Comment on essd-2022-245

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> With the focus of climate policy shifting from pledges to implementation, there is an increasing need to track progress on climate change mitigation at country level, especially for the land-use sector. Despite new tools and models offering unprecedented monitoring opportunities, striking differences remain in estimations of anthropogenic land-use CO₂ fluxes between the national greenhouse gas inventories (NGHGIs) used to assess compliance with the Paris Agreement, and the Global Carbon Budget and IPCC assessment reports, both based on global bookkeeping models (BMs). Recent evidence showed that these differences are mainly due to inconsistent definitions of anthropogenic forest CO₂ fluxes. In particular, the part of the land sink that is caused by the indirect effects of human-induced environmental change (e.g., fertilization effect on vegetation growth due to increase atmospheric CO₂ concentration, climate change) on managed lands is treated as non-anthropogenic by BMs, while in most cases is considered anthropogenic in NGHGIs. In addition, countries use a broader definition of managed land than BMs. Building on previous studies, we implement an approach that adds the CO₂ sink due to environmental change from countries&rsquo; managed forest area (estimated by Dynamic Global Vegetation Models, DGVMs) to the original land-use flux from BMs. This sum is expected to be conceptually more comparable to NGHGIs. Our analysis uses updated and more comprehensive data from NGHGIs than previous studies and provides model results at a greater level of disaggregation in terms of land categories (i.e., forest land, deforestation, organic soils, other land uses) and countries. Our results confirm a large difference in land use CO₂ fluxes between the ensemble mean of the BMs, estimating a source of 4.3 GtCO₂ yr^-1 globally for the period 2001&ndash;2020, and NGHGIs, which estimate a sink of -1.7 GtCO₂ yr^-1. Most of this 6.0 GtCO₂ yr^-1 gap is found on forest land (3.8 GtCO₂ yr^-1), with differences also for deforestation (1.1 GtCO₂ yr^-1), other land uses (1.0 GtCO₂ yr^-1), and to a lesser extent for organic soils (0.1 GtCO₂ yr^-1). By adding the DGVM ensemble mean sink arising from environmental change in managed forests (-5.1 GtCO₂ yr^-1) to BMs estimates, the gap between BMs and NGHGIs becomes significantly smaller both globally (residual gap: 0.9 GtCO₂ yr^-1) and in most regions and countries. The remaining differences mostly reflect smaller net emissions from deforestation and agricultural land in the NGHGIs of developing countries than in the BMs. By reconciling most of the differences between NGHGIs and global models (BMs and DGVMs), offering a blueprint for operationalizing future comparisons, and identifying areas to be further investigated, this study represents an important step forward for increasing transparency and confidence in land-use CO₂ flux estimates at the country level. This is crucial to support land-based mitigation investments and assess the countries&rsquo; collective progress under the Paris Agreement&rsquo;s Global Stocktake.