Editing IPCC:AR6/WGI/Chapter-5 (section)

==== 5.4.5.2 Evaluation of Historical Carbon Cycle Simulations in Concentration-driven Runs ====

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This section evaluates concentration-driven historical simulations of changes in land and ocean cumulative carbon uptake, against observation-based estimates from the Global Carbon Project (GCP; [[#Le%20Quéré--2018a|Le Quéré et al., 2018a]] ). For each model, common historical land-use changes were prescribed ( [[#Jones--2016a|Jones et al., 2016a]] ).

Figure 5.23 shows global annual mean values from CMIP6 concentration-driven runs for 1850 to 2014. The ocean carbon cycle models reproduce historical carbon uptake well, with the model range for the global ocean carbon sink in 2014 (2.3–2.7 GtC yr <sup>–1</sup> ) clustering around the central GCP estimate of 2.6 ± 0.5 GtC yr <sup>–1</sup> . Simulated cumulative ocean carbon uptake (1850–2014) ranges from 110 to 166 GtC, with a model mean of 131 ± 17 PgC, which is lower than the GCP estimate of 150 ± 25 GtC (Figure 5.23a). This suggests that CMIP6 models may slightly underestimate historical ocean carbon uptake ( [[#Watson--2020|Watson et al., 2020]] ).

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'''Figure 5.23 |''' '''CMIP6 Earth system model (ESM) concentration-driven historical simulations for 1850 to 2014, compared to observation-based estimates from the global carbon project (GCP).''' '''(a)''' Cumulative ocean carbon uptake from 1850 (PgC); '''(b)''' cumulative land carbon uptake from 1850 (PgC). Only models that simulate both land and ocean carbon fluxes are shown here. Further details on data sources and processing are available in the chapter data table (Table 5.SM.6).

The land carbon cycle components of historical ESM simulations show a larger range, with simulated cumulative land carbon uptake (1850–2014) spanning the range from –47 to +21 GtC, compared to the GCP estimate of –12 ± 50 GtC (Figure 5.23b). This range is due in part to the complications of simulating the difference between carbon uptake by intact ecosystems and the direct release of carbon due to land-use change ( [[#Hajima--2020a|Hajima et al., 2020a]] ). There is ''high confidence'' that the land continues to dominate the overall uncertainty in the projected response of the global carbon cycle to climate change.

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