The land use, land-use change, and forestry (LULUCF) sector is important for achieving global climate targets. That is both reflected in scientific models exploring climate mitigation strategies and by a majority of countries including land-based emissions reductions and removals in their latest NDC submissions. At the same time, scientific models and the national greenhouse gas inventories (NGHGIs) submitted to the UNFCCC differ in how they estimate land-based emissions. This is due to different definitions of what constitutes “managed” land and anthropogenic emissions removals on that land. The resulting gap between models and country reports is 4-7 Gt CO2 per year. It needs to be taken into account to make model outputs and country reports comparable and ensure policy makers are working towards the correct goal.

Models differentiate between direct and indirect carbon fluxes, where direct ones result from changes in land cover or management and are typically considered to be anthropogenic. Indirect fluxes result from human-induced environmental changes (e.g., an increase in atmospheric carbon dioxide amplifying carbon fertilization). They are accounted for both on managed and un-managed land but are considered to be “natural” carbon sinks. In contrast, NGHGI reporting conventions use an area-approach to define anthropogenic fluxes, considering all fluxes on managed land to be anthropogenic, including indirect ones. Furthermore, countries may define areas to be managed that may be considered un-managed in models. As a result, countries report larger anthropogenic sinks than models typically do.

Utilizing OSCAR, a reduced-complexity climate model with an explicit representation of the land carbon cycle, this gap was bridged. We estimated current and future indirect emissions and used these estimates to align models with aggregate estimates from NGHGIs. Based on this, we updated future LULUCF fluxes and resulting mitigation benchmarks. The results highlight the importance of this re-alignment: When using the new NGHGI-aligned fluxes, more ambitious mitigation becomes necessary as the additional negative flux moves the net-zero timing forward by 1-5 years for 1.5°C pathways compared to results based on the un-aligned modelling conventions. Emissions reductions increase by 3-6 percentage points and cumulative CO2emissions are 15-18% lower. 2°C pathways show similar trends.

This also shows in the increased deployment of carbon dioxide removal (CDR) in the LULUCF sector. The additional land-based CDR between 2020 and 2030 in 1.5°C pathways is 30% higher when using the re-aligned fluxes. Combined with non-land CDR, an additional 2.8 Gt CO2is captured each year compared to 2020 levels, more than doubling the total annual CDR to 5 Gt CO2per year in the 1.5°C pathways. While vastly dominant at nearly 100% between 2020 and 2030, land-based sinks’ importance for CDR shrinks over time, falling to about 30% by 2100. These results stress the need to work towards climate target-appropriate CDR deployment goals.

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