3.2.3.3 Carbon dioxide removal

With some exceptions (Riahi et al., 2021), the bulk of emissions pathways for reaching ambitious temperature goals still exceed the near-term carbon budget, lead to temperature overshoot, and are brought down in the latter half of the century by a speculative scale of novel carbon sinks (IPCC AR5, 2014; IPCC, 2018; IPCC AR6, 2022). Carbon removal is emerging as a key pillar of climate assessments and policy. IPCC AR6 argued across all three working groups that carbon removal will play an essential role in strategies that limit warming to no more than 1.5ºC and is an important feature of “well below 2ºC” scenarios. Correspondingly, countries increasingly integrate carbon sinks into their net-zero goals, NDCs (Hale et al., 2022) and mid-century strategies (Smith. H.B., Vaughan, and Forster, 2022). For now, they predominantly repurpose land use and ecosystem management practices as carbon removal. Engineered carbon removal prototypes and practices are piloted at small scales, but these remain immature or speculative as socio-technical systems (Sovacool, Baum, and Low, 2023). The prospects for scaling to the multi-gigaton levels foreseen in integrated assessment modelling are doubtful, with only limited attention so far to the demand side and policy beyond research and development (Nemet et al., 2018). It is uncertain if these can reach the scale envisioned in pathways in line with well below 2ºC or 1.5ºC. Hence some filtering of plausible emissions pathways to not rely on excessive carbon removal is necessary.

It is important to recognise that carbon removal is understood as playing two roles. First, it can balance residual, recalcitrant emissions in a net-zero state. The currently projected scale of such residuals and removals is substantial at close to 20 percent of current emissions (Buck et al., 2023). The second role is to reverse overshoot of carbon budgets (reducing ultimate outcome temperatures). The more removal capacity required for the first task, the greater the challenge of providing sufficient, rapid, sustainable capacity for the second.

The development of removal approaches also requires careful governance to avoid their use as a substitute for achievable mitigation, rather than a supplement. One analysis of the risk of mitigation deterrence through carbon removal estimates as much as 1.4ºC additional warming (over the 1.5ºC goal) could result. (McLaren, 2020)

Assessment of the relationship between carbon removal and tipping points is nascent. While large-scale CDR efforts might have desirable effects on global temperatures, it faces significant scaling challenges and would likely operate more slowly than many other mitigation approaches. These challenges likely limit its potential as a prevention tool in comparison to GHG emission reduction.

Carbon removal techniques could also have other positive and negative effects on ecosystems and hence tipping point risks. For example, some carbon-removal approaches, such as forest conservation and afforestation, could increase forest resilience and counteract tipping dynamics. But opposite effects are also possible. At scale, most carbon-removal techniques compete for land and/or low-carbon energy supplies, with negative effects on both sustainability and justice (Smith et al., 2015; McLaren, 2012). Moreover, large-scale conversion of natural forests for the purpose of Bioenergy with Carbon Capture and Storage (BECCS) might increase ecosystem vulnerability and the possibility of forest loss, and afforestation in drylands and grassland ecosystems could make tipping more likely in those ecosystems (see Chapters 1.3.2.4 and 1.3.2.5). Proposals for large-scale oceanic carbon removal through alkalinisation or fertilisation also raise questions about their interactions with tipping point drivers, effectiveness and ecosystem disruption (Fakhraee et al., 2023; Tagliabue et al., 2023). Overall, there is so far limited research on the nature and net balance of such effects.