PNNL

Abstract

The feedback between planetary warming and soil carbon loss has been the focus of considerable scientific attention in recent decades, due to its potential to accelerate anthropogenic climate change. The soil carbon temperature sensitivity is traditionally estimated from short-term respiration measurements either from laboratory incubations or field measurements that cannot distinguish between plant and microbial respiration. To address these limitations of previous approaches, we developed a new method to estimate temperature sensitivity (Q10) of soil carbon directly from warming-induced changes in soil carbon stocks measured in 47 field experiments across the world. Variations in warming magnitude and control carbon stock explained some of the differences in field-warmed carbon stock (R2=0.42), revealing a global Q10 of 1.9 ([1.4, 2.5] 95%CI). We then propagated these estimates over the 21st century using a post-hoc correction of 20 CMIP5 Earth system model outputs to reflect this field-derived Q10 range. This reduced the multi-model mean soil carbon stock changes from the previous value of 78 ±139 Pg-carbon (weighted mean ± 1 SD), to -7±84 Pg-carbon with a Q10 driven 95% CI of 159±144 to -151±139 Pg-carbon. On average, incorporating the field-derived Q10 values into Earth system model simulations led to reductions in the projected amount of carbon sequestered in the soil over the 21st century. However, the considerable parameter uncertainty led to extremely high variability in soil carbon stock projections within each model; intra-model uncertainty driven by the measured Q10 was as great as that between model variation. Model-data integration is necessary to capture useful scientific uncertainty; this study demonstrates that high uncertainty in CMIP5 Earth system model soil carbon projections is unlikely to be resolved with better model-data integration.