Pathway
Water
Carbon removal capacity: theoretical potential > 10 Gt CO2/year
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Covering 70% of the Earth's surface, oceans store more carbon than the atmosphere and terrestrial biosphere combined. Oceans have absorbed a quarter of our emissions over the past 150 years and buffered 90% of the excess heat in the atmosphere². This has come at a cost for marine ecosystems: oceans have become 30% more acidic, which has degraded almost 50% of tropical reef systems³. Warmer waters have reduced natural ocean circulation, preventing the flow of nutrients rich water from ocean depths that sustain marine biota. The vast amount of available space and the absence of land use competition mean marine CDR pathways could provide an opportunity to sequester CO₂ at billion ton (Gt) scale.
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The natural carbon cycle has been unbalanced by human activity. CO₂ is cycled between the air to the ocean at 330 Gt CO₂ a year. Humans emit around 40 Gt CO₂ from fossil use and land-use change each year. Oceans have buffered this increase of CO₂ sequestering an additional 9 Gt CO₂ a year. A Gt CO₂ is a billion tonnes of CO₂. [Adapted from Global Carbon Project⁴]
Ocean CO2 is stored in coastal ecosystems, as living marine biomass, in sediments on the seabed, as organic carbon and dissolved inorganic carbon. Approaches for increasing the oceans capacity to hold carbon spans from biological, to chemistry-based and engineering solutions. Biological approaches can include blue carbon restoration. Restoring the 7 million hectares of degraded wetlands has the potential to sequester 1 gigaton of CO2 by 2050⁵. Ocean alkalinity enhancement (OAE) approaches use basic chemistry by introducing minerals that react with CO2 forming bicarbonate or carbonate ions. OAE approaches could both store carbon securely and reduce ocean acidification. Ocean engineered solutions face the challenge of withstanding tough offshore environments.
Currently there are large knowledge gaps limiting our ability to assess the full capacity, risks, co-benefits and costs of many ocean-based carbon removal. Public acceptability is one of the largest constraints on the feasibility of ocean-based approaches. Nonetheless, given its sheer size and dynamics, the ocean offers a promising opportunity to sequester CO2 at scale.
Practices:
Biological approaches:
Macroalgae cultivation
Microalgae cultivation
Coastal wetland restoration
Iron fertilization
Chemical approaches:
Ocean alkalinity enhancement
Coastal enhanced weathering
Hybrid approaches:
Direct ocean capture
Deep sea storage
Artificial upwelling/downwelling
Benefits:
Ocean based carbon removal practices do not compete for space on land.
A co-benefit potential exists to reduce ocean acidification and improve ocean health.
CO2 that reaches the deep ocean/ocean floor can be stored for millennia.
Blue carbon restoration projects can boost marine biodiversity, protect land from coastal erosion and support local communities.
Issues We Care About:
To understand the potential of ocean-based carbon removal, we need reliable methods for blue carbon measurement, verification and reporting.
Under-developed research landscape means uncertainty about the environmental impacts of some practices.
To ensure that CO2 reaches the deep oceans and is stored safely and durably.
Sources
1. https://oceanvisions.org/roadmaps/
2. Special Report on the Ocean and Cryosphere in a Changing Climate, 2019
3. Ocean Acidification, 2012
4. Clobal Carbon Budget, Friedlingstein et al. 2020
5. Project Drawdown, solutions
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