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Carbon Dioxide Removal: An Overview of Methods

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Carbon Dioxide Removal (CDR) projects are becoming increasingly important as we work to combat the climate crisis. These methods, which can be grouped into three categories - biological carbon removal, ocean-based carbon removal, and geological carbon removal - all aim to remove carbon dioxide from the atmosphere and store it in a safe and stable form. In this article, we'll take a look at each of these categories in more detail, and explore the different methods that fall under them.

Biological Carbon Removal

Biological carbon removal methods involve using plants and other organisms to remove carbon dioxide from the atmosphere. These methods include:

  • Afforestation and reforestation: This involves planting trees in areas that were previously without forests. Trees absorb carbon dioxide as they grow, and store it in their wood, leaves and roots.
  • Carbon farming and soil carbon sequestration: This involves using farming practices that promote the growth of healthy soils, which can store more carbon. This can include techniques such as cover cropping, conservation tillage, agroforestry, and regenerative grazing.
  • Agroforestry: This involves integrating trees into agricultural systems to provide multiple benefits, including carbon sequestration.

Ocean-based Carbon Removal

Ocean-based carbon removal methods involve using the ocean to remove carbon dioxide from the atmosphere. These methods include:

  • Blue carbon: This involves protecting and restoring coastal ecosystems such as mangroves, salt marshes, and seagrass beds, which are highly efficient at capturing and storing carbon.
  • Ocean fertilisation: This involves adding nutrients to the ocean to encourage the growth of phytoplankton, which absorb carbon dioxide through photosynthesis.
  • Ocean alkalinity enhancement: This involves adding substances such as lime or crushed silicate rocks to the ocean to make it more alkaline, which can promote the storage of carbon in the deep ocean.
  • Kelp sinking: This involves cultivating kelp and allowing it to sink to the deep ocean seafloor where it can sequester carbon for centuries.

Geological Carbon Removal

Geological carbon removal methods involve storing carbon dioxide underground, for example in depleted oil and gas reservoirs or in deep saline aquifers. These methods include:

  • Enhanced weathering: This involves using natural processes to speed up the weathering of silicate rocks, like olivine, which can remove carbon dioxide from the atmosphere.
  • Carbon mineralization: This involves converting carbon dioxide into solid minerals such as calcite or magnesite.
  • Direct air capture and storage: This involves capturing carbon dioxide directly from the air and storing it underground.
  • Bioenergy with carbon capture and storage (BECCS): This involves growing energy crops such as switchgrass, willow or poplar, burning them to generate electricity, and capturing and storing the resulting carbon dioxide emissions.
  • Biooil and biochar production and storage: This involves creating liquid biofuels or charcoal-like materials through the pyrolysis of biomass. These materials can be stored underground, preventing the carbon they contain from returning to the atmosphere.

All these methods are currently under research, some of them are still in their early stages, while others are being tested and implemented at small scale. Each method has its own advantages, disadvantages and challenges. However, with continued research and investment, these CDR methods have the potential to play a significant role in helping to reduce and remove carbon from the atmosphere.