Biochar is a carbon-rich substance made by heating biomass in an oxygen-free environment, a process called pyrolysis. Biochar can be made from dedicated biomass grown for the purpose, or from sewage and food waste. 

Durability
Added to soil, a large percentage of biochar should last for hundreds to thousands of years (with high pyrolysis temperatures giving greater durability).
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Length of storage
‍Decades, centuries, millennia 
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Speed of drawdown
‍Immediate, 1-3 years, 3-5 years 

‍Co-benefits:
Trials have proved that it improves soil health and increases agricultural yields for many crop types. It can also improve air quality, and provide income for local communities. 

Enhanced Rock Weathering (ERW) accelerates the natural carbon-absorbing chemistry of silicate rocks. Grinding rocks such as basalt into fine fragments creates vastly more surface area for chemical reactions to take place.
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Durability
ERW could be highly scalable. Depending on the minerals in the rocks, it can store a high percentage of carbon rapidly, in one to two years, or more slowly, over a couple of decades

‍Speed of drawdown
‍1-2 years, or a couple of decades

Co-benefits
Spread on agricultural land, the ground-up rock provides nutrients that can increase crop yields. It stores carbon in the form of stable carbonate minerals that will eventually be washed into rivers and the sea.

Soil carbon sequestration (SCS), or carbon farming, increases the ability of agricultural soils to hold carbon. Adding compost and manure and planting extra sets of crops, known as cover crops, can absorb extra CO₂ across the year. Less tilling can help to trap that carbon in the soil. 
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‍Durability
‍Durability may be low, as these practices must be constantly maintained to ensure that organic matter does not release its carbon store. However, durability can be increased to many centuries by adding microbes that mineralise some of the carbon.

Length of storage
‍Several years, centuries

Co-benefits & potential side effects
Soil carbon sequestration can improve the quality of soils, and raise agricultural yields.

Biomass burial is a way to ally with plants to create more durable carbon storage in both land and sea. ‍On land, burying wood can keep it safe from fire and rot, storing the CO₂ rather than it being emitted. ‍In the ocean, kelp or other algae can be buried in deep ocean sediments.

Durability
100+
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Co-benefits
‍Biomass burial can provide safe storage for unused agriculture residue, river runoff and for forest ground biomass which helps eliminate water pollution and mitigate fire risk. 

Afforestation, reforestation and revegetation (ARR) activities include planting trees and allowing vegetation to recolonise land. Afforestation is planting trees on land that has not been forested for several decades at least. Reforestation is restoring land that has been deforested more recently. Revegetation focuses on rebuilding damaged soils, either by deliberate planting and seeding, or by allowing natural recolonisation.

Durability
Decades to centuries.

Length of storage
‍Decades, centuries 

Co-benefits & potential side effects
A diverse array of native species is more resilient, stores more carbon, and brings greater co-benefits than a monoculture – with positive effects on biodiversity, water, soils and climate adaptation. 

Direct air carbon capture and storage (DACCS) chemically extracts CO₂ from the air and stores it safely. The capture technology comes in many varieties, but most commonly uses a solvent to capture the CO₂, then heat to extract the gas from the solvent.

‍Durability 
The concentrated CO₂ can be pumped into deep geological reservoirs, such as depleted oil and gas fields or saline aquifers, where it is expected to be secure for many thousands of years, or incorporated in durable materials (eg. building materials) – some of the most lasting and highly scalable ways to store CO₂.

Length of storage
‍Millennia

Co-benefits & potential side effects
The capture process requires a lot of energy, which must come from a zero or very low-carbon source so it doesn’t cancel out the carbon removed.

Ocean Alkalinity Enhancement (OAE) is a method that speeds up the ocean’s natural carbon sink by adding alkaline substances to seawater. OAE decreases the CO₂ in surface waters which allows for the ocean to uptake more atmospheric carbon. This process naturally happens over geological timescales but accelerating it will make it relevant for the timelines we have to solve for the climate crisis.

‍Durability
The process transforms dissolved inorganic CO₂ in the sea into stable forms of carbon that last around 10,000 years.
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Speed of drawdown
~1 year
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Co-benefits
‍The minerals used can reverse acidification and reduce algal bloom