ISA Fact-check 2024/1 – The carbon cycle in the Area

July 2024

This ISA Fact-check presents a collection of relevant information concerning factors involved in the complex interplay of potential deep-sea mining on carbon cycle processes.

The oceans are facing numerous pressures, such as pollution and climate change mainly originating from land-based activities. Carbon dioxide (CO₂) is a key greenhouse gas that has been accumulating in Earth’s atmosphere at a growing rate, mainly due to the burning of fossil fuels. This contributes to climate change, which brings with it an increase in extreme weather events, acidification of the Earth’s oceans and rising sea levels.

In a recent case heard by the International Tribunal for the Law of the Sea on the obligations of State Parties to the United Nations Convention on the Law of the Sea (UNCLOS) to protect and preserve the marine environment in relation to climate change impacts, anthropogenic greenhouse gas emissions were considered to fall within the definition of “marine pollution” under UNCLOS.^[1] In that sense, States Parties to UNCLOS have specific obligations to take all necessary measures to prevent, reduce and control marine pollution from anthropogenic greenhouse gas emissions and to endeavour to harmonize their policies in this connection (UNCLOS, Article 194, para. 1).

The global carbon cycle is a fundamental component of Earth’s climate system, describing the movement of carbon among the Earth’s atmosphere, oceans, biosphere and geosphere. Understanding this complex cycle is crucial for comprehending the impacts of climate change. The ocean’s surface layer plays a key role by sequestering a quarter of the annual anthropogenic emissions, amounting to 8.7 Gt CO₂ or 30.54 Gt carbon per year (34.9 Gt CO₂ in 2021).^[2] Marine flora, such as phytoplankton, use sunlight to absorb CO₂ and convert it into carbon-based organic matter through photosynthesis. This organic matter is then consumed by marine fauna, playing a pivotal role in global biogeochemical cycles. Coastal ecosystems, such as mangroves, seagrasses and salt marshes, are also vital in capturing and storing carbon with a sequestration capacity 10 times greater than that of tropical rainforests.^[3]

Less than 1 per cent of the CO₂ sequestered in the ocean’s upper layers reaches the deep sea floor annually, amounting to approximately 0.30 gigatons of carbon per year, which is equivalent to 0,35 per cent of the annual global emission in 2021.^[4] As the carbon-based organic matter sinks to the bottom of the ocean, much of it is processed before reaching the ocean floor.

In the upper layers of the deep-sea sediment, microbes and fauna will consume carbon-based organic carbon. At the same time, the remainder becomes buried in an ever-growing sediment layer over 300 metres thick.^[5] This burial is an integral part of the global carbon cycle occurring over millions of years and results in the long-term storage of carbon in the deep-sea.^[6],[7]

Understanding and assessing the potential environmental effects of activities in the Area is a key part of the precautionary approach that the International Seabed Authority applies to carry out its mandate to ensure the effective protection of the marine environment from potential harmful effects due to activities in the Area (UNCLOS, Article 145). As such, any potential interference by deep-sea mining with processes of the global carbon cycle is an important factor to consider.

[1] International Tribunal for the Law of the Sea. 2024. Case no. 31: Request for an advisory opinion submitted by the Commission of small island States on climate change and international law advisory opinion. Available at: https://www.itlos.org/fileadmin/itlos/documents/cases/31/Advisory_Opinion/C31_Adv_Op_21.05.2024_orig.pdf.

[2] Liu, Zhu., Deng, Zhu., Davis, Steve. Ciais, Philippe. et al. Monitoring global carbon emissions in 2021. Nat Rev Earth Environ 3, 217–219 (2022). Available at: https://doi.org/10.1038/s43017-022-00285-w

[3] NOAA. What is Blue Carbon? Available at: https://seagrant.noaa.gov/how-we-work/topics/blue-carbon/#:~:text=Blue%20carbon%20refers%20to%20carbon,in%20the%20sediment%20and%20biomass.

[4] Deep Sea Mining and the Global Carbon Cycle available at: Deep Sea Mining and the Global Carbon Cycle (arcgis.com)

[5] Divins, D.L., Total Sediment Thickness of the World’s Oceans & Marginal Seas, NOAA National Geophysical Data Center, Boulder, CO, 2003. Available at: https://ngdc.noaa.gov/mgg/sedthick/sedthick.html.

[6] Sarmiento, Jorge L., and Nicolas Gruber. 2006. Ocean Biogeochemical Dynamics. Princeton University Press.

[7] Divins, D.L., Total Sediment Thickness of the World’s Oceans & Marginal Seas, NOAA National Geophysical Data Center, Boulder, CO, 2003. Available at: https://ngdc.noaa.gov/mgg/sedthick/sedthick.html.

1/ Carbon storage and cycling in deep-sea sediment

The recovery of polymetallic nodules from the deep-sea floor is likely to impact the upper layers of the sediment to some extent, depending on the technology used. Due to lower productivity and a reduced input of organic matter, deep-sea sediments have an overall low organic carbon content of approximately 0.05 per cent of the dry weight of the sediment. This is significantly less than nearshore sediment environments. For example, organic carbon averaged 5 per cent across mangrove sites.^[8],[9],[10] Most of the carbon stored is not accessible by living organisms for biological processing, which could result in resuspension because it gets sequestered in deep-sea sedimentary rocks.^[11]

Considering these factors, it is inferred that a substantial effect on the global carbon cycle is unlikely given the vastness of the ocean and the fact that exploration areas only cover around 0.4 per cent of the Area, with areas subject to exploitation likely to be considerably less than that.^[12] However, to better understand the impacts, it is crucial to refine our understanding of carbon storage mechanisms in the deep sea. This can be achieved through advanced biogeochemical modelling, such as that undertaken by the Horizon Europe project Ocean-ICU, which is set to conclude in 2027.^[13]

The current literature acknowledges that bacteria play a key role in the short-term cycling of carbon in deep-sea sediment.^[14] Any carbon resuspension resulting from bacterial activities tends to remain near the seabed due to the pressure from the overlying water column. Scientists are currently investigating these processes and estimating the potential amount of carbon resuspension from prospective mining activities.

It is possible that deep-sea mining activities could, at a local scale, impair the carbon cycling ecosystem function by bacteria within the directly impacted area. The restoration of these natural processes is not fully understood. Some studies suggest it may require long recovery periods, while others indicate it takes over 50 years to return to undisturbed levels.^[15],[16] Monitoring this impact is crucial for ecosystem recovery. However, the localized nature and relatively small scale of potential disturbances make a substantial impact on the global carbon cycle unlikely.

[8] Khripounoff, Alexis, Jean-Claude Caprais, Philippe Crassous, and Joël Etoubleau. 2006. “Geochemical and Biological Recovery of the Disturbed Seafloor in Polymetallic Nodule Fields of the Clipperton-Clarion Fracture Zone (CCFZ) at 5,000-m Depth.” Limnology and Oceanography/ 51 (5): 2033–2041. doi:10.4319/lo.2006.51.5.2033.

[9] Burdige, David J. 2007. “Preservation of Organic Matter in Marine Sediments: Controls, Mechanisms, and an Imbalance in Sediment Organic Carbon Budgets?” Chemical Reviews 107 (2): 467–485. doi:10.1021/cr050347q.

[10] Suello, Rey Harvey, Simon Lucas Hernandez, Steven Bouillon, Jean-Philippe Belliard, Luis Dominguez-Granda, Marijn Van De Broek, Andrea Mishell Rosado Moncayo, et al. 2022. “Mangrove Sediment Organic Carbon Storage and Sources in Relation to Forest Age and Position along a Deltaic Salinity Gradient.” Biogeosciences 19 (5): 1571–1585. doi:10.5194/bg-19-1571-2022.

[11] Hilmi, Nathalie, Ralph Chami, Michael D. Sutherland, Jason M. Hall-Spencer, Lara Lebleu, Maria Belen Benitez, and Lisa A. Levin. 2021. “The Role of Blue Carbon in Climate Change Mitigation and Carbon Stock Conservation.” Frontiers in Climate 3 (September). doi:10.3389/fclim.2021.710546.

[12] Orcutt, Beth N., James A. Bradley, William J. Brazelton, Emily R. Estes, Jacqueline M. Goordial, Julie A. Huber, Rose M. Jones, et al. 2020. “Impacts of Deep‐sea Mining on Microbial Ecosystem Services.” Limnology and Oceanography Rom 65 (7): 1489–1510. doi:10.1002/lno.11403.

[13] Cordis. Ocean-ICU Improving Carbon Understanding. Available at : https://cordis.europa.eu/project/id/101083922.

[14] Sweetman, Andrew K., Craig R. Smith, Christine N. Shulse, Brianne Maillot, Markus Lindh, Matthew J. Church, Kirstin S. Meyer, Dick Van Oevelen, Tanja Stratmann, and Andrew J. Gooday. 2018. “Key Role of Bacteria in the Short‐term Cycling of Carbon at the Abyssal Seafloor in a Low Particulate Organic Carbon Flux Region of the Eastern Pacific Ocean.” Limnology and Oceanography 64 (2): 694–713. doi:10.1002/lno.11069.

[15] Haeckel, Matthias, Iris König, Volkher Riech, Michael E. Weber, and Erwin Suess. 2001. “Pore Water Profiles and Numerical Modelling of Biogeochemical Processes in Peru Basin Deep-Sea Sediments.” Deep-Sea Research. Part 2. Topical Studies in Oceanography/Deep Sea Research. Part II, Topical Studies in Oceanography 48 (17–18): 3713–3736. doi:10.1016/s0967-0645(01)00064-9.

[16] Vonnahme, T. R., M. Molari, F. Janssen, F. Wenzhöfer, M. Haeckel, J. Titschack, and A. Boetius. 2020. “Effects of a Deep-Sea Mining Experiment on Seafloor Microbial Communities and Functions after 26 Years.” Science Advances 6 (18). doi:10.1126/sciadv.aaz5922.

2/ Carbon sequestration in midwater ecosystems

The ocean’s water column is stratified into several distinct layers, each characterized by unique physical, chemical and biological properties. Midwater ecosystems extend up to 1,000 metres deep and encompass layers where phytoplankton conduct photosynthesis and marine fauna consume sinking organic matter, serving as a crucial food source for deep-sea organisms.

In most potential exploitation activities, a discharge of water and sediments containing metal compounds from the dewatering of the ore is likely to occur. While specific metals are vital for phytoplankton growth, elevated concentrations can pose toxicological effects. Consequently, experts highlight carbon sequestration by phytoplankton as one of the key ecosystem services potentially impacted by activities in the international seabed area.^[17]

Scientists emphasize the importance of considering midwater ecosystems when evaluating the potential environmental risks of deep-sea mining. To mitigate potential impacts, they recommend delivering dewatering discharge below depths of 1,500 to 2,000 metres in the so-called ‘mesopelagic/bathypelagic transition.’^[18] This approach aims to safeguard phytoplankton photosynthesis and the sinking of organic matter

[17] ISA. 2023. Report on the value of ecosystem services and natural capital of the Area. Available at: https://www.isa.org.jm/wp-content/uploads/2023/06/Report-on-Valuation-of-ecosystem-services.pdf.

[18] J. C. Drazen, K. M. Gjerde, H. Yamamoto. 2020. “Midwater ecosystems must be considered when evaluating environmental risks of deep-sea mining.” PNAS 117 (30) 17455-17460. https://doi.org/10.1073/pnas.2011914117

3/ Emission of greenhouse gases

This topic has not received significant attention in research. Actual data from commercial-scale deep-sea mining operations are unavailable due to the fact that there are no ongoing mining activities. It is understood that the mineral processing of polymetallic nodules will be the primary source of greenhouse gas emissions, accounting for up to 99 per cent of the total emissions, which is on par with those from metal production from terrestrial sources.^[19] Additionally, the combustion of traditional fuels on-board mining and transport vessels will contribute to emissions. Scenarios for these emissions have been developed based on factors including engine loads, specific fuel oil consumption and transport speeds.^[20]

To comprehensively address these climate impacts, they should be evaluated in comparison to traditional shipping and similar land-based mining activities. One effective method is through life cycle assessments, which provide a holistic environmental analysis by considering the entire value chain of the products. Although only a few researchers have applied these methodologies, expanding this research would be valuable.^[21],[22] Future studies should rely on clear data assessment and robust analytical approaches, especially if the industry advances.

[19] Strategy Group Marine Mineral Resources of the German Marine Research Consortium (KDM). 2023. “Deep Sea Mining and the Global Carbon Cycle”. Available at: https://arcg.is/015v092.

[20] Heinrich, Luise, Pradeep Singh, Karen Smith Stegen, and Till Markus. 2024. “Mind the Gap and Close It: Regulating Greenhouse Gas Emissions from Deep-Sea Mining in the Area.” Marine Policy 160 (February): 105929. doi:10.1016/j.marpol.2023.105929.

[21] Alvarenga, R.A.F., N. Préat, C. Duhayon, and J. Dewulf. 2022b. “Prospective Life Cycle Assessment of Metal Commodities Obtained from Deep-Sea Polymetallic Nodules.” Journal of Cleaner Production 330 (January): 129884. doi:10.1016/j.jclepro.2021.129884.

[22] Paulikas, Daina, Steven Katona, Erika Ilves, and Saleem H. Ali. 2020. “Life Cycle Climate Change Impacts of Producing Battery Metals from Land Ores versus Deep-Sea Polymetallic Nodules.” Journal of Cleaner Production 275 (December): 123822. doi:10.1016/j.jclepro.2020.123822.

4/ Access to deep-sea minerals

The 28th Conference of the Parties of the United Nations Framework Convention on Climate Change called on States to triple renewable energy capacity globally. A total of 113 governments pledged to bolster the clean energy transition.^[23] In this context, the International Energy Agency reported that in a scenario that meets the Paris Agreement goals, the total demand for minerals is likely to rise significantly over the next two decades: by over 40 per cent for copper and rare earth elements, 60-70 per cent for nickel and cobalt and almost 90 per cent for lithium.^[24] Mineral resources from the deep sea can play a role in securing this supply for a future sustainable energy mix.

[23] Reuters. 2023. Countries promise clean energy boost at COP28 to push out fossil fuels. https://www.reuters.com/sustainability/climate-energy/over-110-countries-set-join-cop28-deal-triple-renewable-energy-2023-12-02.

[24] IEA. 2021. The Role of Critical Minerals in Clean Energy Transitions, IEA, Paris. Available at: https://www.iea.org/reports/the-role-of-critical-minerals-in-clean-energy-transitions.

In summary, the relationship between potential deep-sea mining and the carbon cycle is complex. Therefore, adopting a holistic approach is crucial when assessing its potential impact within the context of climate change. While no significant global effect on the carbon cycle is anticipated, it remains essential to monitor ecosystem parameters to detect any changes in the services provided by the deep sea and its ecosystems in relation to the carbon cycle. Mandatory baseline surveys conducted by exploration contractors and comprehensive environmental impact assessments are vital in this regard. Moreover, addressing cumulative impacts related to climate change can be effectively achieved through qualitative modelling within established environmental planning tools. This comprehensive approach ensures a thorough evaluation of potential effects.