Study examines carbon dioxide removal (CDR) portfolios, which are key to achieving the Paris Agreement goals of limiting warming to 1.5°C-2°C. It assesses different CDR approaches and their combinations, including BECCS (bioenergy with CCS), afforestation/reforestation, DACCS (direct air CO2 capture), biochar and accelerated weathering.

The study highlights that Diversifying CDR approaches is the most cost-effective strategy to achieve net-zero emissions. Unlike previous focus on BECCS and AFOLU (agriculture, forestry and other land use), the study analyses a broader range of CDR options. It finds that diversification reduces over-reliance on a single approach, reduces land and energy impacts, and has fewer negative side effects.

Main findings of the study:

• Different CDR approaches have different characteristics, including mitigation potential, costs, benefits and adverse side effects.
• IAMs (Integrated Assessment Models) have so far been limited to BECCS and AFOLU, while insufficiently considering DACCS, accelerated weathering and biochar. This contrasts with the current carbon removal market, which is also showing interest in these approaches.
• Large-scale deployment of BECCS leads to controversial trade-offs in the field of energy and AFOLU.
• CDR portfolios are more viable in achieving climate goals, and their composition depends on the availability of resources, technologies and socio-political preferences.
• Strategic and regional selection of CDR reduces costs and allows for more removals.
• More diverse CDR portfolios they reduce negative side effects, thereby limiting technological reliance, institutional differences and ecological damage.

The study uses the EPPA (Economic Projection and Policy Analysis) model to analyze climate scenarios. The model was updated on biochar and accelerated weathering. Biochar is produced by pyrolysis of biomass, which stabilizes the carbon in the biomass. It improves soil quality and can replace fertilizers. Accelerated weathering accelerates natural processes that absorb CO2. Crushed rocks react with CO2, permanently removing it from the atmosphere.

The study compares the costs of different CDR approaches with existing literature and finds that the costs of DACCS are the highest, while BECCS and biochar are more cost-competitive. When evaluating different scenarios, the study found that the largest amount of CDR is achieved in the CDR portfolio scenario, where 31.5 GtCO2 is used annually by 2100. The “Only DACCS” scenario had the largest negative impact on the global economy, while the “Only BECCS”, “Only Biochar” and “Only EW” scenarios had milder impacts.

Other key insights are:

• BECCS and biochar have a significant impact on soil, while biochar requires twice as much land to grow bioenergy crops as BECCS. Conversely, DACCS and EW they have a smaller land footprint.
• DACCS and EW consume electricity, while BECCS and biochar produce it, with BECCS being more efficient than biochar.
• Resource-constrained scenario reduces CDR deployment, but shifts emphasis to BECCS and DACCS, and minimizes impact on land.
• Preservation of natural ecosystems has minimal negative economic impact, and could protect significant areas of natural forests and grasslands.
• CDR deployment delay would lead to a significant increase in carbon prices, highlighting the importance of timely policy and financial action.

The study also highlights the need for further research into modelling the potential agricultural benefits of biochar and accelerated weathering. In conclusion, diversifying CDR approaches, and taking into account regional specificities, is key to cost-effectively achieving net-zero emissions targets while reducing the negative impact on resources. Spring

Glossary of key terms

• Carbon Dioxide Removal (CDR) (Carbon dioxide removal): Activities that remove CO2 from the atmosphere and store it to mitigate climate change.
• Bioenergy with Carbon Capture and Storage (BECCS) (Bioenergy with Carbon Capture and Storage): A technology that uses biomass to produce energy and captures CO2 emissions from the process and stores them underground.
• Afforestation/Reforestation (AFOLU) (Afforestation/reforestation): Planting new trees or restoring existing forests to absorb atmospheric CO2.
• Direct Air Carbon Capture and Storage (DACCS) (Direct Carbon Capture and Storage): A technology that directly captures CO2 from the air and stores it underground.
• Biochar: Charcoal made from biomass through pyrolysis, which stores carbon in the soil and improves its properties.
• Enhanced Weathering (EW) (Rock Weathering): Accelerating the natural process of CO2 absorption by crushing and dispersing silicate rocks.
• Integrated Assessment Model (IAM) (Integrated Assessment Model): A computer model that combines knowledge from different scientific disciplines to analyze complex problems such as climate change.
• Net Zero (Net Zero): A state in which greenhouse gas emissions are balanced by the removal of greenhouse gases from the atmosphere, so that there is no net contribution to global warming.
• Paris Agreement (Paris Agreement): A 2015 international climate change agreement that aims to keep global warming well below 2°C, and preferably below 1.5°C.
• Mitigation Pathway (Mitigation Pathway): A scenario for future greenhouse gas emission reductions and large-scale CDR deployment over time to achieve certain climate goals.
• MRV (Measurement, Monitoring, Reporting and Verification): The process that ensures that greenhouse gas emission reductions are accurately measured, monitored and reported, and that these data are verified.
• EPPA Model: An economic model used to analyze climate strategies and their economic consequences, developed by MIT.
• Syngas: Gas produced from biomass, coal or other materials that can be used to generate energy.
• Pyrolysis (Pyrolysis): A thermochemical process that decomposes organic materials at high temperature in the absence of oxygen.
• Carbon Yield (Carbon yield): The amount of carbon dioxide removed from the atmosphere per hectare used to grow bioenergy crops.
• GDP (Gross Domestic Product): The total value of goods and services produced in a country during a certain period, used as a measure of economic activity.
• Nature-based Removal (Nature-based removal): Removing CO2 from the atmosphere through improved ecosystem functions.
• Techno-economic factors: Factors associated with available technologies and their economic feasibility that influence the introduction of CDR technologies.
• Bio-geophysical characteristics: Characteristics of geographic locations that influence the potential of CDR methods in given regions.