Oceans

Oceans

Carbon removal capacity: theoretical potential > 10 Gt CO₂/year¹.

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.

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.

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. 

• If CO₂ reaches the deep ocean/ocean floor it can be stored for millennia. 

• Blue carbon restoration projects can boost marine biodiversity, protect land from coastal erosion and support local communities.

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

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. IPCC Special Report on the Ocean and Cryosphere in a Changing Climate, 2019

3. Ocean Acidification, 2012

4. Global Carbon Budget, Friedlingstein et al. 2020

5. Project Drawdown, solutions