Editing IPCC:AR6/WGIII/Chapter-7 (section)

==== 7.2.2.3 Implications of Differences in AFOLU CO 2 Fluxes Between Global Models and National Greenhouse Gas Inventories (NGHGIs), and Reconciliation ====

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There is about 5.5 GtCO 2 yr –1 difference in the anthropogenic AFOLU estimates between NGHGIs and global models (this number relates to an IAMs comparison for the period 2005–2015 – see Cross-Chapter Box 6 in this chapter; for comparison with other models see Figure 7.4). Reconciling the differences, in other words, making estimates comparable, can build confidence in land-related CO 2 estimates, for example for the purpose of assessing collective progress in the context of the Global Stocktake (Cross-Chapter Box 6 in this chapter). The difference largely results from greater estimated CO 2 in NGHGIs, mostly occurring in forests ( [[#Grassi--2021|Grassi et al. 2021]] ). This difference is potentially a consequence of: (i) simplified and/or incomplete representation of management in global models ( [[#Popp--2017|Popp et al. 2017]] ; [[#Pongratz--2018|Pongratz et al. 2018]] ), for example, concerning impacts of forest management in biomass expansion and thickening ( [[#Nabuurs--2013|Nabuurs et al. 2013]] ; [[#Grassi--2017|Grassi et al. 2017]] ), (ii) inaccurate and/or incomplete estimation of LULUCF fluxes in NGHGIs ( [[#Grassi--2017|Grassi et al. 2017]] ), especially in developing countries, primarily in non-forest land uses and in soils, and (iii) conceptual differences in how global models and NGHGIs define ‘anthropogenic’ CO 2 flux from land ( [[#Grassi--2018|Grassi et al. 2018]] ). The impacts of (i) and (ii) are difficult to quantify and result in uncertainties that will decrease slowly over time through improvements of both models and NGHGIs. By contrast, the inconsistencies in (iii) and its resulting biases were assessed as explained below.

Since changing the NGHGIs’ approach is impractical, an interim method to translate and adjust the output of global models was outlined for reconciling a bookkeeping model and NGHGIs ( [[#Grassi--2018|Grassi et al. 2018]] ). More recently, an improved version of this approach has been applied to the future mitigation pathways estimated by IAMs ( [[#Grassi--2021|Grassi et al. 2021]] ), with the implications for the Global Stocktake discussed in Cross-Chapter Box 6 in this chapter. This method implies a post-processing of current global models’ results that addresses two components of the conceptual differences in the ‘anthropogenic’ CO 2 flux; (i) how the impact of human-induced environmental changes (indirect effects) are considered, and (ii) the extent of forest area considered ‘managed’. Essentially, this approach adds DGVM estimates of CO 2 fluxes due to indirect effects from countries’ managed forest area (using non-intact forest area maps as a proxy) to the original global models’ anthropogenic land CO 2 fluxes (Figure 7.6).

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'''Figure 7.6 | Main conceptual differences between global models (bookkeeping models, IAMs and DGVMs) and NGHGIs definitions of what is considered the ‘anthropogenic’ land CO''' 2 '''flux, and proposed solution (from''' '''Grassi''' '''et al.''' '''2021). (a)''' Differences in defining the anthropogenic land CO 2 flux by global models (‘land use’) and NGHGIs (‘LULUCF’), including the attribution of processes responsible for land fluxes ( [[#IPCC--2006|IPCC 2006]] ; 2010) in managed and unmanaged lands. The anthropogenic land CO 2 flux by global models typically includes only the CO 2 flux due to ‘direct effects’ (land-use change, harvest, regrowth). By contrast, most NGHGIs consider anthropogenic all fluxes occurring in areas defined as ‘managed’, including also the sink due to ‘indirect effects’ (climate change, atmospheric CO 2 increase, N deposition etc.) and due to ‘natural effects’ (climate variability, background natural disturbances). '''(b)''' Proposed solution to the inconsistency, via disaggregation of the ‘Land Sink’ flux from DGVMs into CO 2 fluxes occurring in managed and in unmanaged lands. The sum of ‘land use’ flux (direct effects from bookkeeping models or IAMs) and the ‘Land Sink’ (indirect effects from DGVMs) in managed lands produces an adjusted global model CO 2 flux which is conceptually more comparable with LULUCF fluxes from NGHGIs. Note that the figure may in some cases be an oversimplification, in other words, not all NGHGIs include all recent indirect effects.

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