In the fight against climate change and achieving the long-term temperature goal of the Paris Agreement, not only rapid and deep reductions in greenhouse gas (GHG) emissions are crucial, but also carbon dioxide removal (CDR) from the atmosphereAccording to the Intergovernmental The Intergovernmental Panel on Climate Change (IPCC) defines CDR as anthropogenic activities that remove CO2 from the atmosphere and permanently store it in geological, terrestrial or oceanic reservoirs, or in products. CDR is essential to balance out residual emissions and achieve net zero GHG emissions in the medium term, and ultimately to reduce temperatures by achieving net negative emissions in the long term. One essential condition for the effectiveness of CDR is its durability.

Understanding CDR Durability

Durability in a scientific context refers to CO2 storage time frame into a non-atmospheric environment. It depends on two main factors: (i) CO2 storage duration and (ii) risk of reversal of such storage, i.e. the release of CO2 back into the atmosphere. However, there is no generally accepted definition of “permanent” or “permanent” CO2 storage, nor a scientifically agreed limit on “durability”.

Storage duration

The length of time that CO2 storage is needed depends on the specific CDR objective. If the goal is to offset anthropogenic emissions, storage must be effective for as long as the emitted GHGs would affect the atmosphere, i.e. thousands of years for fossil CO2. However, CDR also plays a key role in achieving short- to medium-term objectives (decades to centuries). Less durable CDR could help reduce net GHG emissions in the coming decades and achieve the Paris Agreement goal of balancing sources and sinks in the second half of this century. Short-term storage lasting at least a few decades can significantly reduce the size and duration of the temperature peak if the stored carbon is not released back into the atmosphere before global temperatures reach their peak.

The duration of CO2 storage varies depending on the CDR method:

• Technical methods (engineered CDR), such as direct air capture and storage (DACCS) or accelerated rock weathering (ERW), can store CO2 in more chemically and physically stable forms. Some of these, including geological storage, have the potential to store CO2 for thousands of years, especially if the CO2 is trapped under thick, impermeable seals or converted into solid minerals.
• Natural methods (nature-based conventional CDR), such as afforestation/reforestation or peatland/wetland restoration, capture CO2 through photosynthesis and store it in vegetation, soil or sediments. The durability of relevant carbon sinks in biological systems varies, from centuries in woody biomass to millennia in deeper soil layers.

Risk of reversal

Reversal risk refers to the likelihood that stored carbon will be partially or completely released back into the atmosphere. If reversal occurs, the lost carbon no longer contributes to offsetting the long-term climate effects of emissions.

• Geological storage It initially has a relatively high risk of reversal until the CO2 stabilizes (e.g., before mineralization), but then asymptotically approaches a low risk (perhaps close to zero) over time. For example, injection of CO2 into basaltic rocks may lead to its immobilization in carbonate minerals over years.
• Biological methods have a more variable and less predictable reversal risk profile. CO2 removed by plants through photosynthesis can be returned to the atmosphere through respiration, decomposition, or fires. Natural and anthropogenic disturbances, such as fires or deforestation, can increase the risk of reversal. However, strong safeguards and investments in project design and management can help manage these risks.

Sustainability in policies and decision-making

While science debates durability in the range of centuries to millennia, CDR policies and contracts tend to cover much shorter time frames, often only a few decades and almost never more than a century. The challenge for policymakers is to define durability in the context of a particular policy or investment, taking into account factors such as readiness, economic feasibility, political acceptability and social or environmental benefits. Authorities, whether public or private, often determine who is responsible for providing storage and what the consequences of reversal are. These responsibilities and obligations define the terms of financing and regulate the transport and storage of CO2.

Complementary approach to CDR

Since no CDR method currently meets all the criteria for readiness, scalability, sustainability, and durability simultaneously over several centuries, it is essential complementary approachInstead of preferring only technical methods over natural ones, it is recommended diversified CDR portfolio.

• Natural methods they are immediately deployable on a large scale, are relatively inexpensive, and can deliver significant environmental and social benefits, such as improved biodiversity and ecosystem services. Although they are more susceptible to reversal, they can effectively reduce nearby damage.
• Technical methods they offer higher durability and lower risk of reversal.

A combination of natural and technical methods that show complementary time and risk profiles, can be deployed in synergistic packages to balance the conditions of durability, feasibility and social and environmental sustainability. However, it is important to note that no investment in CDR can justify a delay in rapid and sustained reductions in greenhouse gas emissionsOn the contrary, a comprehensive approach to investment in all forms of CDR offers the best prospect of meeting the long-term temperature goal of the Paris Agreement in the context of sustainable development. Spring

Glossary of key terms

• Carbon Dioxide Removal (CDR): Anthropogenic activities that remove CO2 from the atmosphere and permanently store it in geological, terrestrial or oceanic reservoirs or in products. This includes human enhancement of natural removal processes or deployment of carbon removal technologies.
• Durability: In a scientific context, it refers to the time scale of CO2 storage in a non-atmospheric pool. It is a function of (i) the length of storage and (ii) the risk of storage reversal (releasing CO2 back into the atmosphere).
• Permanence / Permanence: A term commonly used in carbon markets to describe the requirement that carbon credits represent permanent climate mitigation benefits. It is often associated with specific contractual or policy timeframes that are often shorter than the scientific understanding of permanent storage.
• Nature-based Conventional CDR / Natural Conventional CO2 Removal: CO2 removal methods that use natural processes (e.g. photosynthesis) to capture and store CO2 in vegetation, soil or sediment. Examples include afforestation/reforestation and peatland/wetland restoration. These methods tend to have a higher risk of reversal.
• Engineered Novel CDR / Engineered Novel CO2 Removal: CO2 removal methods, which involve human-designed technologies to capture CO2 and store it in more geochemically or physically stable forms. Examples include direct air carbon capture and storage (DACCS) and accelerated rock weathering (ERW). These methods tend to have a lower risk of reversal.
• Reversal Risk: The probability that stored carbon will be fully or partially released back into the atmosphere. This risk varies depending on the CDR method, time, geographical location, and risk management approaches used.
• Paris Agreement: An international climate change treaty adopted in 2015 that aims to limit global warming to well below 2°C above pre-industrial levels and to pursue efforts to limit the temperature increase to 1.5°C. Article 4.1. calls for a “balance between anthropogenic emissions by sources and removals by sinks of greenhouse gases in the second half of this century.”
• Net-zero Emissions: A state in which anthropogenic emissions of greenhouse gases into the atmosphere are balanced by anthropogenic removals over a specified period.
• Net-negative Emissions: A condition in which the amount of CO2 removed from the atmosphere is greater than the amount emitted, resulting in a net reduction in atmospheric CO2.
• Scalability: The ability of a CO2 removal method to be deployed and scaled up on the scale needed to significantly reduce atmospheric CO2, often in the gigatons per year range.
• Sustainability: In the context of CO2 removal, it refers to ensuring that CO2 removal activities optimize social and environmental benefits (e.g. for biodiversity, ecosystem services, human health and well-being) and minimize trade-offs.
• Liability: Legal or contractual responsibility for ensuring the durability of carbon storage and for the consequences of reversing the stored carbon. Policies and standards seek to define these obligations within specific time frames.
• Long-Term Low GHG Emission Development Strategies (LT-LEDS): Strategies that parties to the Paris Agreement voluntarily submit to the United Nations Framework Convention on Climate Change (UNFCCC) setting out their long-term greenhouse gas emission reduction goals and pathways to achieve them.
• Bioenergy with Carbon Capture and Storage (BECCS): A CO2 removal method that combines the cultivation of biomass (which absorbs CO2) with combustion for energy, with the CO2 emissions from the combustion being captured and stored, theoretically resulting in net negative emissions.
• Direct Air Carbon Capture and Storage (DACCS): A technology that captures CO2 directly from the ambient air and then stores it, usually underground.
• Enhanced Rock Weathering (ERW): A method of CO2 removal that involves spreading crushed silicate-rich minerals (such as basalt) onto the Earth's surface, where they react with atmospheric CO2 and permanently bind it in carbonate minerals.
• Geological Storage: Storing CO2 in underground geological formations, such as depleted oil and gas fields, saline aquifers, or unmined coal seams.
• Carbon Markets: Systems that allow carbon credits to be traded, where each credit typically represents a tonne of CO2 reduced or removed. The concept of “permanence” is key to the credibility of the credits in these markets.