Abstract
<strong class="journal-contentHeaderColor">Abstract.</strong> Reliable estimates of soil carbon change are required to determine the carbon budgets consistent with the Paris climate targets. This study evaluates projections of soil carbon during the 21<sup>st</sup> century in CMIP6 Earth System Models (ESMs) under a range of atmospheric composition scenarios. In general, we find a reduced spread of changes in global soil carbon (Δ<em>C<sub>s</sub></em>) in CMIP6 compared to the previous CMIP5 model generation. However, similar reductions were not seen in the derived contributions to Δ<em>C<sub>s</sub></em> due to both increases in plant Net Primary Productivity (NPP, named Δ<em>C<sub>s</sub></em>,<em><sub>NPP</sub></em>) and reductions in the effective soil carbon turnover time (<em>τ</em><sub><em>s</em></sub>, named Δ<em>C<sub>s,τ</sub></em>). Instead, we find a strong relationship across the CMIP6 models between these NPP and <em>τ</em><sub><em>s</em></sub> components of Δ<em>C<sub>s</sub></em>, with more positive values of Δ<em>C<sub>s</sub></em>,<em><sub>NPP</sub></em> being correlated with more negative values of Δ<em>C<sub>s,τ</sub></em>. We show that this emergent relationship is the result of 'false priming', which leads to a decrease in the effective soil carbon turnover time as a direct result of NPP increase and occurs when the rate of increase of NPP is relatively fast compared to the slower timescales of a multipool soil carbon model. The inclusion of more soil carbon models with multiple pools in CMIP6 compared to CMIP5, therefore seems to have contributed towards the reduction in the overall model spread in future soil carbon projections.
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