Carbon Dioxide Removal types

Carbon dioxide removal (CDR) techniques are long-term strategies to remove CO2 from the atmosphere and store it on land or in the ocean. 

To curb the current upward trend of global mean temperature until a stabilisation is achieved,  reducing GHG emissions is certainly necessary, but not sufficient. Whether through engineered or natural methods, the removal of carbon dioxide from the atmosphere could bring us closer to the ambitious goal of achieving a climate-neutral world by the middle of this century. 

While there are many  benefits associated with CDR techniques to mitigate climate change, many open questions still exist about their effectiveness, possible downsides, and potential risks.

Ocean Alkalinity Enhancement (OAE)

Increasing ocean alkalinity to expand the carbon storage capacity of seawater

PREMISE

Ocean’s inner alkalinity naturally converts dissolved inorganic CO2 in seawater into bicarbonates and carbonates. The resulting CO2 deficit in seawaters is continuously rebalanced by the absorption of atmospheric CO2 in the ocean. This process occurs naturally over geologic times. 

AMBITION

Artificial addition of alkaline substances to seawater (for instance, the spread of large amounts of alkaline minerals such as olivine and basalt over the open ocean or beaches) may thus increase the capacity of the ocean to sequester atmospheric CO2, while reducing, at the same time, the acidification of the ocean.

Direct Air Carbon Capture and Storage (DACCS)

(Electro)chemically removing CO2 from air

PREMISE

Carbon dioxide can be absorbed directly from the atmosphere using sorbents with a strong affinity for CO2 or organic materials attached to a porous solid support. The carbon absorbed needs to be subsequently stored at geological sites.

AMBITION

DACCS has the potential to provide a significant contribution to global CDR. The high-energy demand at the moment represents the main limitation to the rapid scaling of DACCS plants.

Bioenergy with Carbon Capture and Storage (BECCS)

Burning biomass to generate energy, while capturing and storing CO2

PREMISE

Biomass is any renewable organic material that comes from plants and animals, as well as organic waste from our homes. When growing, biomass naturally extracts carbon from atmospheric CO2. Biomass can be used for combustion, fermentation, or other conversion methods to generate bioenergy (electricity, heat, biofuels, etc.).

AMBITION

Carbon is captured during the conversion of biomass to energy and subsequently stored into geological storage locations or used in the production of inert materials (e.g. concrete). The energy produced has thus the potential of resulting in net-zero CO2 emissions.   

Afforestation / Reforestation (A/R)

Planting / restoring forests to increase long-term carbon storage in biomass and soil

PREMISE

Forests are natural carbon reservoirs. Through the process of photosynthesis, trees and plants produce oxygen by absorbing CO2 from the atmosphere and accumulating it as carbon in vegetation and soils. The more trees and plants there are, the more CO2 is absorbed.

AMBITION

To maximise the ability of forests to sequester CO2, the extent of forested land on the planet must increase. Afforestation (converting abandoned and degraded lands into forests) and reforestation (replanting trees in deforested lands) are therefore natural ways of lowering the amount of CO2 in the atmosphere.

Marine biomass (macroalgae) cultivation

Storing CO2 into organic matter

PREMISE

Macroalgae species (also known as kelp or seaweed) are fast growing primary producers that usually thrive in the shallow waters of coastal areas due to the abundance of both light and nutrients. Because of their efficiency, these algae can store large amounts of carbon into organic matter.  

AMBITION

Macroalgae can be farmed in coastal areas but also on appositely built platforms in the open ocean. The biomass produced can be either used as a source for energy production with carbon capture (BECCS), transformed into relatively stable forms (e.g. biochar) or sunk into the deep ocean where, after natural decomposition, carbon remains stored for thousands of years.

Ocean iron fertilisation

Sequestering CO2 by stimulating phytoplankton growth

PREMISE

Iron is an indispensable nutrient for oceanic primary production. However, iron is scarce over immense areas of the ocean where, nonetheless, other nutrients are relatively abundant. This results in a limitation of primary production and thus of the ability of the ocean to absorb carbon from the atmosphere.

AMBITION

Adding iron to iron-limited regions of the ocean stimulates phytoplankton growth, accelerating CO2 sequestration from the atmosphere. 

Soil and Land Carbon Management

Increasing carbon retention and storage in soils

PREMISE

Soil and land are natural carbon reservoirs. CO2 is naturally sequestered from the atmosphere through photosynthesis and stored in the form of soil organic carbon (SOC), which is also highly beneficial for the soil’s health and fertility.

AMBITION

Improving soil and land management could incentivise the retention of carbon as SOC. This approach doesn't require land use conversions (e.g., afforestation, reforestation), but rather implies converting more soils and lands into growing plantations.

Enhanced weathering on land

Carbon vegetation uptake with alkaline minerals and nutrient supply

PREMISE

Weathering is the deterioration of rocks, soils, and minerals on the surface of the Earth through contact with water, atmospheric gases, and biological organisms. In this process, CO2 is removed from the atmosphere by reaction with alkaline minerals converting CO2 dissolved in natural waters into carbonates and bicarbonates.

AMBITION

By applying finely ground minerals over the land surface, the weathering process is accelerated favouring the absorption of CO2 in natural waters. At the same time, other elements released during the mineral dissolution could stimulate biological productivity, further enhancing the storage of carbon on land. 

Artificial upwelling

Mechanically pumping deep ocean water to the surface to increase CO2 storage.

PREMISE

Upwelling is a natural process where the winds and the rotation of the earth naturally determine the motion of dense, cold water from the deep ocean to the upper ocean layers. Deep water is rich in nutrients and stimulates phytoplankton growth while sequestering CO2 from the atmosphere.  

AMBITION

By mechanically pumping nutrient-rich deep ocean water to the surface, the carbon flux from the air-sea-interface to the ocean’s depth can be increased and the CO2 sequestration enhanced.

Blue carbon enhancement

Increase carbon sediment burial with mangroves and wetlands.

PREMISE

Mangroves, tidal marshes, seagrass meadows are tropical trees that grow in coastal ecosystems. They naturally sequester and store significant amounts of carbon from the atmosphere and ocean, therefore called ‘blue’ carbon. 

AMBITION

Enhancing the growth of coastal habitats would increase the sequestration and embedding of blue carbon in ocean sediments.