The deep ocean can be used for carbon dioxide removal from our atmosphere. Here are the opportunities, risks and constraints. By Phil Williamson, University of East Anglia The ocean and climate are inextricably linked. More than 90 percent of the extra heat from global warming has gone into the deep blue sea, with associated climate-driven changes reducing the productivity of fisheries and threatening more than 600 million people through sea-level rise. Yet the ocean offers opportunities: it could also be a source of solutions rather than just a victim of climate change. One possibility is and storing it in the deep. Doing so on a large scale would help deliver the net zero goal of the Paris Climate Change AgreementBut considerations beyond just the financial commitments of research and development, costed at US$2.5 billion, will need to be taken into account. Either biology or chemistry could be used to remove carbon dioxide from the atmosphere to store it in the ocean. The most straightforward approach is a ‘nature-based solution’, restoring the natural functions and processes already provided by our environment. Protecting marine ecosystems by ending overfishing and other conservation actions can reinstate the role of top predators in marine food webs, particularly those of the deep ocean — found in depths beyond 200 metres. The resulting increase in the abundance of larger organisms, such as fish and whales would transfer organic carbon from surface waters to deeper layers through faeces and deadfall, slowing the return of that carbon to the atmosphere. The need to end overfishing through improved ocean protection is internationally recognised by UN Sustainable Development Goal 14, and associated targets of the Convention for Biological Diversity (CBD) and the World Parks Congress. The main problem in using this approach for climate mitigation is that the additional carbon removed from the atmosphere on a long-term basis is likely to be relatively small. The removal also cannot be easily attributable to any single country. Whilst ocean ecosystem restoration is undoubtedly worthwhile for the many non-climatic benefits it can provide, signatories of the Paris Climate Change Agreement are required to communicate their emission reduction strategies and quantify their national contributions. In theory, much greater CO2 drawdown is possible if the growth of marine phytoplankton (microscopic floating algae) can be increased by ocean fertilisation, for parts of the ocean where dissolved nutrients in seawater are in short supply. Directly adding relatively small amounts of biologically-available iron, or larger amounts of nitrogen and phosphorus, would stimulate phytoplankton blooms, converting inorganic carbon dioxide to plant biomass. Pumping up nutrient- rich water from the deep using underwater pipes, referred to as artificial upwelling, could also be used to achieve the same effect. These ideas were first suggested in the early 1990s, and have been investigated by patch-scale field experiments and modelling studies, however, many key issues remain unresolved. These include: whether fisheries might benefit or suffer if ocean productivity is increased in this way; whether there might be other damaging side effects; for example, reducing productivity elsewhere, potentially many thousands of kilometres away (linked by ocean circulation patterns) and the uncertain over the next 50-100 years. Concerns that there might be damaging side effects resulted in a non-binding moratorium on ocean fertilisation in 2008, followed by more detailed regulation of marine geoengineering — defined as “ .” Marine geoengineering is now limited to strictly-controlled research under the Convention on the Prevention of Marine Pollution by Dumping of Wastes and Other Matter, referred to as the London Convention and London Protocol (LC/LP). There are also further restrictions in the Southern Ocean, where iron additions would potentially have greatest climatic effectiveness under various agreements. The large-scale cultivation of seaweed (macroalgae) is another biological option for ocean-based climate mitigation. Seaweed photosynthesis would remove carbon dioxide from seawater, followed by managed long-term storage of carbon fixed in its biomass. This approach would be most efficiently carried out in shallow waters, scaling-up the cultivation and harvesting procedures of existing widely used for food production in China, South Korea and Japan. Cultivation in deeper water has also been proposed, but requires a very large area (of or more than 40 percent of global shelf seas) for climatically-significant carbon removal. Deliberate sinking of the harvested seaweed to the deep ocean seafloor would then provide carbon storage.