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

==== 5.4.5.3 Evaluation of Latitudinal Distribution of Simulated Carbon Sinks ====

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This distinction between the relatively high fidelity with which the ocean carbon sink is simulated, and the much wider range of simulations of the land carbon sink, is also evident in the zonal distribution of the sinks (Figure 5.24). We compare the ESM simulations to estimates from three atmospheric inversion models: Copernicus Atmosphere Monitoring Service (CAMS; [[#Chevallier--2005|Chevallier et al., 2005]] ), Carbon Tracker 2017 ( [[#Peters--2007|Peters et al., 2007]] ) and Model for Interdisciplinary Research on Climate Atmospheric Transport Model (MIROC-ATM4; [[#Saeki--2017|Saeki and Patra, 2017]] ). The ocean carbon cycle components of CMIP6 ESMs are able to simulate the tropical CO <sub>2</sub> source and mid-latitude CO <sub>2</sub> sink, with relatively small model spread (Figure 5.24a). The CMIP6 ensemble (red wedge) simulates a larger ocean carbon sink at 50°N and a weaker sink in the Southern Ocean, than the inversion estimate, but with some evidence of a reduction in these residual errors compared to CMIP5 (blue wedge). The spread in inversion fluxes arises primarily from differences in the atmospheric CO <sub>2</sub> measurement networks and from transport model uncertainties.

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[[File:241de5eb0312fe9520a7064d39e739db IPCC_AR6_WGI_Figure_5_24.png]]

'''Figure 5.24 |''' '''Comparison of modelled zonal distribution of contemporary carbon sinks against atmospheric inversion estimates for 2000–2009''' : '''(a)''' ocean carbon uptake; '''(b)''' net land uptake. Latitude runs from 90°S (i.e., –90°N) to 90°N. Positive uptake represents a carbon sink to ocean/land while negative uptake represents a carbon source. The land uptake is taken as net biome productivity (NBP) and so includes net land-use change emissions. The bands show the mean ±1 standard deviation across the available inversions (black bands, 3 models), CMIP5 Earth system models (ESMs) (blue bands, 12 models for the ocean, 12 models for the land), and CMIP6 ESMs (red bands, 11 models for ocean, 10 models for land). Further details on data sources and processing are available in the chapter data table (Table 5.SM.6).

It has been previously noted that AR5 models tended to overestimate land uptake in the tropics and underestimate uptake in the northern mid-latitudes, compared to inversion estimates. The inclusion of nitrogen limitations on CO <sub>2</sub> -fertilization within CMIP6 models was expected to reduce this discrepancy ( [[#Anav--2013|Anav et al., 2013]] ). There is indeed some evidence that the CMIP6 ensemble (red wedge in Figure 5.24b) captures the northern land carbon sink more clearly than CMIP5 (blue wedge in Figure 5.24b), but there remains a tendency for the ESMs to place more of the global land carbon sink in the tropics than the mid-latitudes, compared to the inversion estimates. Based on a consistent signal across CMIP6 ESMs, there is ''medium confidence'' that land carbon cycle models continue to underestimate the Northern Hemisphere land carbon sink, when compared to estimates from atmospheric inversion ( [[#Ciais--2019|Ciais et al., 2019]] ).

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