Carbon dioxide removal


Carbon dioxide removal is a process in which carbon dioxide is removed from the atmosphere by deliberate human activities and durably stored in geological, terrestrial, or ocean reservoirs, or in products. This process is also known as carbon removal, greenhouse gas removal or negative emissions. CDR is more and more often integrated into climate policy, as an element of climate change mitigation strategies. Achieving net zero emissions will require first and foremost deep and sustained cuts in emissions, and then—in addition—the use of CDR. In the future, CDR may be able to counterbalance emissions that are technically difficult to eliminate, such as some agricultural and industrial emissions.
CDR includes methods that are implemented on land or in aquatic systems. Land-based methods include afforestation, reforestation, agricultural practices that sequester carbon in soils, bioenergy with carbon capture and storage, and direct air capture combined with storage. There are also CDR methods that use oceans and other water bodies. Those are called ocean fertilization, ocean alkalinity enhancement, wetland restoration and blue carbon approaches. A detailed analysis needs to be performed to assess how much negative emissions a particular process achieves. This analysis includes life cycle analysis and "monitoring, reporting, and verification" of the entire process. Carbon capture and storage are not regarded as CDR because CCS does not reduce the amount of carbon dioxide already in the atmosphere.
As of 2023, CDR is estimated to remove around 2 gigatons of per year. This is equivalent to about 4% of the greenhouse gases emitted per year by human activities. There is potential to remove and sequester up to 10 gigatons of carbon dioxide per year by using those CDR methods which can be safely and economically deployed now. However, quantifying the exact amount of carbon dioxide removed from the atmosphere by CDR is difficult.

Definition

Carbon dioxide removal is defined by the IPCC as: "Anthropogenic activities removing from the atmosphere and durably storing it in geological, terrestrial, or ocean reservoirs, or in products. It includes existing and potential anthropogenic enhancement of biological or geochemical sinks and direct air capture and storage, but excludes natural uptake not directly caused by human activities."
Synonyms for CDR include greenhouse gas removal, ''negative emissions technology, and carbon removal. Technologies have been proposed for removing non- greenhouse gases such as methane from the atmosphere, but only carbon dioxide is currently feasible to remove at scale. Therefore, in most contexts, greenhouse gas removal means carbon dioxide removal.
The term geoengineering is sometimes used in the scientific literature for both CDR or SRM, if the techniques are used at a global scale. The terms geoengineering or climate engineering'' are no longer used in IPCC reports.

Categories

CDR methods can be placed in different categories that are based on different criteria:
  • Role in the carbon cycle ; or
  • Timescale of storage

    Concepts using similar terminology

CDR can be confused with carbon capture and storage, a process in which carbon dioxide is collected from point-sources such as gas-fired power plants, whose smokestacks emit in a concentrated stream. The is then compressed and sequestered or utilized. When used to sequester the carbon from a fossil fuel-fired power plant, CCS reduces emissions from continued use of the point source, but does not reduce the amount of carbon dioxide already in the atmosphere.

Role in climate change mitigation

Use of CDR reduces the overall rate at which humans are adding carbon dioxide to the atmosphere. The Earth's surface temperature will stabilize only after global emissions have been reduced to net zero, which will require both aggressive efforts to reduce emissions and deployment of CDR. It is not feasible to bring net emissions to zero without CDR as certain types of emissions are technically difficult to eliminate. Emissions that are difficult to eliminate include nitrous oxide emissions from agriculture, aviation emissions, and some industrial emissions. In climate change mitigation strategies, the use of CDR counterbalances those emissions.
After net zero emissions have been achieved, CDR could be used to reduce atmospheric concentrations, which could partially reverse the warming that has already occurred by that date. All emission pathways that limit global warming to 1.5 °C or 2 °C by the year 2100 assume the use of CDR in combination with emission reductions.

Critique and risks

Critics point out that CDR must not be regarded as a substitute for the required cuts in greenhouse gas emissions. Oceanographer David Ho formulated it like this in 2023 "We must stop talking about deploying CDR as a solution today, when emissions remain high—as if it somehow replaces radical, immediate emission cuts.
Reliance on large-scale deployment of CDR was regarded in 2018 as a "major risk" to achieving the goal of less than 1.5 °C of warming, given the uncertainties in how quickly CDR can be deployed at scale. Strategies for mitigating climate change that rely less on CDR and more on sustainable use of energy carry less of this risk.
The possibility of large-scale future CDR deployment has been described as a moral hazard, as it could lead to a reduction in near-term efforts to mitigate climate change. However, the 2019 NASEM report concludes: "Any argument to delay mitigation efforts because NETs will provide a backstop drastically misrepresents their current capacities and the likely pace of research progress."
CDR is meant to complement efforts in hard-to-abate sectors rather than replace mitigation. Limiting climate change to 1.5 °C and achieving net-zero emissions would entail substantial carbon dioxide removal from the atmosphere by the mid-century, but how much CDR is needed at country level over time is unclear. Equitable allocations of CDR, in many cases, exceed implied land and carbon storage capacities. Many countries have either insufficient land to contribute an equitable share of global CDR or insufficient geological storage capacity.
Experts also highlight social and ecological limits for carbon dioxide removal, such as the land area required. For example, the combined land requirements of removal plans as per the global Nationally Determined Contributions in 2023 amounted to 1.2 billion hectares, which is equal to the combined size of global croplands.

Permanence

Forests, kelp beds, and other forms of plant life absorb carbon dioxide from the air as they grow, and bind it into biomass. However, these biological stores are considered volatile carbon sinks as the long-term sequestration cannot be guaranteed. For example, natural events, such as wildfires or disease, economic pressures and changing political priorities can result in the sequestered carbon being released back into the atmosphere.
Biomass, such as trees, can be directly stored into the Earth's subsurface. Furthermore, carbon dioxide that has been removed from the atmosphere can be stored in the Earth's crust by injecting it into the subsurface, or in the form of insoluble carbonate salts. This is because they are removing carbon from the atmosphere and sequestering it indefinitely and presumably for a considerable duration.

Current and potential scale

As of 2023, CDR is estimated to remove about 2 gigatons of per year, almost entirely by low-tech methods like reforestation and the creation of new forests. This is equivalent to 4% of the greenhouse gases emitted per year by human activities. A 2019 consensus study report by NASEM assessed the potential of all forms of CDR other than ocean fertilization that could be deployed safely and economically using current technologies, and estimated that they could remove up to 10 gigatons of per year if fully deployed worldwide. In 2018, all analyzed mitigation pathways that would prevent more than 1.5 °C of warming included CDR measures.
Some mitigation pathways propose achieving higher rates of CDR through massive deployment of one technology; however, these pathways assume that hundreds of millions of hectares of cropland are converted to growing biofuel crops. Further research in the areas of direct air capture, geologic sequestration of carbon dioxide, and carbon mineralization could potentially yield technological advancements that make higher rates of CDR economically feasible. Investing in nature-based solutions is considered a way to buy time for the advancement of engineered carbon removal methods, enabling their full deployment in the second half of the 21st century.

Methods

Overview listing based on technology readiness level

The following is a list of known CDR methods in the order of their technology readiness level. The ones at the top have a high TRL of 8 to 9, the ones at the bottom have a low TRL of 1 to 2, meaning the technology is not proven or only validated at laboratory scale.
  1. Afforestation/ reforestation
  2. Soil carbon sequestration in croplands and grasslands
  3. Peatland and coastal wetland restoration
  4. Agroforestry, improved forest management
  5. Biochar carbon removal
  6. Direct air carbon capture and storage
  7. Bioenergy with carbon capture and storage
  8. Enhanced weathering
  9. Blue carbon management in coastal wetlands
  10. Ocean fertilization, ocean alkalinity enhancement that amplifies the oceanic carbon cycle
The CDR methods with the greatest potential to contribute to climate change mitigation efforts as per illustrative mitigation pathways are the land-based biological CDR methods and/or bioenergy with carbon capture and storage. Some of the pathways also include direct air capture and storage.