The relationship of annual Rh (g C m<sup>−2</sup>) with annual precipitation (Pr, mm), net primary productivity (NPP, g C m<sup>−2</sup>), and soil organic carbon (SOC, kg C m<sup>−2</sup>) from CCSM4 (left panels) and observations (right panels)

<p><strong>Figure 3.</strong> The relationship of annual Rh (g C m<sup>−2</sup>) with annual precipitation (Pr, mm), net primary productivity (NPP, g C m<sup>−2</sup>), and soil organic carbon (SOC, kg C m<sup>−2</sup>) from CCSM4 (left panels) and observations (right panels). The slope, coefficient of determination, and the significance level of the regression with a paired Student's <em>t</em> test are given in each panel.</p> <p><strong>Abstract</strong></p> <p>Soil microbial respiration (Rh) is a large but uncertain component of the terrestrial carbon cycle. Carbon–climate feedbacks associated with changes to Rh are likely, but Rh parameterization in Earth System Models (ESMs) has not been rigorously evaluated largely due to a lack of appropriate measurements. Here we assess, for the first time, Rh estimates from eight ESMs and their environmental drivers across several biomes against a comprehensive soil respiration database (SRDB-V2). Climatic, vegetation, and edaphic factors exert strong controls on annual Rh in ESMs, but these simple controls are not as apparent in the observations. This raises questions regarding the robustness of ESM projections of Rh in response to future climate change. Since there are many more soil respiration (Rs) observations than Rh data, two 'reality checks' for ESMs are also created using the Rs data. Guidance is also provided on the Rh improvement in ESMs.</p>