In the pursuit of climate change mitigation and achieving the long-term temperature goal of the Paris Agreement (Article 2.1.a), rapid and deep reductions in greenhouse gas (GHG) emissions are essential across all sectors. The very However, reducing emissions is not enough; achieving “a balance between anthropogenic emissions by sources and removals by sinks of greenhouse gases in the second half of this century” (Article 4.1 of the Paris Agreement) will require carbon dioxide removal (CDR)CDR is defined as “anthropogenic activities that remove CO2 from the atmosphere and permanently store it in geological, terrestrial or oceanic reservoirs or in products”. Durability – along with scalability and sustainability – is a key condition for the success of CDR and depends on (i) the duration of CO2 storage and (ii) the risk of reversal of such storage.

Durability from a Climate Science Perspective: Storage Duration and Risk of Reversal

The concept of “permanence” in relation to CDR refers to the timescale over which CO2 is stored outside the atmosphere. A sufficiently long storage period for CO2 is essential for climate change mitigation. If the sole objective of CDR is to offset anthropogenic emissions, storage must be effective for as long as the emitted greenhouse gases would disturb the atmosphere, i.e. thousands of years for fossil CO2. However, CDR also plays a key role in achieving short- to medium-term goals (decades to centuries). Less persistent CDR can contribute to achieving net reductions in GHG emissions in the coming decades and reduce the magnitude and duration of temperature peaks, thereby reducing negative impacts on nature and people.

CO2 storage time varies considerably between CDR methods depending on the physical and chemical stability of the carbon storage forms.

• Technological (engineering) methods of CDR, such as direct air CO2 capture and storage (DACCS) or accelerated rock weathering (ERW), can store CO2 for thousands of years, especially if the CO2 is trapped under a thick, impermeable seal or converted into solid minerals.
• Natural (conventional) CDR methods, which capture CO2 through photosynthesis and store it in plant biomass and soil organic matter (e.g. afforestation/reforestation, peatland/wetland restoration), can store carbon for centuries in woody biomass and up to millennia in deeper soil layers.

Risk of reversal refers to the probability that stored carbon will be fully or partially re-released into the atmosphere.

• Geological carbon storage has a relatively high risk of reversal initially until CO2 stabilizes, but over time this risk asymptotically approaches zero. For example, immobilization of CO2 in basalt rocks can occur over years.
• On the contrary, the reversal risk profile for CDR involving biological storage is more variable and less predictable. Newly established ecosystems may be vulnerable in their initial stages and later gain increased resilience, but factors such as fires, pests, deforestation or agricultural practices contribute to an increased risk of reversal. However, strong safeguards and investments in project design and management can help manage the risks of reversal in natural methods.

Sustainability in Policy Making: Accountability and Commitments

Policymakers must address the challenge of sustainability when creating incentives for the implementation of CDR. When formulating policies, they must define sustainability in the context of a specific policy or investment, taking into account feasibility, political acceptability, and social and environmental benefits in addition to science.

CDR plays a key role in national and corporate goals of net zero GHG or CO2 emissions. Depending on how CDR is integrated into these goals and what the specific mitigation objective is, permanence will need to be defined and responsibility for CO2 storage assigned. It is important to note that current CDR policies and contracts tend to cover much shorter time frames (rarely exceeding a few decades and almost never more than a century) than those considered by science when discussing permanence. In the event of a reversal of CO2 storage, the lost carbon would need to be fully replaced, for example through additional CDR efforts.

A complementary approach to implementing CDR methods

Instead of favoring technological CDR methods over natural ones, comprehensive and a diversified portfolio of CDR methods is more likely to mitigate risks (including risks of reversal and sustainability) while increasing the scale of removal. No CDR method currently meets all the criteria for readiness and scalability, sustainability and durability over several centuries.

• Technological methods they offer greater durability and lower risk of reversal.
• Conventional natural methods however, they are more immediately deployable at scale, are more cost-effective and deliver significant co-benefits for people and nature, such as enhancing biodiversity and ecosystem services.

Natural and technological CDR methods that exhibit complementary time and risk profiles can be deployed in synergistic packages to balance the conditions of durability, feasibility and social and environmental sustainability. Policies should incentivize time- and context-relevant combinations of CDR methods that are initiated immediately, maximize scalability and are sustained over the long term to lead to the expected climate stabilization. However, it is crucial to emphasize that no investment in CDR can justify delaying rapid and sustained greenhouse gas emission reductions. Spring

Glossary of key terms

• Anthropogenic Emissions (Anthropogenic Emissions): Greenhouse gas emissions that result from human activity, such as the burning of fossil fuels.
• Bioenergy with carbon capture and storage (BECCS): A CDR method that involves growing biomass, burning it to produce energy, and then capturing and storing CO2 emissions.
• Biochar: A form of charcoal made from biomass that is added to soil to improve its fertility and store carbon.
• Net-Zero Emissions: A state in which any remaining greenhouse gas emissions to the atmosphere are balanced by equivalent removals from the atmosphere.
• Net-Negative Emissions: A state in which more greenhouse gases are removed from the atmosphere than are emitted, leading to an overall decrease in atmospheric concentrations.
• DACCS (Direct Air Carbon Capture and Storage): A technology that captures CO2 directly from the ambient air and then stores it.
• Durability: The length of time CO2 remains stored outside the atmosphere and the risk of its release. This is a graduated concept in climate science.
• ERW (Enhanced Rock Weathering): A process that accelerates the natural chemical weathering of rocks that bind CO2 from the atmosphere.
• Geological Reservoirs: Underground geological formations, such as salt formations or depleted oil and gas fields, that are used for the permanent storage of CO2.
• The Paris Agreement's global temperature goal: The goal is to limit global warming to well below 2°C above pre-industrial levels and to pursue efforts to limit it to 1.5°C.
• IPCC (Intergovernmental Panel on Climate Change): Intergovernmental Panel on Climate Change, the main international body for assessing climate change.
• Kyoto Protocol: An international treaty that set binding targets for reducing greenhouse gas emissions for industrialized countries.
• LT-LEDS (Long-Term Low GHG Emission Development Strategies): Long-term low-carbon development strategies that countries submit under the Paris Agreement.
• Nature-based Conventional CDR: Methods that use or enhance natural processes to capture and store carbon (e.g. afforestation, wetland restoration).
• Novel Engineered CDR: Methods that use carbon capture and storage technology (e.g. DACCS, ERW).
• Overshoot scenarios: Scenarios in which global temperature temporarily exceeds the target limit (e.g. 1.5°C) before decreasing again through extensive CDR.
• Paris Agreement: A key international treaty on climate change.
• Permanence (Permanence): A concept in carbon markets that refers to the long-term (often centuries) storage of carbon to provide lasting benefits for climate change mitigation.
• Reversal Risk: The probability that previously stored carbon will be re-released into the atmosphere.
• Sinks: Processes, activities or mechanisms that remove a greenhouse gas, aerosol or greenhouse gas precursor from the atmosphere.
• Sources: Processes or activities that release greenhouse gases into the atmosphere.
• Sustainable Development: Development that meets the needs of the present without compromising the ability of future generations to meet their own needs.
• Synergistic CDR Portfolios: Combining different CDR methods with complementary time and risk profiles to optimize overall benefits and minimize risks.