Overview
The subtle shift in emphasis away from a focus on reducing and eliminating GHG emissions (as in the quantified emission limitation and reduction targets of the Kyoto Protocol) towards net zero framing has shone a new light on the role and potential of GHG removal (GGR) or CDR in climate change mitigation[1]. Today, there is growing acceptance that CDR methods are needed to offset some ongoing, hard-to-abate, residual anthropogenic GHG emission sources to achieve and thereafter maintain a state of net zero (IPCC 2022).
The aim of this study is to provide a synthesised technical assessment of carbon dioxide removal (CDR) methods and review their measurement, reporting, and verification (MRV) features and accounting aspects.
Summary
- Reasons persist for being both positive and circumspect about the role of CDR in climate action. On the one hand, CDR seems essential for achieving the Paris Agreement. On the other hand, CDR poses significant challenges for climate policy.
- A number of MRV efforts are underway today by private and public actors in both the voluntary carbon markets (VCM) and compliance (regulated) carbon markets. The discussions and outcomes from these processes will have clear ramifications for the way in which MRV approaches and methodologies will be used to calculate and verify CDR actions in future. Yet, at time of writing, much debate and uncertainty remain regarding the best means of doing so.
- The technical review of CDR methods highlights that questions remain over the foundational science (except relating to engineered geological storage and biological capture and storage) and/or social acceptance underpinning most CDR methods. As such, the view that a portfolio of CDR methods is likely to be needed to meet the Paris Agreement is reaffirmed.
- For CDR methods such as soil organic carbon (SOC), enhanced weathering (EW) and ocean-based CDR, challenges persist for monitoring CO2 flux rates, carbon (C) stock changes and the fate and behaviour of C carriers in the environment. Despite indications that a CO2 drawdown effect can be expected, monitoring of field trials have in many cases been unable to corroborate these hypotheses.
- Recent focus has been almost exclusively on developing project-based methodologies, which are inherently consequential and rely on baseline scenarios and counterfactuals that are notoriously difficult to establish. Thus, related additionality testing in credit schemes has been subject to widespread criticism.
- Two critical issues for the current approach to CDR crediting/certification exist:
- Policy utility: If certified/credited CDR activities do not create a symmetrical[1] and equivalent amount of CDR in the host country national greenhouse gas inventory (NGHGI) – a problem that can be termed ‘inventory visibility’ – then any policy designed to incentivise CDR will lack political utility.
- Long-term responsibility: Host countries are ultimately the ‘underwriter of last resort’ for carbon reversal, i.e. they will need to take very long term responsibility for the carbon storage and manage the risk of carbon reversal. Better understanding of the liability of any carbon reversals may impair countries’ willingness to host CDR activities, a problem that can be further exacerbated by inventory visibility.
- Ocean-based CDR methods pose unique challenges in these regards: since the C reservoir falls outside of national MRV frameworks, any CO2 drawdown will be neither visible in any country’s NGHGI nor subject to any host country monitoring that could offer to accept liability for carbon reversal. Until these challenges are addressed, ocean-based CDR will not meet UNFCCC accounting standards.