Climate action requires new accounting guidance and governance frameworks to manage carbon in shelf seas
Accounting guidelines exist for the recording of carbon flows in terrestrial and coastal ecosystems. Shelf sea sediments, while considered an important carbon store, have yet to receive comparable scrutiny. Here, we explore whether effective management of carbon stocks accumulating in shelf seas could contribute towards a nation’s greenhouse gas emissions reduction targets. We review the complexities of carbon transport and fate in shelf seas, and the geopolitical challenges of carbon accounting in climate governance because of the transboundary nature of carbon flows in the marine environment. New international accounting guidance and governance frameworks are needed to prompt climate action.
Integration of natural capital into physical and economic accounts is being examined by many countries worldwide1. An important aim of such integrated accounts is to support the implementation of the UN Sustainable Development Goals (SDGs) and multilateral environmental agreements such as the UN Framework Convention on Climate Change (UNFCCC). Carbon sequestration, commonly defined in the natural sciences literature as the processes of carbon capture and storage, is an important element of such accounts because it may contribute towards a nations’ greenhouse gas emissions reduction contributions within the 2015 Paris Agreement and previous obligations under the UNFCCC Article 4 (Parties commitments to mitigate climate change)2.
Vegetated terrestrial systems such as forests sequester, capture and store carbon in their biomass and the soil beneath them. A similar process occurs in the marine environment where vegetated marine systems, such as salt marshes, mangroves and seagrass meadows, capture and store carbon. Not all carbon fixed by these systems (i.e. turned into biomass) becomes stored in the soil where it was produced; a fraction is eventually transported and stored (i.e. buried) in coastal or offshore shelf sea sediments. Shelf sea sediments not only store detritus generated by terrestrial and coastal vegetation, but also carbon inputs generated from phytoplankton productivity and other carbon sources (e.g. macroalgae) throughout the shelf seas and adjacent ocean environment. However, we still do not know what fraction of the accumulating organic matter, herewith termed as particulate organic carbon (POC), is derived from each of the potential sources listed above. Wide regional variations in the composition of the accumulating sediment are to be expected due to multiple and divergent organic matter sources3,4,5. Information is lacking also on the amount of organic carbon produced that becomes ultimately buried in shelf sea sediments.
Terrestrial vegetated ecosystems, including coastal ecosystems, which are classified as managed lands and which sequester carbon, are recognised by the UNFCCC through various governance mechanisms6. Marine ecosystems, in contrast, are less well represented and managed7. General obligations stated in Article 4 of the UNFCCC require all Parties to compile inventories of emissions by sources and removals by sinks based on specific Intergovernmental Panel on Climate Change (IPCC) guidelines. Given the sediments’ organic carbon density (8.8 mgC cm−3) and the large area of shelf seas (Table 1), which is ~7% of the global marine area8,9, and considering that they are a potentially manageable carbon store, we suggest that carbon sequestration in shelf sea sediments should be considered within the scope of both IPCC inventory and environmental–economic accounting methodologies. Proper management of the carbon currently sequestered into these invisible stores, in fact, may play a significant role in mitigating climate change. We consider the opportunities, challenges and benefits of the preservation of shelf sea sedimentary carbon as a contribution to climate change mitigation by considering the following critical questions: How is ownership of carbon stocks in shelf sea sediments distributed? How should these carbon stocks be measured and registered in national carbon accounts? What arrangements can be made to ensure the conservation of the carbon stocks?
This review outlines the uncertainties about carbon sources in the marine environment and carbon transport, and the potential scale of carbon storage in shelf seas, and then examines the potential governance and management of this carbon. Current evidence suggests that, due to transport of carbon in organic matter (i.e. POC) by water currents, the long-term burial of carbon in shelf sea sediments may occur in different territorial waters to those in which it is produced (Fig. 1). This has important consequences for the physical and economic accounting of carbon.
We explore options to address the complexities of organic carbon production, transport and distribution in shelf seas within the technicalities of physical and economic accounting systems for carbon reporting looking at (i) common pool resources10 and (ii) the governance and management of carbon sequestered in shelf sea sediments.
We summarise the most relevant insights from biogeochemistry, environmental economics, governance and management of carbon in the marine environment. These elements are all fundamental to understand: (i) the fate of carbon in the marine environment; (ii) the national physical inventory and economic accounting of this carbon; (iii) the economic value of carbon sequestration in the marine environment; and (iv) the national and international governance for the sustainable management of this natural resource. We use the well-studied North Sea11 as a real-world example to illustrate the complexities involved. We draw this material together to show the scale of the carbon stores in the marine environment and contemplate how they might be incorporated into carbon management schemes in order to contribute to meeting the 2015 Paris Agreement goals of limiting the global temperature rise well below 2 °C, possibly below 1.5 °C2.
Carbon stocks in shelf sea sediments
The world’s oceans currently take up as much as 25% of anthropogenic carbon dioxide (CO2) emissions (1.9 × 1015 g y−1)12,13. While other gases such as nitrous oxide and methane contribute to climate change, we only focus on the fate of CO2 in the marine environment because CO2 emissions from anthropogenic activities contributed the greatest proportion to the increase of greenhouse gases over the period 1970–201014,15. The CO2 is stored throughout the water column where it can be isolated from air–sea exchange for periods of decades to centuries, while other storage occurs via carbon burial in marine sediments.
Most CO2 emitted by human activities is trapped within the ocean and stored in the oceanic water column with a residence time of 100–1000s of years16. The remaining carbon is stored in marine sediments with residence times of between 100s and millions of years. In ocean sediments on the shelf slope (200–1000 m), or those deeper than >1000 m water depth, storage is extensive17,18.
Shelf sediments, defined here as those deposited in <200 m water depth, are less extensive (7.6% of the global marine area)9 but globally sequester as much carbon as tropical forests (Table 1). Shelf sediment stores are vulnerable to human activities such as trawling, marine mining and oil and gas exploration, and <200 m water depths have the potential to release CO2 into the atmosphere within a year of their disturbance, assuming that the water column is well mixed. The interface between the shelf and land is the location for shallower coastal wetland ecosystems, such as mangrove, tidal marsh and seagrass meadows. Their areal extent is relatively small (<1% of the global area)19,20,21, but they accumulate and store the most carbon per unit area19. Because of their locations and characteristics, they are most vulnerable to anthropogenic disturbance and any CO2 released can be directly emitted into the atmosphere20,22. While there are IPCC guidance for the management of some coastal wetlands ecosystems in national GHG inventories, there is currently a lack of any guidance for regions beyond the coastal zone which would be applicable for management of shelf sea sediments. This is predominantly because of a lack of mapping of the sedimentary environments and the need for further scientific evidence to support the effect of management.
Every year, the primary productivity of coastal plant communities and phytoplankton in surface waters, as well as the delivery of terrestrial organic matter results in seasonally and annually variable flux of carbon to the sea floor in the form of POC3. In cases where there is either restricted transport of carbon inputs by coastal circulation or where continental shelves lie predominantly within the domain of a single country, the carbon can be deposited locally within the jurisdiction of one country. This is the case, for example, on the west coast of the US, the northern Chinese margin seas and Siberian shelf seas. In other cases, depending on the local topography, hydrography and physical characteristics of the water column where the carbon has arrived from or been produced, the carbon can be transported for many hundreds of km and move across national borders where continental shelf areas are shared and along pathways of water circulation and transport. This is the case in the North Sea where UK, Belgian, Dutch, German and Danish borders are all within close proximity of each other, and so water moves rapidly through national waters (Fig. 2). Whether the carbon fractions remain within or move across shared boundaries will also depend on timescales of transport and hydrographic features such as residual currents, stratification and wave events, which dictate surface or near bed transport, or other features such as seasonal jets23,24,25,26,27,28,29. On shelves, residual currents are typically in the order of 10−2 to 10−1 m/s, resulting in transport distances of a few km to a few tens of km per day. Particulate transport becomes different and difficult to predict when hydrodynamic conditions are quiet enough, e.g. at neap tides, or during quiet weather, for POC to settle temporarily to the seabed. A proportion may get buried by physical processes or biological activity, and biologically processed until long-term burial or it is remobilised by an erosion event30,31,32. These different scenarios in the transport rates and bed interaction of POC illustrate that transit times across boundaries can be days to years depending on hydrography (including seasonal and inter annual variability), sedimentology, biogeochemical processes and proximity to boundaries. From a carbon storage and Exclusive Economic Zone (EEZ) point of view, an approach could be to, for a particular case, identify different zones where, in comparison to burial rate, transport of mobile POC (in the water column or as bed-load) is likely to be quick, intermittent, slow or very slow to negligible. Such areas would have different status in terms of the transport of POC generated in situ (or moved in from elsewhere) and hence associated sensitivity to anthropogenic disturbance in the sense of carbon storage.
We focus on the North Sea example to show how challenging accounting for governance and management of carbon stocks in the marine environment can be. However, worldwide, most other shelf seas or continental shelf margins that are bordered by multiple countries present similar challenges.
The fate of carbon is driven by hydrodynamics and sedimentary processes. Throughout their vertical and horizontal transport, different carbon fractions can be remineralised, depending on their resistance to decomposition, the hydrodynamics and the biological communities present. The proportion of each of these sources that is delivered to the sediment, as well as the rate at which they accumulate and are stored, can vary with environmental conditions. In the North Sea, one of the main well-established current patterns carries water and POC anti-clockwise (Fig. 2), from the north-west along the UK and northern European coast to the main location of burial in the deep Norwegian trench11,26,33. Thus, depending on the regional geomorphology and hydrography, carbon removed from the atmosphere in one location (e.g. one specific country territorial sea or EEZ), may be processed during transport, causing the release of CO2, sedimentation of carbon or advection, with the remaining carbon being ultimately stored in a different location (e.g. another specific country EEZ). All shelf sea systems are important stores of organic carbon, but the sites of storage within these systems varies widely with their different geographic settings18,34,35. Storage of riverine or coastal vegetative material is influenced by shelf width and circulation36. The North Sea is an example of a geological passive margin with a wide shelf. Convergent plate margins (for example Taiwan), by contrast, create narrow shelves where fluvial or coastally derived carbon may be exported off shelf and stored at depths >200 m37.
When organic matter reaches the sea floor as POC, whether produced locally or advected there, it contributes to carbon that has already accumulated there. Some of the organic matter will be fresh and/or easily decomposed, while some will be refractory or older and more resistant to decomposition31,38. Various physical and biological mechanisms lead to the incorporation and eventual burial of the deposited organic carbon30,31,39,40,41. Thus, the amount of carbon stored in marine sediments for significant periods of time depends on the detrital carbon concentration in the water, accumulation rate in the sediment, controls on remineralisation rates and bio-physical pathways within the bed17,42. Direct or indirect disturbances of the seabed or changes in water column conditions can affect many of these processes and thus the overall carbon stock, storage rates and capacity17,30,43,44,45. Anthropogenic activities in the territorial waters of a country containing significant carbon stocks could result in the release of carbon that has been undisturbed for centuries or longer. Such carbon could subsequently be re-deposited locally, transported by currents and deposited elsewhere, or be remineralised, potentially leading to CO2 release into the atmosphere. The likelihood of this release occurring depends on the environmental conditions that generated that stock. The fate of this carbon (i.e. whether it is retained within a nation’s EEZ) will ultimately be controlled by biologically and physically mediated transport processes. Thus, features like fjords may enhance local deposition, submarine canyons may enhance off shelf transport43,46 and strong current flows may enhance long range transport.
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