Ocean Currents

Throwing the Ocean Carbon Sink at the Climate Crisis

Confronting climate change by preserving and enhancing the living ocean carbon sink

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Fishes living in the middle depths of the ocean might be responsible for transferring as much as 10% of carbon from the surface waters to deep ocean © Amanda Cotton

This blog was co-written by guest blogger, Oswald Schmitz. Oswald is Professor of Ecology in the Yale School of the Environment. He is a former member of Ocean Conservancy’s Board of Directors and continues to serve in a science advisory capacity. His current research and conservation interests center on developing nature-based climate solutions.

There is something exhilarating about seeing a whale announce its return from the ocean depths by blowing a plume of warm breath into the air. It marks the end of a dive cycle in which these gentle giants descend to the ocean depths to capture prey and return to the ocean surface to catch breaths of fresh air and rest. And, while resting near the surface, they also release another kind of plume into the water in the form of fecal matter. These fecal plumes are rich in nitrogen and iron providing marine algae living in the sunlit surface waters the supplies of nutrients essential to drive photosynthesis, and thereby convert carbon dioxide (CO2) that has been absorbed into the water from the atmosphere into “living carbon.”

The oceans are replete with plants and animals that can help mitigate climate change by removing and retaining a portion of CO2 from the atmosphere. For example, marine algae, the ocean’s mightiest CO2 consumer yet, is also one of its smallest, converting upwards of 42 gigatons of the gas into living plant biomass each year. This is a huge number, approximately 1.5 times more CO2 than all that is released in the emissions from fossil fuel burning by the top 20 emitting countries globally. But not all of that carbon stays in the ocean, otherwise, a big piece of our climate change problem would be solved. But we can take steps to help maximize carbon retention in the ocean.

The next decade presents a formidable challenge to reducing the build-up of greenhouse gasses in the atmosphere by 7.6 percent per year if we hope to keep the planet from warming more than 1.5°C above pre-industrial levels and avoid irreversible harm. Arresting human-caused emissions by decarbonizing the economy is a key and immediately obvious solution. However, enlisting the roles of marine species and sediments as part of a portfolio of “natural carbon solutions” may further help us to lower the risk of overshooting the 1.5°C threshold. Indeed, research on animal effects in a wide variety of terrestrial, freshwater and marine ecosystems reveals that animal species greatly influence carbon uptake and storage within those ecosystems. Clearly, there is much untapped potential in the world’s living creatures.

As an advocacy organization, Ocean Conservancy works to identify and advance science-based solutions to environmental threats. The organization has been exploring the extent to which implementing nature-based ocean strategies are a realistic—and impactful—option for mitigating climate change. Ocean Conservancy has interviewed more than a dozen scientists and policy experts to dive deeper into this opportunity. These experts agree that the ocean has a vital role to play in mitigating climate change. But they note that there are still considerable hurdles that need to be overcome at the nexus of science and policy to better understand the opportunities and limitations for integrating natural ocean carbon sequestration into a policy portfolio for mitigating climate change. Here are a few examples:

  • Marine sediments are a huge repository of organic carbon. But agitating sediments exposes the carbon to oxygen, resulting in some of its conversion back to CO2 that could circulate back up to the ocean surface and be released to the atmosphere. Scientific analyses are still needed to quantify the amount of carbon that is reactivated by bottom trawling, deep-sea mining and similar activities in different parts of the ocean so that countries can prioritize and protect areas that hold the highest storage potential.
  • Biodiversity primes the carbon pump. An ecosystem with a wide variety of marine life and a healthy balance of predators and prey species may improve the capacity of the ocean to take up, store and reuse carbon. A case in point is the food chain in which sea otters that prey on sea urchins prevent the urchins from devouring coastal marine kelp forests, which normally draw down, store and transfer CO2 to the deep ocean. A better understanding of other such food chain effects— and biodiversity in general—on carbon storage, especially in the deep ocean, is needed to develop effective policy to conserve marine biodiversity in service of climate goals.
Oceanic Blue Carbon
Oceanic blue carbon includes the carbon stored through the actions of marine life, from krill to whales. Understanding how marine vertebrates contribute to carbon storage can help recognize their potential role in climate change mitigation. © GRID Arendal
  • Fishes are a critical component of the ocean carbon pump machinery. Recent estimates show that since 1950 industrial fisheries have removed 318 million metric tons of certain large-bodied fish such as tunas and mackerels. This is equivalent to releasing 37.5 megatons of fish-embodied CO2 from the ocean. And this estimate may only be the tip of the iceberg. Fishes living in the middle depths of the ocean might account for as much as 10% of carbon transfer from surface waters to the deep ocean. This role of ocean biology is something that ocean carbon models do not yet adequately incorporate. Furthermore, there needs to be an accounting of the large amount of carbon contained in calcite, the hard-mineral body parts that fish create, which sink to the deep ocean and accumulate in seafloor sediments. The global carbon bound up in calcite produced by fish and echinoderms is equivalent to the amount of carbon emitted from the burning of fossil fuels by the countries of Brazil, the UK and Australia.
  • Marine protected areas (MPAs) may be able to protect ocean carbon stores. Protected areas were originally developed to help ensure breeding stocks of fishes and other species remained healthy and could provide a sustained supply of catchable fish outside of the protected area boundaries. But MPAs may also serve a new purpose. By preventing human activities, such as industrial fishing or seafloor disturbance, enhanced marine biodiversity and undisturbed marine sediments could more effectively store carbon within MPAs. But the amount of carbon stored in existing marine protected areas is still largely unknown because these tools are currently not considered a potential carbon sink solution.
  • Certain fishing strategies can help enhance ocean carbon storage or avoid carbon loss. Rebuilding fish stocks, reducing fish discards and avoiding bottom trawling in areas with carbon-rich sediments can retain and perhaps increase carbon stored in ocean systems. Going forward, fishery management strategies and catch rates could be tuned to balance the provision of edible protein with the removal of carbon sequestration services. Implementing such an approach depends on appropriate carbon pricing and devising equitable ways to compensate fishermen for contributing to carbon storage.

In addition, through these interviews with experts, we have identified two concrete actions that would advance the field and address key disconnects in international governance:

  • The Intergovernmental Panel on Climate Change (IPCC) provided guidance on the inclusion of wetlands in national greenhouse gas inventories in 2013 and the IPCC’s Sixth Assessment Report Working Groups (WG) are now assessing the state of knowledge on ocean carbon sinks and carbon sequestration by natural and engineered systems more generally (WGI, WGII, WGIII). We recommend a special focus on the 6th Assessment report being written now (or a special report by IPCC in the 7th Assessment Report cycle) to assess the most recent evidence on biological carbon storage, the role of biodiversity and greenhouse gas inventory methods relevant to natural systems. This synthesis of evidence and existing methodologies will support decision-makers’ work to integrate the ocean into climate mitigation policy, including into Nationally Determined Contributions.
  • Further action is needed to create coherence across ocean and climate regimes, a major topic of discussion at the recent United Nations Framework Convention on Climate Change (UNFCCC) Dialogue on the Ocean and Climate Change mandated by the last UN climate summit. Toward this objective, we recommend a “bridging agreement” be co-developed by the UNFCCC Paris Agreement, Convention on Biodiversity Diversity and United Nations Convention on the Law of the Sea. Such an agreement is needed to recognize and protect the role of ocean life in drawing down ocean carbon and avoiding carbon loss in the future. This bridging mechanism would formally acknowledge and enhance the common interest of each agreement by requiring biodiversity protections in the ocean as a climate mitigation strategy. Ideally, this policy bridge would be complete before the IPCC synthesizes the science on nature-based carbon storage in either the 6th or 7th Assessment reports and the collective goal-setting by each entity begins.

Stabilizing the climate will require throwing everything we have at the crisis, including a societal transition away from fossil fuels to renewable energy. But preserving and enhancing natural carbon sinks, including those on land and in the ocean, must be part of this global effort. The ocean alone cannot solve the climate crisis but the ocean can be a vital part of the solution if humanity strives to protect the marine biodiversity and habitats that are most important to drawing down and storing the carbon found in the living ocean.

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