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

=== Cross-Chapter Box 6 | Implications of Reconciled Anthropogenic Land CO 2 Fluxes for Assessing Collective Climate Progress in the Global Stocktake ===

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'''Authors:''' Giacomo Grassi (Italy/European Union), Joeri Rogelj (Belgium/Austria), Joanna I. House (United Kingdom), Alexander Popp (Germany), Detlef van Vuuren (the Netherlands), Katherine Calvin (the United States of America), Shinichiro Fujimori (Japan), Petr Havlík (Austria/the Czech Republic), Gert-Jan Nabuurs (the Netherlands)

The Global Stocktake aims to assess countries’ collective progress towards the long-term goals of the Paris Agreement in the light of the best available science. Historic progress is assessed based on NGHGIs, while expectations of future progress are based on country climate targets (e.g., NDCs for 2025 or 2030 and long-term strategies for 2050). Scenarios consistent with limiting warming well-below 2°C and 1.5°C developed by IAMs (Chapter 3) are expected to play a key role as benchmarks against which countries’ aggregated future mitigation pledges will be assessed. This, however, implies that estimates by IAMs and country data used to measure progress are comparable.

In fact, there is about 5.5 GtCO 2 yr –1 difference during 2005–2015 between global anthropogenic land CO 2 net flux estimates of IAMs and aggregated NGHGIs, due to different conceptual approaches to what is ‘anthropogenic’. This approach and its implications when comparing climate targets with global mitigation pathways are illustrated in this Box Figure 1a–e.

By adjusting the original IAM output (Cross-Chapter Box 6, Figure 1a) with the indirect effects from countries’ managed forest (Cross-Chapter Box 6, Figure 1b, estimated by DGVMs, see also Figure 7.6), NGHGI-comparable pathways can be derived (Cross-Chapter Box 6, Figure 1c). The resulting apparent increase in anthropogenic sink reflects simply a reallocation of a CO 2 flux previously labelled as natural, and thus does not reflect a mitigation action. These changes do not affect non-LULUCF emissions. However, since the atmosphere concentration is a combination of CO 2 emissions from LULUCF and from fossil fuels, the proposed land-related adjustments also influence the NGHGI-comparable economy-wide (all sector) CO 2 pathways (Cross-Chapter Box 6, Figure 1d).

This approach does not imply a change in the original decarbonisation pathways, nor does it suggest that indirect effects should be considered in the mitigation efforts. It simply ensures that a like-with-like comparison is made: if countries’ climate targets use the NGHGI definition of anthropogenic emissions, this same definition can be applied to derive NGHGI-comparable future CO 2 pathways. This would have an impact on the NGHGI-comparable remaining carbon or GHG budget (i.e., the allowable emissions until net zero CO 2 or GHG emissions consistent with a certain climate target). For example, for SSP2-1.9 and SSP2-2.6 (representing pathways in line with 1.5°C and well-below 2°C limits under SSP2 assumptions), carbon budget is 170 GtCO 2 -eq lower than the original remaining carbon budget according to the models’ approach (Cross-Chapter Box 6, Figure 1e). Similarly, the remaining carbon (or GHG) budgets in [[IPCC:Wg3:Chapter:Chapter-3|Chapter 3]] (this report), as well as the net zero carbon (or GHG) targets, could only be used in combination with the definition of anthropogenic emissions as used by the IAMs (Cross-Chapter Box 3 in Chapter 3). In the absence of these adjustments, collective progress would appear better than it is.

Cross-Chapter Box 6

The UNEP’s annual assessment of the global 2030 ‘emission gap’ between aggregated country NDCs and specific target mitigation pathways ( [[#UNEP--2020|UNEP 2020]] ), is only affected to a limited degree. This is because some estimates of global emissions under the NDCs already use the same land-use definitions as the IAM mitigation pathways ( [[#Rogelj--2017|Rogelj et al. 2017]] ), and because historical data of global NDC estimates is typically harmonised to the historical data of global mitigation pathway projections ( [[#Rogelj--2011|Rogelj et al. 2011]] ). This latter procedure, however, is agnostic to the reasons for the observed mismatch, and often uses a constant offset. The adjustment described here allows this mismatch to be resolved by drawing on a scientific understanding of the underlying reasons, and thus provides a more informed and accurate basis for estimating the emission gap.

The approach to deriving a NGHGI-comparable emission pathways presented here can be further refined with improved estimates of the future forest sink. Its use would enable a more accurate assessment of the collective progress achieved and of mitigation pledges under the Paris Agreement.

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'''Cross-Chapter Box 6, Figure 1 | Impact on global mitigation pathways of adjusting the modelled anthropogenic land CO 2 fluxes to be comparable with National Greenhouse Gas Inventories (NGHGIs) (from Grassi et al. 2021).''' 

'''(a)''' The mismatch between global historical LULUCF CO 2 net flux from NGHGIs (black), and the original (un-adjusted) modelled flux historically and under future mitigation pathways for SSP2 scenarios from Integrated Assessment Models (IAMs, Chapter 3). '''(b)''' Fluxes due to indirect effects of environmental change on areas equivalent to countries’ managed forest (i.e., those fluxes generally considered ‘anthropogenic’ by countries and ‘natural’ by global models). '''(c)''' Original modelled (solid line) LULUCF mitigation pathways adjusted to be NGHGI-comparable (dashed line), for example, by adding the indirect effects in panel b. The indirect effects in panel b decline over time with increasing mitigation ambition, mainly because of the weaker CO 2 fertilisation effect. In panel c, the dependency of the adjusted LULUCF pathways on the target becomes less evident after 2030, because the indirect effects in countries’ managed forest (which are progressively more uncertain with time, as highlighted by the grey areas) compensate the effects of the original pathways. '''(d)''' NGHGI-comparable pathways for global CO 2 emissions from all sectors including LULUCF (obtained by combining global CO 2 pathways without LULUCF – where no adjustment is needed – and the NGHGI-comparable CO 2 pathways for LULUCF ( [[#Gütschow--2019|Gütschow et al. 2019]] ; [[#Grassi--2017|Grassi et al. 2017]] ). '''(e)''' Cumulative impact of the adjustments from 2021 until net zero CO 2 emissions or 2100 (whatever comes first) on the remaining carbon budget.

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