Carbon removal is a process that extracts carbon dioxide (CO2) directly from the atmosphere. It aims to durably store CO2 in reservoirs like geological formations, terrestrial ecosystems, or oceans. Its purpose is to reduce atmospheric CO2 concentration, addressing global warming and climate impacts. It actively mitigates climate change by drawing down existing greenhouse gases.
Understanding Carbon Removal
Carbon removal, also referred to as carbon dioxide removal (CDR), takes CO2 directly from the atmosphere. This distinguishes it from carbon capture and storage (CCS), which prevents new CO2 emissions from large point sources. CCS intercepts CO2 before release, while CDR addresses CO2 already emitted.
Removing existing CO2 is necessary for achieving climate targets, like the Paris Agreement. Even with significant emissions reductions, CO2 will remain in the atmosphere for hundreds to thousands of years. CDR methods aim for “net-negative” emissions, meaning more CO2 is removed than emitted, to stabilize global temperatures.
Nature-Based Carbon Removal Approaches
Nature-based carbon removal approaches use biological and geological processes to draw down and store atmospheric CO2. These methods often offer environmental benefits beyond carbon sequestration. They are implemented on land or within aquatic systems.
Afforestation and Reforestation
Afforestation involves establishing new forests in long-unforested areas. Reforestation focuses on replanting trees in recently cleared or degraded areas. Both processes enhance carbon sequestration as growing trees absorb CO2 through photosynthesis, storing it in their biomass and soil. These practices contribute to climate regulation and support biodiversity by creating new habitats.
Soil Carbon Sequestration
Soil carbon sequestration, also known as “carbon farming” or “regenerative agriculture,” involves managing land to increase soil carbon. Practices like no-till or low-till farming reduce soil disturbance, retaining carbon. Planting cover crops and incorporating diverse crop rotations enhance soil fertility and organic matter, increasing storage. Managed grazing and applying organic amendments like compost or crop residues further build soil organic carbon levels.
Blue Carbon Initiatives
Blue carbon refers to carbon stored in coastal and marine ecosystems, like mangroves, tidal marshes, and seagrass meadows. These ecosystems efficiently sequester CO2 from the atmosphere and ocean. They store large quantities of carbon in plant biomass and deep, water-logged sediments. For instance, over 95% of the carbon in seagrass meadows is stored in their soils, where low oxygen levels slow decomposition and allow carbon to remain trapped for thousands of years.
Technological Carbon Removal Approaches
Technological carbon removal approaches use engineered systems and industrial processes to extract and store atmospheric CO2. These methods offer distinct mechanisms for achieving carbon removal.
Direct Air Capture (DAC)
Direct Air Capture (DAC) technology filters CO2 directly from ambient air. Fans draw air into the system, where CO2 binds with liquid solvents or solid sorbents. Once captured, heat or a vacuum releases the CO2 in a concentrated stream, regenerating the solvents or sorbents for reuse. The separated CO2 can then be permanently stored underground in geological formations or converted into products like building materials.
Bioenergy with Carbon Capture and Storage (BECCS)
Bioenergy with Carbon Capture and Storage (BECCS) combines bioenergy generation from biomass with carbon capture technology. Biomass (e.g., plants, trees) absorbs CO2 as it grows. When biomass is converted into heat, electricity, or fuels, CO2 emissions from combustion or fermentation are captured before entering the atmosphere. The captured CO2 is then transported and stored underground in geological formations.
Enhanced Weathering
Enhanced Weathering (EW) accelerates Earth’s natural rock weathering process, removing atmospheric CO2. This involves spreading finely ground silicate rocks, like basalt or olivine, onto agricultural land or other surfaces. When rainwater (containing dissolved atmospheric CO2) contacts these crushed rocks, a chemical reaction occurs. This reaction converts the CO2 into stable bicarbonate ions that are transported by waterways into the ocean, where they precipitate as carbonate minerals, locking away carbon for thousands to tens of thousands of years.
Carbon Mineralization
Carbon mineralization is a process where CO2 reacts with minerals to form solid, stable carbonate minerals. This natural process can be accelerated by human intervention. One approach involves injecting concentrated CO2 deep underground into reactive rock formations, like basalt, where it solidifies within a few years. Another method, ex-situ mineralization, combines CO2 with alkaline feedstocks like mine tailings or industrial by-products to create carbonated aggregates for use in products like concrete, permanently storing carbon.
The Role of Carbon Removal in Climate Action
Carbon removal is recognized as a component of global climate strategies. It is necessary for achieving net-zero and potentially net-negative emissions, where more CO2 is removed than emitted. This is because even with aggressive emissions reductions, some sectors, like agriculture and aviation, may have emissions difficult to eliminate. Carbon removal complements, rather than replaces, deep emissions cuts, addressing historical CO2 accumulation and hard-to-abate emissions.
To limit global warming to 1.5°C, the Intergovernmental Panel on Climate Change (IPCC) estimates billions of tons of CO2 will need to be removed annually by mid-century, reaching 7-9 gigatonnes annually by 2050. Permanent storage, where captured CO2 is stored for centuries or millennia, underpins the long-term effectiveness of these methods. Scaling up carbon removal technologies and nature-based solutions is part of a comprehensive strategy to stabilize global temperatures.