Greenhouse Gas Removal Briefing Paper

What is greenhouse gas removal?

It is possible to capture greenhouse gases from the atmosphere and store them in various natural or artificial reservoirs. This process is best established for carbon dioxide (CO₂) removal. There are many ways to remove carbon, some involving innovative technologies and others harnessing natural processes.

It is important to distinguish between emissions removal and emissions reduction. An activity can only be classified as GGR if it is removing existing greenhouse gases from the atmosphere and then storing them. For example, preventing emissions from a gas-fired power station from entering the atmosphere through their capture and storage would be considered emissions reduction (resulting in reduced, or at best, zero emissions). Similarly, a process that removes carbon dioxide from the atmosphere and uses this as an ingredient for a synthetic fuel, which when burned results in the return of the carbon dioxide to the atmosphere, is not greenhouse gas removal. In contrast, nature-based or technical approaches that remove greenhouse gases already present in the atmosphere and which store them constitute GGR (resulting in a net negative contribution).

Why do we need GGR?

Simply put, reaching global net zero emissions by 2050 is not possible without carbon removal. In fact, the term ‘net’ on a global basis implies use of greenhouse gas removal to compensate for those emissions which cannot be avoided.

• Reaching absolute zero emissions is not currently feasible, particularly for ‘harder to abate’ sectors including heavy industry (cement, steel, chemicals and aluminium) and heavy-duty transport (shipping, trucking and aviation).
• IPCC projections show that all pathways to limit global warming to 1.5°C will require 100-1000 GtCO₂ of carbon dioxide removal over the 21^st century.

For governments that have set net zero targets, including the UK, deploying carbon removal methods will be an important climate mitigation solution, alongside emission reduction practices. For businesses, carbon removal can play a role in reaching net zero targets, alongside action to reduce absolute carbon emissions.

Greenhouse gas removal solutions

GGR can take numerous forms from new technologies to land management practices. The various removal options can be divided into nature-based or enhanced natural processes; and those which require engineering and technology. The list of natural and technological solutions in this section is not exhaustive but covers the majority of the most widely considered GGR approaches.

Image 2: Negative emission technologies

Nature-based and enhanced natural solutions

Nature-based solutions for removing carbon from the atmosphere include afforestation, reforestation, and restoration of coastal and marine habitats.

Photosynthesis removes carbon dioxide naturally, and trees are an effective way of storing carbon removed from the atmosphere by photosynthesis. Expanding, restoring and managing forests to encourage more carbon uptake can leverage the power of photosynthesis, converting carbon dioxide in the air into stored carbon.

Enhanced natural processes include land management approaches to increase the carbon content in soil through modern farming methods.

Afforestation

Afforestation involves the repurposing of land use by growing forests where there were none before. While afforestation can capture large amounts of atmospheric carbon over time, it requires significant land resources for which there may be competition for other uses. It can also pose risks to biodiversity and ecosystem services. Afforestation, reforestation and forest management are already widely practiced around the world and are well understood.

Reforestation

Reforestation is the practice of re-establishing a forest where there was one in the past. While eventually leading to net negative emissions, in the short-term reforestation can cause emissions of greenhouse gases to the atmosphere. Whilst trees absorb carbon via photosynthesis, they also emit volatile organic compounds (VOCs) which can react with the atmosphere forming ozone and lengthening the lifetime of atmospheric methane, both of which are greenhouse gases. There are also challenges of land competition which could lead to emissions elsewhere.

Forest Management

Forest Management involves the optimisation of existing forests for the purpose of greenhouse gas removal and storage in biomass, and avoids the risks of land competition associated with either the creation of new forests or reintroduction of forests on land now used for other purpses, such as agriculture. Research-led forest management can protect biodiversity and ecosystem services while increasing GGR potential.

Agroforestry

Agroforestry is a method whereby trees are reintroduced into an arable landscape to improve soil organic carbon, to reduce erosion, improve biodiversity and, normally (but not always) to provide separate income from that derived from arable cultivation. There is extensive literature about benefits to biodiversity and ecosystem services but estimates of GGR efficiency and cost are not yet available.

Farming of macroalgae

Farming of macroalgae (kelp) allows photosynthetic capture of CO₂ in marine habitats, potentially overcoming terrestrial land shortages. Coastal areas may have the nutrients required for this growth but farming in the open ocean is likely to require additional nutrient inputs, especially if farmed intensively. Research into kelp farming as a GGR solution is still in its infancy and pilots are required to validate removals efficacy as well as potential ecological impacts.

Restoration of wetland, peatland and coastal habitats

Restoration of wetland, peatland and coastal habitats can help to ensure these important habitats continue to draw CO₂ from the air. Peat, permafrost and other wetland habitats are long-term carbon stores but face threats from agriculture and other human activity. Coastal kelp, seagrass and mangrove habitats are significant ‘blue carbon’ resources but are globally threatened and could be restored. There is a significant knowledge base and high level of readiness surrounding protection and restoration methods, but this has not typically focused on GGR (and protection activities in themselves do not result in GGR).

Soil Carbon Seqestration

Soil Carbon Sequestration is the process of removing CO2 from the atmosphere by changing land management practices in such a way as to increase the carbon content of soil. This can involve management of vegetation, animals, water, nutrients, fire and tillage. After as little as a few decades soil can become saturated and require ongoing maintenance to avoid CO₂ being re-emitted. There is also limited evidence of efficacy in the UK context and risk of possible increased emissions of Nitrous Oxide (N₂O). This GGR solution is ready for implementation and many of the practices are already being used in various contexts.

Biochar

Biochar – charcoal produced from biomass – can be added to soils to enable carbon to remain stored for hundreds or thousands of years. There are risks to soil health due to the accumulation of heavy metals and organic contaminants. Research has indicated that biochar addition to soil may be an irreversible process, and the long-term effects are not well understood. Furthermore, there is potential competition for biomass with other GGRs including BECCS and Construction. Biochar as a GGR solution has been piloted but not yet widely applied in the UK context.

Enhanced weathering

Enhanced weathering can accelerate natural processes that absorb CO₂ (for example, by adding very fine mineral silicate rocks to soils). Immaturity of the technique means GGR potential in various local UK environments is not yet fully understood, nor are the potential impacts on soil and water quality. The required minerals would need to be mined, transported and processed at a large scale, which would likely cause a series of negative environmental impacts. Reversibility has not been demonstrated and further research is required on long-term effects. This GGR solution needs to be piloted in the field as current experience is primarily lab-based.

Ocean fertilisation

Ocean fertilisation is the practice of adding nutrients (primarily iron) to the ocean to increase its capacity to absorb CO₂ by encouraging the growth of phytoplankton, which are single-celled, photosynthetic microorganisms. Though the principles of ocean fertilisation approaches are well understood, further research is required to understand the actual carbon cycle, the potential risks, and associated costs before any consideration of this approach as a potential GGR solution at scale.

Technical solutions

While natural biological and chemical processes draw CO₂ and other greenhouse gases out of the atmosphere, there are also engineering-based GGR solutions. These approaches are only beginning to become feasible but offer promising additional removals potential when combined with renewable electricity and/or sustainable biomass (e.g. from Afforestation).

Direct air capture (DAC)

DAC technologies extract CO₂ directly from the atmosphere. The CO₂ can be permanently stored in deep geological formations or saline aquifers (thereby achieving negative emissions) – referred to as Direct Air Capture with carbon capture and storage (DACCS); or it can be used in the production of fuels, chemicals, building materials and other products containing CO₂. When the use of the CO₂ involves its release back into the atmosphere (such as through use as a synthetic fuel) this is not a greenhouse gas removal process but an emissions reduction one (since it has replaced a fossil fuel). There are currently 15 small-scale direct air capture plants operating worldwide, capturing more than 9,000 tCO₂/year, with a 1 Mt CO₂/year capture plant in advanced development in the United States. Externalities include waste products such as absorbents used for CO₂ capture.

Methane capture from sites such as disused coal mines is already carried out where the gas is emitted in a high enough volume and concentration to ensure financial viability. As methane is a significantly more potent greenhouse gas than CO₂, a question for research is whether methane can be photo-catalytically converted to CO₂ at sites where it would otherwise be vented into the atmosphere.

Bioenergy with carbon capture and storage (BECCS)

BECCS involves the capture and permanent storage of CO₂ from processes where biomass is burned to generate energy. This can include power plants using biomass (or a mix of biomass and fossil fuels) or wastes e.g. from pulp mills for paper production.

BECCS enables carbon removal because biomass absorbs CO₂ as it grows, and this CO₂ is not re-released when it is burned. Instead, it is captured and injected into deep geological formations, removing it from the natural carbon cycle.

Bioenergy from biomass-based power plants is a mature technology, as is CO₂ capture in other applications, but the combination is largely still at the demonstration stage. There are a number of BECCS facilities operating around the world today, capturing CO₂ from industrial processes (for example, ethanol production) and biomass-based power generation. In the UK, Drax has begun a pilot project to capture CO₂ from its biomass-fuelled power plant. If successful, this could become the world’s first negative emissions power plant.

A challenge for BECCS is that some bioenergy feedstocks are unsustainable and may also compete with other land uses.

Negative Emissions Construction

There are potential negative emissions opportunities within the construction sector including negative emissions concrete (produced using oxide and carbonate reactions) and potentially negative emissions timber (from nature-based solutions). In particular, the potential role for passive greenhouse gas removal by building materials in-situ should be considered in addition to the more commonly discussed areas of negative emissions at the point of manufacture/construction. The ability to source enough domestic timber of appropriate quality is not guaranteed, and the processing and transportation of materials may reduce the overall GGR potential. For construction to make a significant impact as a GGR solution, building requirements and safety and quality assurance would need to be adjusted to enable the appropriate scale.

Market readiness, cost and removal potential

Analysis from Vivid Economics shows the UK will require circa 100 MtCO2 of GGR to reach net zero by 2050, with Scotland playing a pivotal role. Even in a conservative scenario, net zero is achievable with GGR in Scotland representing a third of UK GGR by 2050.

The technology immaturity, uncertainty related to negative emissions potential and the feasible scale of different GGR methods (in terms of tonnes of emissions removed per year), are the subject of ongoing debate. However, it is possible to get a sense of the potential role of different GGR options in reducing greenhouse gases. As the GGR process is best established for carbon dioxide (CO₂) removal, this section will focus on carbon removal.

The following graphic illustrates the technology readiness level (TRL), projected cost (in US$ per tonne of CO₂ removed) and global CO₂ removal potential (in GtCO₂) for different carbon removal solutions based on data provided in the Royal Society’s Greenhouse Gas Removal report, 2018. The TRLs indicate the technology maturity on a scale from 1 to 9, with 9 being the most mature.

Note: Agroforestry, macroalgae farming and methane capture are not included as research has yet to identify removal potential and cost for these solutions.

Greenhouse gas removal mechanisms

It is important to consider the different ways in which GGR can be deployed in the net zero transition. From a business perspective, in some cases or sectors, GGR may be achieved via direct application within a business’s own operations. Businesses can also look within their own supply (or value) chain for GGR opportunities, or they may need to consider indirect GGR mechanisms where emissions are offset by funding GGR activity in another location.

Direct application

Directly deploying GGR techniques is an option for businesses that have the capacity to start removing GHGs using their existing assets.

• Farmers can use nature-based solutions including agroforestry or soil carbon sequestration.
• Any landowner might reforest areas of land (recognising the need to take careful consideration of the type of species to gain the additional biodiversity benefits). 
• Factories may be able to capture carbon at the source of the emissions – noting that this would be an emissions reduction, rather than greenhouse gas removal (from the atmosphere).

Insetting

As opposed to carbon offsetting where an organisation pays for projects to capture atmospheric carbon dioxide in an external location, carbon insetting refers to an organisation investing in sustainable practices within its own value chain. Carbon insets support the implementation of practices, often through agroforestry and tree-planting projects, that sequester carbon, promote climate resilience, protect biodiversity and restore ecosystems.

Offsetting

Carbon offsetting is a process that involves either a reduction in emissions to, or removal from, the atmosphere of carbon dioxide or other greenhouse gases in order to compensate for emissions made elsewhere. Any activity that reduces emissions or involves removal of greenhouse gases from the atmosphere could constitute a carbon offset.

In the context of GGR, a company can pay for emissions reduction or greenhouse gas removal in another location and then count the associated reduction in emissions or removal of CO₂ towards its own climate targets.

Whilst offsetting does not remove GHGs from the atmosphere which an organisation has directly released, it is an effective means of getting finance to flow to projects which can reduce, and remove, emissions.

Carbon credits

Emissions reductions from offsetting projects should first be verified for accuracy by a third party. After verification, they are sold as a carbon credit or unit which represents a certain volume of emissions reductions.

Carbon credits allow companies to meet climate targets by purchasing credits for their emissions from an external source where emissions are either reduced, removed or avoided. This may then be counted towards a company’s own emissions targets.

A number of carbon offset standards and markets exist, including under mandatory (compliance) schemes and voluntary programs. Compliance markets are created and regulated by mandatory national, regional, or international carbon reduction regimes – namely those linked to national climate commitments under the UNFCCC. Voluntary markets function outside of compliance markets and enable companies and individuals to purchase carbon offsets on a voluntary basis with no intended use for compliance purposes.

For further information, read Chapter Zero’s explainer on carbon offsetting.

When should a business be thinking about GGR?

For heavy emitting industries and harder-to-abate sectors that, due to technical barriers, cannot reduce their emissions quickly enough to meet net zero targets, GGR provides an additional option to accelerate the transition to net zero emissions. Such sectors include heavy industry (cement, steel, chemicals and aluminium) and heavy-duty transport (shipping, trucking and aviation).

Some companies are well placed to directly deploy GGR techniques using existing resources, for example in certain industrial processes or in the agricultural sector.  

A company may also apply carbon insetting along their value chain, as explained above. Insetting can generate GHG emissions reductions and carbon storage/removal helping an organisation to meet net zero targets, whilst simultaneously generating positive impacts for communities, landscapes and ecosystems.

For some companies, the ability to capture and value carbon may present an opportunity to become carbon negative, removing more CO₂ from the atmosphere than they emit.

GGR may also present new revenue and growth opportunities for businesses by tapping into future trillion-dollar markets for products which use GHGs as an input.

As business leaders realign company strategies towards value-led approaches with long-term sustainability a core objective, they should consider:

• Innovation, supply chain and customer opportunities and risks presented by GGR approaches.
• How competitive advantage could be enhanced by engaging with GGR innovators to help them scale-up.
• What GGR approaches could be adopted to accelerate achievement of corporate climate commitments, and expand them.
• How GGR investment can align with community sustainability objectives.

UK policy outlook for GGR

Recent calls for evidence from the UK’s Department for Business Energy and Industrial Strategy (BEIS) and National Infrastructure Commission (NIC) indicate that the UK is seriously considering carbon removals as part of its wider portfolio of solutions to meet its legally binding commitment to net zero emissions by 2050, and plans set out in its Net Zero Strategy.

The scale-up of greenhouse gas removals to the level necessary to achieve net zero will require funding, public support, and a stable long-term policy framework.

Public acceptance

Public acceptability must be considered for any potential deployment of GGR at scale. The proposed GGR solutions have a variety of risk profiles and externalities, and the publics’ acceptance of these factors will be critical for the long-term success and continued deployment of the solutions. There is a need to ensure innovation is undertaken responsibly whilst also acting with haste.

A programme of public engagement surrounding GGRs should be considered. Carbon credits for offsetting and CCS have suffered from poor public perception, branded as ‘ways to let ourselves off the hook of reducing carbon emissions’ or even as ‘geoengineering’. As highlighted in this paper, GGRs are not an alternative to emissions reduction solutions, and it is important that their role is framed appropriately.

Risks to other sectors

Deploying GGR at scale could pose a number of risks to other sectors. Direct air capture will require a large amount of energy when deployed at scale, whilst BECCS and biochar would have impact on land and water in terms of requirements for forestation. Some nature-based solutions also pose risks to biodiversity and ecosystem services.

A policy framework should recognise the key benefits and risks of different methods and use the full range of different technologies needed to tackle an issue of this scale. Doing so will allow government to make the most efficient use of land and resources, and avoid potential acute negative impacts caused by deploying technologies in isolation at a large scale.

Knowledge and skills gaps

A sufficiently experienced base of GGR practitioners has not yet been developed.

Many of the skills in sectors that will need to be phased out, such as coal and oil, could be transferred into the emerging GGR industry and therefore the risk of job losses could be mitigated with new, ‘green’ jobs.

Summary and next steps

Greenhouse Gas Removal activities will form an essential component of the global transition to net zero by 2050, as well as longer-term mitigation of climate change. Despite this, there are significant technological and research challenges yet to be addressed, and the current readiness levels of most GGR solutions are insufficient. A rapid programme of research and development to address remaining knowledge gaps, accelerate new GGR technologies and to fund, construct and evaluate pilot projects is urgently required in order to meet net zero targets.

Published November 2021

Authors and contributors:

• Harriet Harthan, Nick Scott and Emily Farnworth, Centre for Climate Engagement, Hughes Hall, University of Cambridge
• Anthony Lindley, Policy Intern, Centre for Science and Policy, University of Cambridge
• Dr Shaun Fitzgerald and Katie Parker, Centre for Climate Repair, University of Cambridge
• Ana Patricia Santana, PhD Candidate, University of Cambridge