Apparent ecosystem carbon turnover time: Uncertainties and robust features

The turnover time of terrestrial ecosystem carbon is an emergent ecosystem property that quantifies the strength of land surface on the global carbon cycle-climate feedback. However, observation- and modelingbased estimates of carbon turnover and its response to climate are still characterized by large uncertainties. In this study, by assessing the apparent whole ecosystem carbon turnover timesas the ratio between carbon stocks and fluxes, we provide an update of this ecosystem level diagnostic and its associated uncertainties in high spatial resolution (0.083) using multiple, state-of-the-art, observation-based datasets of soil organic carbon stock (Csoil), vegetation biomass (Cveg) and gross primary productivity (GPP). Using this new ensemble of data, we estimated the global median to be 43C7 -7 yr (medianCdifference to percentile 75 -difference to percentile 25) when the full soil is considered, in contrast to limiting it to 1m depth. Only considering the top 1m of soil carbon in circumpolar regions (assuming maximum active layer depth is up to 1 m) yields a global median of 37C3 -6 yr, which is longer than the previous estimates of 23C7 -4 yr (Carvalhais et al., 2014). We show that the difference is mostly attributed to changes in global Csoil estimates. Csoil accounts for approximately 84% of the total uncertainty in global estimates; GPP also contributes significantly (15 %), whereas Cveg contributes only marginally (less than 1 %) to the total uncertainty. The high uncertainty in Csoil is reflected in the large range across state-of-the-art data products, in which full-depth Csoil spans between 3362 and 4792 PgC. The uncertainty is especially high in circumpolar regions with an uncertainty of 50% and a low spatial correlation between the different datasets (0:2 < r < 0:5) when compared to other regions (0:6 < r < 0:8). These uncertainties cast a shadow on current global estimates of in circumpolar regions, for which further geographical representativeness and clarification on variations in Csoil with soil depth are needed. Different GPP estimates contribute significantly to the uncertainties of mainly in semiarid and arid regions, whereas Cveg causes the uncertainties of in the subtropics and tropics. In spite of the large uncertainties, our findings reveal that the latitudinal gradients of are consistent across different datasets and soil depths. The current results show a strong ensemble agreement on the negative correlation between and temperature along latitude that is stronger in temperate zones (30-60 N) than in the subtropical and tropical zones (30 S-30 N). Additionally, while the strength of the -precipitation correlation was dependent on the Csoil data source, the latitudinal gradients also agree among different ensemble members. Overall, and despite the large variation in , we identified robust features in the spatial patterns of that emerge beyond the differences stemming from the data-driven estimates of Csoil, Cveg and GPP. These robust patterns, and associated uncertainties, can be used to infer -climate relationships and for constraining contemporaneous behavior of Earth system models (ESMs), which could contribute to uncertainty reductions in future projections of the carbon cycle-climate feedback. The dataset of is openly available at https://doi.org/10.17871/bgitau.201911 (Fan et al., 2019).