What is Greenhouse Gas Removal (GGR)?

Greenhouse Gas Removal (GGR) is also known as ‘negative emissions’, or in the case of carbon dioxide (CO₂) specifically, ‘carbon dioxide removal (CDR)’. It is an umbrella term for techniques that capture greenhouse gases from the air and store or chemically convert them with some degree of permanence. Unlike techniques to reduce emissions, which prevent greenhouse gases from entering the air, GGR techniques take back greenhouse gases that are there already. 

There is a wide variety of approaches and methods for GGR, but they have two stages in common: a stage of capturing the greenhouse gas and a stage of storing or converting it.

For CO₂ capture, it can happen through biological processes, industrial chemical processes, or rock weathering processes. Depending on the technique, the carbon can be stored in trees, plant matter, in the soil, deep underground, in the oceans, or in long-lived products.[1] 

Interest in GGR is rising rapidly in response to the urgent need to achieve net-zero emissions (see ‘Why pursue GGR?’). Some GGR techniques have been around for a long time, such as planting trees and biochar. But most are new technologies that are starting to emerge around the world.  

You can find out more about different types of GGR under the ‘What are the main GGR methods?’ section.  

References

[1] Greenhouse gas removal, Royal Society and Royal Academy of Engineering, September 2018, p.8 

We know with a high degree of certainty that climate change is happening and is a result of greenhouse gas emissions caused by human activity. The accumulation of CO₂ in the air, generated by human activity, has warmed the world by about 1°C since pre-industrial times and the impacts are being felt across the world, albeit not uniformly. In the Paris Agreement, world leaders set the goal to limit warming to well below 2°C, and pursue efforts to stay below 1.5°C. This will limit, but not completely avoid, raising the risk of heatwaves, flooding and extreme weather events, among many other potential impacts including extinction of species.[2]  

To achieve the 1.5°C goal, net CO₂ emissions need to be reduced rapidly this decade, and as close to zero as possible by the middle of the century. The state at which the greenhouse gases going into the atmosphere are balanced by removal out of the atmosphere is known as ‘net zero’. 

We can reduce emissions now through a range of mechanisms, including improving efficiency, switching to zero-carbon electricity, electrifying transport and heating, and making behavioural changes.  

However, even if we make great progress in eliminating emissions there are likely to be some industries (e.g. aviation and agriculture) which will still be emitting some greenhouse gases in 2050. Achieving net zero will therefore involve actively removing carbon from the atmosphere (see ‘Shouldn’t we just focus on reducing greenhouse gas emissions?’ section).  

GGR also opens up the possibility of net negative emissions in the further future. This would begin to reverse some aspects of climate change, but some changes (such as sea level rise) would still continue in their current direction for decades to millennia.[3] 

References

[2] Hoegh-Guldberg, O., D. Jacob, M. Taylor, M. Bindi, S. Brown, I. Camilloni, A. Diedhiou, R. Djalante, K.L. Ebi, F. Engelbrecht, J.Guiot, Y. Hijioka, S. Mehrotra, A. Payne, S.I. Seneviratne, A. Thomas, R. Warren, and G. Zhou, 2018: Impacts of 1.5ºC Global Warming on Natural and Human Systems. In: Global Warming of 1.5°C. An IPCC Special Report on the impacts of global warming of 1.5°C above pre-industrial levels and related global greenhouse gas emission pathways, in the context of strengthening the global response to the threat of climate change, sustainable development, and efforts to eradicate poverty [Masson-Delmotte, V., P. Zhai, H.-O. Pörtner, D. Roberts, J. Skea, P.R. Shukla, A. Pirani, W. Moufouma-Okia, C. Péan, R. Pidcock, S. Connors, J.B.R. Matthews, Y. Chen, X. Zhou, M.I.Gomis, E. Lonnoy, T.Maycock, M.Tignor, and T. Waterfield (eds.)]. In Press.

[3] IPCC, 2021: Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press. In Press. FAQ 5.3, p. 1272. 

GGR should be pursued responsibly, taking into account wider impacts, public values and interests. Responsible innovation is a central aim of CO₂RE. Our work will build on UKRI’s responsible innovation AREA framework to ‘Anticipate’ GGR impacts, ‘Reflect’ on the framings of GGR, ‘Engage’ in meaningful two-way engagement and ‘Act’ on its findings to inform the trajectory of GGR innovation itself. Read more about our work on public perceptions. 

We will facilitate public participation on decision-making, such as understanding people’s preferences around GGR, and engage with society on ethical issues and concerns about the effects of GGR. The Hub is also working with the Demonstrator projects to support embedding this approach in their work.

The amount of GGR we will need depends on our choices: how much we choose to limit the rise in global temperature, and how much we choose to reduce emissions.   

The State of Carbon Dioxide Removal report notes that across IPCC pathways, 420-1100 billion tonnes of CDR will be required by 2100 to limit global warming to 1.5°C with no or limited overshoot.[4] For context, the world emits around 40 billion tonnes of CO₂ every year. 

The amount and mix of GGR in the pathways vary; for example, some rely more on capturing carbon and storing it underground, whereas others rely more on agriculture and land use. 

Other estimates are developed based on a ‘bottom-up’ perspective on which sources of emissions will be most difficult to avoid. These suggest a lower level of removals may be required in 2050 for net zero, perhaps around 1.5 to 3.1 billion tonnes of CO₂ per year.[5] Nevertheless, this would still require a rapid scale up from the present. 

In the UK, it is estimated that we will need to capture and store 83-169 million tonnes of CO₂ every year by 2050, according to the Climate Change Committee.[6] For context, the UK’s emissions in 2019 were around 522 million tonnes. The UK government’s Net Zero Strategy expects deployment of “engineered” removals to reach 75-81 million tonnes of CO₂ per year by 2050, but does not go into any more detail about precisely what mix of techniques they expect this to involve. 

The amount of GGR we will need is based on estimates, and there is some uncertainty in how much CO₂ different methods could remove and how quickly they could be deployed. Many proposals for GGR are at a very early stage in their development and have not yet been proven to work at full scale. For more on this, have a look at the ‘Which GGRs are most promising?’ section. 

References

[4] Smith, S. M., Geden, O., Nemet, G., Gidden, M., Lamb, W. F., Powis, C., Bellamy, R., Callaghan, M., Cowie, A., Cox, E., Fuss, S., Gasser, T., Grassi, G., Greene, J., Lück, S., Mohan, A., Müller-Hansen, F., Peters, G., Pratama, Y., Repke, T., Riahi, K., Schenuit, F., Steinhauser, J., Strefler, J., Valenzuela, J. M., and Minx, J. C. (2023). The State of Carbon Dioxide Removal – 1st Edition. The State of Carbon Dioxide Removal. doi:10.17605/OSF.IO/W3B4Z

[5] Bergman, A. & Rinberg, A. (2021) The Case for CDR: From Science to Justice. CDR Primer. eds: Wilcox, J. et al.

[6] Sixth Carbon Budget, Committee on Climate Change, December 2020, p.80. Available at: https://www.theccc.org.uk/publication/sixth-carbon-budget/

Getting to net zero means we need very deep and rapid cuts in emissions. Many ways to cut emissions are less costly and more scalable than GGR currently. 

Nevertheless, it is increasingly recognised that GGR will play a crucial role in achieving net zero and limiting global warming. We will need emissions cuts and removals, not one or the other.  

Even if we make great progress in eliminating emissions, we are likely to still be left with some in 2050. Achieving net zero global emissions on that timescale, which is necessary for keeping global warming to around 1.5°C, will therefore involve active removals as well. Responsible innovation and deployment take time. In order to have the option of using them at scale by 2050, we need to start work on them now.  

GGR will also be imperative for achieving net negative emissions (where removals exceed emissions). This may be needed in some countries as part of a global net zero plan to allow room for other countries for whom reaching net zero proves more difficult. Some argue that the UK should reach net negative emissions, because over the course of history we have released far more CO₂ per person than most other countries. Net negative emissions may even become a global goal, if we reach too dangerous a level of warming. 

GGR should not be pursued as a substitute for decarbonisation. Many of the GGR technologies are not yet commercially viable and face implementation challenges. At scale they may conflict with other goals for sustainable development.[7] Nevertheless, several GGR methods hold promise (see ‘Which GGRs are most promising?’ section) and a growing number are being tested. This reinforces the case for pursuing GGR development in an environmentally and socially robust manner, underpinned by appropriate governance and social permissions.  

References

[7] IPCC. (2018): Summary for Policymakers. In: Global warming of 1.5°C. An IPCC Special Report on the impacts of global warming of 1.5°C above pre-industrial levels and related global greenhouse gas emission pathways, in the context of strengthening the global response to the threat of climate change, sustainable development, and efforts to eradicate poverty [Masson-Delmotte, V. et al (eds)]. World Meteorological Organization, Geneva, Switzerland, 32 pp

There are many methods for GGR – some are at a very early stage in their development and others, such as tree-planting, are very well-established. It is sensible at this stage to look at a broad and diverse portfolio of GGRs, which helps to maximise the removal potential and minimise over-reliance on any one technique. Scenarios from the Climate Change Committee show that methods in both land and industry sectors methods will likely be needed to reach net zero in the UK by 2050.  

The Royal Academy of Engineering and the Royal Society’s report (PDF) on Greenhouse Gas Removal lists 12 GGR methods that are estimated to have global potential on the billion tonnes of CO₂ scale.[8] Note that these are themselves broad categories covering a range of approaches. 

• Biochar is a stable, long lived, charcoal-like product from the burning of biomass in the absence of oxygen (pyrolysis). Biochar is carbon-rich and can be spread on farmland, potentially storing carbon in the soil for an extended period. The GGR-D Programme’s biochar Demonstrator project is looking to address the uncertainties concerning the extent and scope of deployment of biochar. 
• Bioenergy with carbon capture and storage (BECCS) involves using biomass for energy, capturing the CO₂ emissions and storing them, usually underground. The GGR-D Programme’s perennial biomass crops Demonstrator project, PBC4GGR, is investigating the potential for plants like willow and miscanthus to support BECCS in the UK. 
• Building with biomass is about using forestry materials in building, which extends the time of carbon storage of natural biomass and enables additional forestry growth.  
• Direct air capture and carbon storage (DACCS) involves using engineered processes to capture atmospheric CO₂ for subsequent storage, usually underground.  
• Enhanced terrestrial weathering is where crushed rocks spread on land react with CO₂ to remove it from the atmosphere. The GGR-D Programme’s enhanced rock weathering Demonstrator project is exploring amending soils with crushed calcium and magnesium rich silicate rocks from quarry waste. 
• Forestation is about growing new trees and improving the management of existing forests. As forests grow they absorb CO₂ from the atmosphere and store it in living biomass, dead organic matter and soils. The GGR-D Programme’s woodland creation and management Demonstrator project, NetZeroPlus, is gathering evidence, addressing knowledge gaps and allowing decision makers to explore the GGR consequences of different tree-planting options. 
• Habitat restoration can increase carbon storage in habitats such as peatlands and coastal wetlands. This also prevents carbon release through further degradation, often providing a number of other co-benefits. The GGR-D Programme’s peatland restoration Demonstrator project, GGR-Peat, is working with natural processes to recreate, and where possible enhance, the environmental conditions that lead to peat formation. 
• Low-carbon concrete can be created by altering the constituents, manufacture or recycling method of concrete to increase its storage of CO₂.  
• Mineral carbonation involves accelerating the conversion of silicate rocks to carbonates either above or below the surface to provide permanent storage for CO₂.  
• Ocean alkalinity is increasing ocean concentration of ions like calcium to increase uptake of CO₂ into the ocean, and reverse acidification.  
• Ocean fertilisation is applying nutrients to the ocean to increase photosynthesis and remove atmospheric CO₂.  
• Soil carbon sequestration involves changing agricultural practices such as tillage or crop rotations to increase the soil carbon content.  

References

[8] Greenhouse gas removal, Royal Society and Royal Academy of Engineering, September 2018 

It is sensible at this stage to consider a broad and diverse portfolio of GGRs. Some proposals for GGR are at a very early stage in their development, others are more established. They vary in cost, how much they can remove and in the resources they require (such as land, water and energy). Some carry risks, for instance due to costs of land conversion and biodiversity loss, particularly if deployed on very large scales. Some bring co-benefits, for instance generating power, reversing biodiversity loss or providing useful materials. Some may prove unsuccessful in scaling up.  

Importantly, there is still much scope for innovation and improvement, and some techniques will work better in some contexts than others. Understanding these factors better is part of the research aims of the GGR-D Programme. 

CO₂RE is generating ways of evaluating different GGR options in a balanced and multi-disciplinary way, providing tools for government, businesses and people to make better decisions. Find out more about our research. The five GGR-D Programme Demonstrator projects, alongside other projects funded by the Department for Business, Energy & Industrial Strategy (BEIS), are investigating different GGR methods and the results will be used to shape longer-term government decision-making on the most effective technologies. 

A number of businesses and start-ups are engaging in carbon removal, and the UK government is supporting the field through policies, and research and development. 

The government’s 2021 Net Zero Strategy states that GGRs are “essential” to compensate for the residual emissions arising from the most difficult activities to decarbonise. It states that a portfolio approach that supports innovation, demonstration and commercialisation of a wide range of engineered GGR solutions is needed this decade. England, Scotland, Wales and Northern Ireland also have their own plans and targets for planting more trees and restoring peatland, because in the UK, agricultural and land use policies are devolved. 

The UK government and its research arm, UKRI, are investing £100 million into research, development and demonstration of GGR across multiple programmes. This includes CO₂RE and the five GGR Demonstrator projects. It also includes a competition on Direct Air Capture and other GGR technologies launched by the government’s Department for Business, Energy & Industrial Strategy (BEIS).    

To find out more about the current publicly-funded GGR initiatives in the UK, take a look at the UK GGR Landscape map developed by the Carbon Removal Centre and CO₂RE.  

The transition to net zero will require us to capture and permanently store significant amounts of CO₂. This is not just for carbon removal from the air, but also for important activities in society that are most difficult to decarbonise, such as the production of steel, cement and ammonia. One long-term, low-risk option is geological carbon storage. 

Geological carbon storage is a process where either pure or dissolved CO₂ is injected (or “sequestered”) underground in order to isolate it from the atmosphere for the very long term, i.e., more than 10,000 years. Several varieties of geological carbon storage have been deployed around the world, with many more planned. In the UK, 5 projects are in early development, located offshore near industrial areas of the country.  

Large pilot projects that have taken place around the world have shown that storing CO₂ underground can be done safely. Once stored, there are many methods available to detect if leakage occurs, and suitable regulation can ensure appropriate monitoring of storage reservoirs will be done for many decades.[9]  

CO₂ is a gas found throughout nature and is dangerous to humans only in very high concentrations. Many large storage projects are offshore and hold little to no risk to the public, and there have been no detectable CO₂ leaks from geological carbon storage projects both onshore and offshore to date. 

References

[9] Ringrose, P.S. (2020). How to Store CO2 Underground: Insights from early-mover CCS Projects. 

To achieve net zero emissions, we must rapidly reduce emissions this decade, and as close to zero as possible by the middle of the century. Even with great progress on reducing emissions, there will likely still be residual emissions that are difficult to eliminate fully. Achieving net zero will therefore involve actively removing greenhouse gases from the atmosphere (Greenhouse Gas Removal). Figure 1 demonstrates how emission reductions and removals work alongside one another to achieve net zero.

Figure 1: Demonstrates how a combination of emission reductions and Greenhouse Gas Removal helps achieve net zero emissions.

Carbon offsetting involves compensating for one’s own emissions by investing in or carrying out projects or activities that reduce or remove an equivalent amount of greenhouse gases elsewhere. Offsetting is included in many climate strategies, particularly in an attempt to compensate for emissions that are currently very difficult to reduce completely. Types of offsetting include via emission reductions, avoided emissions, and carbon removal/GGR offsetting.

A crucial difference between offsetting and a removal that is not used as part of an offsetting strategy is that offsetting does not change the net balance of carbon in the atmosphere. Offsets are not aimed at lowering overall atmospheric CO₂ concentrations; at best, they aim to balance out emissions to prevent an increase in net emissions. Removals, on the other hand, are projects designed to reduce the amount of carbon in the atmosphere. Together with emission reductions, as shown in Figure 1, they can help achieve net zero emissions.

Still, removal-based projects are available for offsetting purposes, alongside those that aim to avoid or reduce emissions. Most offsets utilised today are from emission reduction or avoided emissions projects. However, expert consensus is that offsetting needs to shift away from such offsets to durable Greenhouse Gas Removal. This is because only removals can balance out emissions to reach net zero. Figure 2 demonstrates the outcomes of these different offsetting approaches.

Figure 2: Demonstrates offsetting via emission reduction/avoidance vs carbon removal/GGR offsetting.

In line with the aim of reaching net zero, actors still need to reduce their emissions as much as possible before using offsets, especially as the performance of offsets cannot be guaranteed. The Oxford Principles for Net Zero Aligned Offsetting (revised 2024) outline overarching principles to help ensure offsetting works towards rather than undermines global net zero in future.

Trees are vital for our planet, enhancing biodiversity, improving our health and providing recreational places, as well as absorbing CO_2. They represent a cost-effective way of removing carbon emissions from the air. Planting more trees, and protecting the forested areas we already have, will play a key role in achieving net zero. However, the seemingly simple solution of planting more trees is more complicated than it might first appear.

Competition for land and water is an important concern when planting trees for GGR. Achieving large amounts of GGR with reforestation and afforestation would require large amounts of land – approximately 27.5 million hectares for 1 billion tonnes of CO₂ removed,[10] which is bigger than the UK’s total land area. Even if we maximised the amount of vegetation all land on Earth could hold, we would only store enough carbon to offset about ten years of greenhouse gas emissions at current rates. After that, there could be no further increase in carbon capture.[11]

Other issues include the permanence of these carbon sinks. Although some fire is a natural process which maintains the health of the ecosystem, the increasing frequency and severity of wildfire caused by climate change means the risk of carbon release caused by fire is increasing.[12] Other risks include extreme weather and pests. Planting trees where they do not belong can harm ecosystems and biodiversity and even release greenhouse gases into the atmosphere. A singular focus on rapid carbon removal might generate undesirable side effects, for example on native wild species. The answer is to attempt to understand all of the positive and negative effects of planting different tree species in different places so that the best possible decisions can be made.[13]

Forests are one of many carbon-rich ecosystems. We must do more to protect and restore existing forests and woodlands, alongside peatlands, coastal and marine habitats amongst others , if we are to achieve the maximum climate change mitigation and biodiversity benefits.[14]

As with all GGR methods, tree-planting has advantages and potential disadvantages, so it makes sense to both examine the right place for the right tree and look at a broad portfolio of methods for removing CO₂ from the atmosphere rather than putting all our hopes on any single one (in addition to reducing emissions as much as possible).

References

[10] B Anderegg, J Freeman, R Jacobson & M Torn (2021), “The Building Blocks of CDR Systems: Forest Carbon” CDR Primer, edited by J Wilcox, B Kolosz, J Freeman

[11] B Waring, M Neumann, I C Prentice, M Adams, P Smith and M Siegert (2020), “Forests and Decabonization – Roles of Natural and Planted Forests”, Frontiers in Forests and Global Change, 3.

[12] Stephens, S, Agee, J, Fulé, P, North, M, Romme, W, Swetnam, T & Turner, M (2013), “Managing Forests and Fire in Changing Climates”, Science (New York, N.Y.). 342. 41-42. 10.1126/science.1240294.

[13] CAJ Girardin, S Jenkins, N Seddon, M Allen, SL Lewis, CE Wheeler, BW Grisom and Y Malhi (2021), “Nature-based solutions can help cool the planet – if we act now”, Nature, 593(7858), pp.191-194.

[14] Strassburg, B.B.N., Iribarrem, A., Beyer, H.L. et al. Global priority areas for ecosystem restoration. Nature 586, 724–729 (2020).

The GGR sector – which currently barely exists – will need to scale to the multi-billions of tonnes in annual removal globally, and multi-millions in the UK, within a few decades. This is an unprecedented challenge.  

In the UK alone, this burgeoning industry needs to be capable of removing 83-169 million tonnes of CO₂ ever year by 2050, according to the Climate Change Committee.[15] Success will require key technologies to fall in cost, as well as new business models, economic policies and legal and regulatory frameworks, all in a manner that is acceptable to the public and socially robust.  

CO₂RE has identified six broad societal needs in order to achieve GGR at scale by 2050 in the UK: 

1. A clear vision for GGR. GGR is an emerging sector that will need to be integrated with, and additional to, broader initiatives to stop climate change.
2. Public support for GGR, as it will have big implications for communities and the environment. People need to be given a say in decisions which affect them, and GGR should be done in a way that is fair and brings other benefits.
3. Innovation, because many approaches are at an early stage of development and are expensive. Government support and effective policies can stimulate innovation and bring costs down.
4. Long-term, government-backed incentives for GGR are crucial to support the fledgling market.  
5. Strong and robust monitoring, verification and reporting to track the deployment and effectiveness of GGR.  
6. Tools for businesses, policymakers and others to make better decisions on GGR as it scales up. 

You can read more about the six priorities in an op-ed in the Conversation by CO₂RE directors Professor Cameron Hepburn and Dr Steve Smith.  

References

[15] Sixth Carbon Budget, Committee on Climate Change, December 2020, p.80. Available at: https://www.theccc.org.uk/publication/sixth-carbon-budget/ 

Actions by individuals can make a difference in fighting climate change, both in reducing emissions and shaping the future of GGR.  

By reading this, you’ve already made a start. GGR requires industry and governments to come together to overcome the challenges, but it also requires a social mandate from the public, so why not engage with the issues and make your voice heard? Greater understanding of public attitudes around GGR is needed, including which methods should be pursued, and how they should be incentivised and governed. As a member of the public, you could tell your MP, local councillors and mayor that you think action on climate change is important and share your views on GGR. You could engage with CO₂RE by attending our events or signing up to our newsletter to hear about opportunities. As mentioned in ‘How to pursue GGR?’, public participation is a key part of our work. Finally, why not explore the growing body of public information on GGR, such as The Carbon Removal Show podcast or the Science Museum’s Our Future Planet exhibition.

GGR is not a substitute for drastically reducing emissions and that is another area where individuals can make a difference. The Grantham Institute for Climate Change and the Environment at Imperial College London lists 9 things you can do about climate change. Personal action is key for raising the importance of issues to policymakers and businesses. Again, making your voice heard by those in power, particularly by politicians and corporations, shows that you care about this issue and is one way of putting pressure on them to change. Examples of how you can reduce your own carbon footprint include eating less meat and dairy, cutting back on flying and driving, and investing your money responsibly.