Abstract

Most emission scenarios consistent with the Paris Agreement target of limiting global warming to 1.5°C include net negative CO₂ emissions in the second half of this century, i.e. CO₂ removal (CDR) from the atmosphere exceeds CO₂ emissions. These pathways differ significantly with regards to their: a) CDR efficiency — the net CO₂ removal; b) timing — the potential for net CO₂ removal occurring at the right time to meet the net-zero targets; and c) permanence — the net CO₂ removal from the atmosphere for a sufficiently long length of time.

Here, we adapted the MONET framework to compare the CDR efficiency, timing, and permanence of a non-exhaustive portfolio of archetypal CDR pathways representing afforestation/reforestation (AR), biochar, bioenergy with carbon capture and storage (BECCS), direct air capture of CO₂ with storage (DACCS) and enhanced weathering (EW) (see Fig. 1). We showed that, in the case of BECCS, the carbon footprint of biomass feedstocks contribute to up to 26% reduction in CDR efficiencies, especially when high-moisture biomass feedstock is adopted. Upstream activities, such as biomass cultivation and processing, are responsible for the largest share of CO₂ emissions. By contrast, biomass supply chain emissions have a mild impact on the overall CDR efficiency of biochar, which is mainly affected by the overall C yield of pyrolysis processes, by almost 50%. AR is subject to a range of catastrophic events, specifically wildfires, which risk can be assessed through their frequency and severity. Ongoing forestry management could help reduce this risk, thus contributing to increase the overall CO₂ removal potential of this CDR pathway. The CDR efficiency of AR declines by more than half in warm and dry climates (i.e., subtropical and tropical), whereas it remains unchanged in cold and humid climates (i.e., boreal). Consequently, AR’s permanence is overall very likely to decrease significantly over time, and to become very low. Finally, the CDR efficiency of DACCS and EW is affected by the carbon intensity of the energy used in the CO₂ capture process and in the grinding of the rock, respectively. We also observed a trade-off between the rock size adopted in EW processes, with smaller rock leading to higher CDR removals, and the higher energy consumption associated with rock grinding, leading to lower CDR removals.

Plain-language Summary

CDR options differs in term of CO₂ removal efficiency. Importantly, the CO₂ removal efficiency of all CDR options is intertwined with their timing and permanence, but comparative quantitative analyses remain a lacuna in the literature. As CDR is expected to be deployed at a commercial-scale, and the service that gets remunerated is the permanent removal of CO₂ from the atmosphere, there is a need to understand how impactful removal of CO₂ is, now and over time. This study addresses this knowledge gap by identify and quantify their key sources of CO₂ leakages, and discuss the impact of time, both in terms of timing and permanence, on the CO₂ removal efficiency of these CDR methods.