Carbon dioxide removal, or CDR, is a collection of techniques aimed at addressing the carbon dioxide already present in the atmosphere. This process involves capturing CO2 gas and securing it in a way that prevents its release back into the environment. The goal of any CDR method is to durably store this captured carbon in various reservoirs, including those found in the earth, oceans, or even within manufactured products.
Differentiating Removal from Reduction
Understanding the distinction between carbon dioxide removal and emissions reduction is important for grasping climate strategies. Emissions reduction focuses on preventing new carbon dioxide from entering the atmosphere at its source. This can be likened to turning down the faucet of a bathtub that is beginning to overflow. The primary goal is to slow the rate at which the problem worsens by decreasing the output of greenhouse gases from power plants, vehicles, and industrial facilities.
Carbon dioxide removal, on the other hand, is like opening the drain to lower the water level that is already too high. This process targets the carbon dioxide that has accumulated in the atmosphere. Even if all emissions were to stop today, the existing concentration of atmospheric CO2 would continue to affect the climate. For this reason, scientific assessments indicate that both turning down the faucet (reduction) and draining the tub (removal) are necessary to meet long-term climate goals.
Nature-Based Removal Approaches
Nature offers several pathways for drawing down atmospheric carbon dioxide using biological processes. Afforestation, the planting of new forests, and reforestation, the restoration of depleted ones, are prominent examples. Through photosynthesis, trees absorb CO2, converting the carbon into organic matter that becomes their trunks, branches, and roots, effectively locking it away in their biomass. A single mature tree can absorb over 48 pounds of CO2 annually.
Soil carbon sequestration aims to increase the carbon content of agricultural lands. Practices like no-till farming, where the soil is not disturbed after a harvest, and planting cover crops between cash crop seasons help incorporate more organic matter. This not only improves soil health but also enhances its capacity to hold carbon, preventing it from returning to the atmosphere.
Coastal and marine ecosystems also contribute to carbon removal through a process known as “blue carbon” sequestration. Environments such as mangrove forests, tidal salt marshes, and seagrass meadows are particularly effective at capturing CO2. When these plants die, their carbon-rich organic material becomes buried in the waterlogged, low-oxygen soils, where it decomposes very slowly. This allows these coastal habitats to store large quantities of carbon for extended periods.
Technological Removal Approaches
Alongside nature-based methods, a suite of engineered technologies is being developed to perform carbon dioxide removal. Direct Air Capture (DAC) is one of the most discussed technological solutions. These systems use large fans to pull in ambient air and pass it over chemical contactors that selectively bind with CO2 molecules. The captured carbon dioxide is then released in a concentrated stream for storage or use.
Bioenergy with Carbon Capture and Storage (BECCS) is a hybrid approach that begins with natural photosynthesis. In a BECCS system, biomass like trees or switchgrass is cultivated, absorbing CO2 as it grows. This biomass is then combusted at a power plant to generate electricity. The resulting CO2 emissions from combustion are captured before they can escape and are subsequently prepared for long-term storage.
A different technological pathway is Enhanced Weathering, which accelerates a natural geological process. Certain types of rocks, such as basalt or olivine, naturally react with CO2 in the air and water, eventually forming stable carbonate minerals. Enhanced weathering involves grinding these rocks to increase their surface area and spreading the dust over large areas of land or ocean. This speeds up the chemical reactions that draw CO2 out of the atmosphere.
The Role of Carbon Storage
For any removal method to be effective, the captured carbon must be durably stored. Without reliable sequestration, the captured CO2 could simply leak back into the air, nullifying the initial effort. The goal is to isolate the CO2 from the atmosphere for centuries or even millennia.
One of the most developed methods for long-term storage is geological sequestration. This involves injecting highly compressed CO2 into deep underground rock formations. Suitable sites include depleted oil and gas reservoirs or deep saline aquifers, where the CO2 is injected thousands of feet below the surface and trapped by overlying layers of impermeable rock.
Another storage approach is mineralization, which involves reacting captured CO2 with specific types of rock to create a solid carbonate mineral. By exposing CO2 to minerals rich in compounds like magnesium oxide and calcium oxide, it can be chemically converted into materials similar to limestone. This process locks the carbon into a stable, solid form, providing a permanent storage solution.
Global Scale and Feasibility
Bringing carbon dioxide removal to a scale that can meaningfully alter climate trajectories presents considerable considerations. Current global CDR efforts remove a small fraction of total annual emissions. Expanding these technologies to the necessary levels will require surmounting significant hurdles related to cost, energy, and land use.
Technological approaches like Direct Air Capture are currently expensive and demand substantial amounts of energy to operate their systems. Bioenergy with Carbon Capture and Storage (BECCS) faces challenges related to the vast areas of land required to grow sufficient biomass, which could compete with food production and biodiversity.
Nature-based solutions also come with their own set of feasibility questions. While planting trees is a well-understood method, the carbon storage is only as permanent as the forest itself, vulnerable to fires, disease, or land-use changes. Similarly, the effectiveness of soil sequestration can vary greatly depending on agricultural practices and climate conditions.