Carbon dioxide removal (CDR), CO₂ removal or carbon removal refers to the human-driven extraction of the greenhouse gas CO₂ from the atmosphere combined with long-term storage. The goal of carbon removal procedures is to decrease atmospheric carbon dioxide levels and to mitigate or even reverse man-made global warming.
Assessment of removal methods
To qualify as carbon removal method we evaluate possible solutions based on the following criteria:
|Net negative emissions||The removal method must capture and store more carbon dioxide than is produced by it's construction and operation.|
|Long term storage||Captured CO₂ must be locked away for the long-term. Any captured gas that is used for industrial purposes (e.g. for carbonated drinks) is released back into the atmosphere, therefore not negative emissions.|
|Additionality||The process must not have taken place anyway without the purchasing of the carbon removal. Essentially meaning that this method only works because we are purchasing from it.|
These points are generally calculated and assessed in complex life-cycle analysis - that is often verified by independent third parties - so that we can be sure a removal method matches the above criteria.
Making sure negative emissions really are negative
All negative emissions purchased result from after the removal of any emissions from the construction and operation of a method. e.g. if a removal method sequesters 8 tonnes of carbon dioxide but it needed to generate 3 tonnes to do so, only the remaining 5 tonnes are available to purchase.
There is no "silver bullet" or single method that has the potential to reverse the volume of CO₂ that has been emitted over the last centuries. We believe in supporting a variety of approaches to remove carbon and aim to make as many as possible available through the CDR Platform.
Trees naturally sequester carbon dioxide to convert into what they are made of - biomass. As long as the tree stands (i.e. does not burn or die and decompose), the carbon dioxide is locked out of the atmosphere. Trees also have the added benefit of providing natural ecosystems that can support local wildlife and communities.
We must be careful to plant only native species (no mono-cultures) and ensure that there is no deforestation happening before planting trees.
How many trees?
We purchase the planting of trees but how do we turn this into kg of CO₂ removed?
- We use an average CO₂ sequestration rate of 10.875kg of CO₂/tree/year.
- We only take the first 5 years of the trees' carbon sequestration into account. What gets sequestered after this is a gift to our planet and we don’t sell that.
- To mitigate chances of loss due to disease or fires we double plant (meaning we buy double the number of trees that we need to).
Therefore for every two trees we purchase we assign a removal value of 54.375kg (10.875kg * 5 years).
More information on the logic and references behind these numbers is available via our sister project - Carbon Removed.
Direct air capture and storage (DACS)
Direct air capture uses large fans to pull ambient air over filters that capture carbon dioxide. This highly concentrated carbon dioxide can then be stored to produce negative emissions.
Storage can be pumping the gas underground into empty caverns however these need to be monitored to make sure the CO₂ does not leak back into the atmosphere. An alternative way is to pump CO₂ into basalt rock where it naturally reacts, turning into solid stone - naturally stable this does not need any long-term monitoring.
By seeding natural buoys with macroalgae (seaweed/kelp) and floating them into the ocean, the macroalgae can grow in the surface waters and capture carbon dioxide via photosynthesis. Over time the macroalgae grow, eventually getting so heavy that they sink to the deepest depths of the ocean. Here the pressure from the water slows/stops decomposition so the CO₂ remains locked away for thousands of years.
Bio-oil underground injection
Waste biomass is typically burned or left to decompose resulting in the already captured CO₂ returning to the atmosphere. However by heating the waste-biomass to high temperatures (>500ºC) without oxygen - a process called "fast pyrolysis" - it is possible to turn it into bio-oil.
Bio-oil is a liquid similar to crude oil but due to certain qualities is a poor fuel substitute. But by pumping it into large, underground wells it can make excellent geological storage keeping the carbon locked away for millennia.
Enhanced weathering (olivine distribution)
The mineral olivine reacts naturally with atmospheric CO₂. When amplified this process is a carbon dioxide removal method called "enhanced weathering". It involves grinding olivine to a fine sand (to increase the surface area), spreading the sand (for example on walking paths) and waiting.
Over time the olivine reacts with water and the air to sequester CO₂ and produce stable silicates.
Like bio-oil, biochar is produced by heating biomass without oxygen (pyrolysis) where it turns into a solid, coal-like substance. The produced biochar can then be used to lock away carbon dioxide in long-term storage.