Net zero weakens the land carbon sink but land-based mitigation compensates

Abstract Land ecosystems absorb about a fifth of human CO₂ emissions, mostly because rising CO₂ stimulates plant growth. National greenhouse gas inventories count this land carbon sink on managed land as anthropogenic. Mitigation pathways count only the land-use flux, the emissions and removals from land use. To match inventories, external sink estimates are added to pathways afterwards. Here we compute both components inside the integrated assessment framework LPJmL-MAgPIE-REMIND for six CMIP7 scenarios, with the land’s response to climate change and CO₂ included. In scenarios reaching net-zero CO₂ around mid-century, atmospheric CO₂ declines and the sink on managed land weakens. We show that the sink weakens later than in Grassi et al. and does not become a source, unlike in Gidden et al. Land-based mitigation, such as afforestation, turns the land-use flux from a net emission today into net removals that by 2100 more than compensate for the weaker sink. Inventories count the sum of the sink and the land-use flux. In our net-zero scenarios, this sum peaks in 2050–2070, then loses about 25% by 2100. With our sink counted, net-zero CO₂ comes 2 to 7 years earlier than with the land-use flux alone, and at most 3 with either external estimate.