The impacts of ocean- and land-based Carbon Dioxide Removal on Planetary Boundaries

To meet the Paris Agreement targets we need large-scale deployment of carbon dioxide removal (CDR). Most of the literature focuses on the removal potential of CDR and the direct effect on temperature. Nevertheless, to design sustainable and robust mitigation strategies, we need to explore and quantify broader impacts of CDR on the Earth system. Although these effects on the Earth system have been identified, there is no comprehensive understanding of their magnitude if CDR is implemented on a large scale. The Planetary Boundary (PB) framework aims to maintain a safe operating system for humans. The PB framework, used with an Earth system model, could be used to systematically assess the positive or negative effects of different CDR methods on the Earth system features that are critical to human welfare.

We use the University of Victoria Earth System Climate Model (UVic ESCM) to simulate large-scale ocean alkalinity enhancement (OAE), artificial upwelling (AU), reforestation (REF), and bioenergy with carbon capture and storage (BECCS), under the SSP1-2.6 control scenario. Our goal is to assess the efficiency and sustainability of the selected CDR methods. Therefore, we use the PB framework to quantify CDR’s impact on the PBs of climate change, ocean acidification, land system change, biochemical flows, freshwater change, and biosphere integrity. By doing so, we can assess whether, after implementing CDR, the PB control variables stay below the boundaries (safe operating space) or go beyond it to the increasing or high-risk zone.

Our preliminary results show the impacts of OAE and REF on radiative forcing, CO₂ concentration, ocean acidification, and land system change. In all the future scenarios, the radiative forcing level falls in the high-risk zone (3.00 Wm^-2). In 2300, OAE and REF reduce the radiative forcing to 1.7 Wm^-2, which gets closer to the upper end of the zone of increasing risk (1.5 Wm^-2), but still in the high-risk zone. In 2100, the CO₂ concentration decreases in all the future scenarios, getting closer to the upper end of the zone of increasing risk (450 ppm), only reached by REF. In 2300, the CO₂ concentration further decreases, falling within the zone of increasing risk, with OAE and REF CO₂ concentration of 383 and 387 ppm, respectively. REF almost reaches the PI forest coverage (92% in 2100, 96% in 2300), while OAE has a negligible impact on the land system change, staying, nevertheless, within the PB (75% of PI forest coverage). OAE has the highest impact on ocean acidification, quantified as surface ocean saturation state with respect to aragonite (Ωarag). OAE increases the surface ocean’s aragonite saturation state (2.63 Ωarag) close to the PB (2.75 Ωarag) in 2100, and it allows staying within the PB in 2300 (2.97 Ωarag). REF shows a similar increase in 2100, but a slightly smaller increase in 2300 (2.86 Ωarag) compared to OAE.

To conclude, only in the far future, large-scale CDR will help stay within the PB or upper PB of most of the explored control variables; however, CDR impacts are mainly minor compared to SSP1-2.6.