Apparent ecosystem carbon turnover time: uncertainties and robust features
Authors: Fan Naixin , Koirala Sujan , Reichstein Markus , Thurner Martin , Avitabile Valerio , Santoro Maurizio , Ahrens Bernhard , Weber Ulrich , Carvalhais Nuno 
Published: October 2020

Abstract. 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 modeling-based 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 times (τ) as 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 τ to be 43-7+7 yr
(median-difference to percentile 25+difference to percentile
75) when the full soil is considered, in contrast to limiting it to 1 m
depth. Only considering the top 1 m of soil carbon in circumpolar regions (assuming
maximum active layer depth is up to 1 m) yields a global median τ of
37-6+3 yr, which is longer than the previous estimates of 23-4+7 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 τ 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 τ 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 τ mainly in
semiarid and arid regions, whereas Cveg causes the uncertainties of τ in
the subtropics and tropics. In spite of the large uncertainties, our findings reveal that the
latitudinal gradients of τ are consistent across different datasets and soil depths. The
current results show a strong ensemble agreement on the negative correlation between τ 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 τ–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 τ, we identified robust features in the spatial patterns of
τ 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 τ–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 τ is openly available at
https://doi.org/10.17871/bgitau.201911 (Fan et al., 2019).