Carbon Dioxide Removal

The human-driven extraction and permanent removal of the greenhouse gas CO₂ from the atmosphere to reverse climate change.

Defining Carbon Dioxide Removal

Carbon dioxide removal, CO₂ removal or carbon removal refers to the human-driven extraction of the greenhouse gas CO₂ from the atmosphere combined with its permanent storage. The goal of carbon removal procedures is to decrease atmospheric carbon dioxide levels and to mitigate or even reverse man-made global warming. Known carbon dioxide removal techniques include:

  • Afforestation (planting trees): Trees naturally sequester carbon dioxide to convert into what they are made of – biomass.
  • Direct air capture and storage: Chemical processes are used to separate CO₂ from ambient air. The separated gas is injected into underground geological formations and prevented from escaping with physical and geochemical trapping (turning the gas into stone).
  • Enhanced weathering: Geoengineering approaches that use the dissolution of natural or artificially created minerals to remove CO₂ are referred to as enhanced weathering.
  • Blue carbon: Like trees, the world’s ocean ecosystems – mangroves, salt marshes, seagrasses and algae – are sequestering carbon dioxide through plant growth.
  • Carbon farming: Agricultural methods aimed at sequestering atmospheric carbon into the soil, crop roots, wood and leaves are catagorised as carbon farming.
  • Ocean fertilisation: Here purposeful introduction of nutrients to the upper ocean increase marine food production and remove carbon dioxide.


Carbon Dioxide Removal with Trees

Trees, like all plants, naturally sequester carbon dioxide (CO₂) to convert into what they are made of - biomass. Plants convert CO₂ as they grow over time - the bigger the tree, the more CO₂ was sequestered by that tree. This is why afforestation - the conversion of bare or cultivated land into forest - is a carbon dioxide removal method. If the tree dies and decomposes or is burned, all the biomass is converted back into CO₂, which is released back into the atmosphere. Thus for carbon sequestration through trees, it is important that those trees survive and are not cut down and burned.

CO₂ Sequestration Rate of Trees

The sequestration rate differs between tree characteristics like species - mangroves for example, sequester up to 4 times more CO₂ than other rainforest trees; geographical location of the tree - tropical regions sequester more than cold regions; and age - young trees can’t grow as fast as older trees and therefore sequester less.

On average young trees (up to 10 years old) absorb about 6 kg of CO₂ per year or 0.5 kg of CO₂ per month. From 10 years onwards they absorb about 22 kg of CO₂ per year or 1.8 kg of CO₂ per month 1.

Direct Air Capture and Storage

Defining Direct Air Capture

The direct air capture, abbreviated as DAC, technology uses chemical processes to separate the CO₂ from the ambient air. This is a still young technology and there is a growing number of companies trying to commercialise it including Climeworks - one of our partners - and Carbon Engineering.

Defining Direct Air Capture and Storage

The separated gas is then injected into underground geological formations, called geological storage, and prevented from escaping with physical and geochemical trapping. The advantage of this process is that a huge amount of CO₂ is removed in a short period of time without needing a lot of space or water however as a very new technology it is currently very expensive.

  1. Urban Forestry Network - Trees Improve Our Air Quality ↩︎

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