routes, driven by the Canada–US Joint Arctic Leaders’ Statement to integrate Indigenous perspectives into shipping policies [180]. Collaboration among Arctic states is crucial for managing the environmental impacts of Arctic shipping, including its influence on the Arctic carbon cycle. By working together and integrating best practices into shipping policies, Arctic states can significantly reduce carbon emissions, thereby preserving the Arctic’s unique ecosystem and maintaining the balance of its carbon cycle for future generations.

4.3. Carbon-Based Maritime Transportation/Pollution Modelling

Global shipping’s extensive reach and pollution rates significantly impact the carbon cycle. This influence can be direct, as shipping contributes to atmospheric CO₂, or indirect, through pollutants that affect radiative forcing by sulphur oxides, alter primary production processes, and impact the efficiency of the oceanic biological pump. These pollutants can lead to changes in ocean chemistry and biological activity, further influencing the carbon cycle. The following models explore the climate impacts of black carbon and pollutants from shipping. Climate models considering maritime shipping impacts and their translation to carbon cycle feedback are scarce, revealing a gap in modelling. This presents promising avenues for future research.

Table 3. Climate-based maritime shipping pollution models.

No.	Model	Description	Advantages	Disadvantages	Modelling Applicability	Reference
1	MIROC-ES2L	Earth System Model for long-term climate impact on nitrogen cycle and ocean nutrients	Inclusion of black carbon effects in oceanic biological pump Black carbon input in atmospheric module	Pollutant consideration in NPP of plankton only Black carbon iron excludes atmospheric chemistry interaction No exclusive marine shipping black carbon quantification	Yes, global results of iron uptake are due to black carbon emissions	[183]
2	CNRM-CM3.3	Transportation-induced climate impact general circulation model	Non-CO2 pollutant effect consideration Sea-level rise linked to shipping CO2 emissions Pollution-induced AMOC changes	Partial carbon-cycle simulation by model	Partially, results are reported by latitude	[181]
3	OsloCTM2	Atmospheric circulation model for radiative forcing from shipping emissions	Ship-type-based analysis of shipping pollution Traffic-driven emission atmospheric distribution consideration	Partial carbon-cycle simulation by model No oceanic components in the model	No, only global temperature results are provided	[182]

shows a comparison of the models, while detailed introductions of selected models are given in Section S3. While this is a relatively small sample of models, several observations can still be drawn. Two of these models, CNRM-CM3.3 [181] and OsloCTM2 [182], sourced a wide array of shipping pollution data which could distinguish pollutants by the transportation sector and type of ship. This suggests that the lack of shipping pollution carbon-cycle research is not due to a data shortage. MIROC-ES2L [183] is a model that includes a carbon-cycle code and does not differentiate between the sources of black carbon, allowing for a separate examination of the impacts of maritime shipping. Furthermore, only OsloCTM2 included ship distribution data within the model, implying that incorporating traffic data could be complex. Interestingly, each of these models focuses on non-CO₂ shipping pollutants. MIROC-ES2L analyzed the impact of iron from black carbon emissions and sulphur and phosphorous compounds on net primary production and the biological pump, showing that iron availability increased phytoplankton and carbon uptake, decreasing atmospheric carbon concentrations. OsloCTM2 further demonstrated that CO₂ reduction policies contribute to an overall warming effect in the short term (<25 years) due to reductions in non-CO₂ pollutants that have negative radiative forcing effects. From the numerous climate change impacts investigated by Olivié, Cariolle, Teyssèdre, Salas, Voldoire, Clark, Saint-Martin, Michou, Karcher, and Balkanski [181], such as global temperature and sea-level rises, it is evident that shipping pollution has a quantifiable effect on the climate. It is curious why few models correlate these impacts to the carbon cycle. Shipping pollution modelling could benefit from linking these impacts to carbon-cycle feedback.

5. Conclusions

The review highlights the underexplored oceanic processes in the carbon cycle, with an emphasis on the Arctic region. Compared to terrestrial studies, research on the carbon cycle in the Arctic Ocean, especially focusing on its deep-water carbon sequestration in cold conditions, is markedly insufficient but is essential for understanding the global carbon sink. Based on an in-depth analysis, this review has identified several key research gaps that are pivotal for guiding future directions:

(1) Observational Data Shortfall: The notable lack of observational data in the Arctic Ocean, especially in real-time data collection, when compared to terrestrial carbon-cycle studies, underscores an urgent need for a more comprehensive understanding of the oceanic carbon cycle. This shortfall is primarily attributed to limited research infrastructure and funding constraints in the Arctic region.

(2) Data Collection Challenges: Data collection in the Arctic, whether in situ or via remote sensing, faces unique challenges due to the region’s remoteness and harsh environmental conditions. Notably, the absence of real-time Argo profiling floats, due to sea ice cover, complicates distinguishing between snow, ice, and clouds.

(3) Quantification of Anthropogenic Emissions: Despite the oceans’ significant role in absorbing anthropogenic carbon emissions, there is a clear deficiency in the study of the oceanic carbon cycle. Key gaps include the lack of methods to directly quantify anthropogenic emissions in the marine environment and uncertainties in measuring air–sea carbon fluxes, particularly considering changes in sea-ice melting rates.

(4) Marine Transport Emissions: The research predicts trends in marine transport emissions, identifying the North Atlantic high seas as one of the top regions for rapid emission growth, in stark contrast to the Arctic region, where emissions are almost negligible.

(5) Policy Gaps: The review also underscores the policy gaps impacting effective emission reduction strategies. These include a significant reliance on voluntary and market-based emission control measures in many policies, the need for regulations specifically tailored to harsh environments, limited sharing of environmental impact data, and insufficient engagement with indigenous communities.

(6) Carbon-Cycle Modelling Gaps: The gaps identified include the absence of specialized regional SCMs (simple climate models) for the Arctic Ocean, over-reliance on oceanic sub-modules in ESMs (Earth System Models), limited availability of Arctic Ocean climate data sets for model initialization, and a scarcity of socio-economic analyses in carbon-cycle research.

The following actions are recommended to address these gaps:

(1) Enhanced Research Infrastructure: Investment in advanced research infrastructure and increased funding for the Arctic region to support comprehensive observational data collection.

(2) Innovative Data Collection Methods: The development and deployment of innovative data collection methods, including real-time Argo profiling floats adapted for icy conditions, to overcome the unique challenges the Arctic environment poses.

(3) Improved Quantification Techniques: Advancement in techniques for directly quantifying anthropogenic emissions in the marine environment and improved methods for measuring air–sea carbon fluxes.

(4) Regulatory and Policy Improvements: Development of stringent regulations tailored explicitly to harsh Arctic environments, and to induce increased sharing of environmental impact data and enhanced engagement with indigenous communities. Additionally, shifting from voluntary and market-based emission control measures to more robust regulatory frameworks is necessary.

(5) International Cooperation and Public Awareness: Strengthening international cooperation and increasing public awareness of climate and carbon-cycle policies to foster a collaborative approach to addressing the identified research gaps.

Addressing these gaps presents significant opportunities for advancing future research, particularly in developing regional models. More detailed discussions on modelling are provided in Section S4. This review highlights the evident shortcomings in Arctic oceanic carbon-cycle research. Addressing these gaps is crucial for advancing our understanding of climate change and enhancing our preparedness to tackle these challenges in the future.