Terrestrial biosphere model performance for inter‐annual variability of land‐atmosphere CO₂ exchange

Abstract

Interannual variability in biosphere-atmosphere exchange of CO 2 is driven by a diverse range of biotic and abiotic factors. Replicating this variability thus represents the 'acid test' for terrestrial biosphere models. Although such models are commonly used to project responses to both normal and anomalous variability in climate, they are rarely tested explicitly against inter-annual variability in observations. Here, using standardized data from the North American Carbon Program, we assess the performance of 16 terrestrial biosphere models and 3 remote sensing products against long-term measurements of biosphere-atmosphere CO 2 exchange made with eddy-covariance flux towers at 11 forested sites in North America. Instead of focusing on model-data agreement we take a systematic, variability-oriented, approach and show that although the models tend to reproduce the mean magnitude of the observed annual flux variability, they fail to reproduce the timing. Large biases in modeled annual means are evident for all models. Observed interannual variability is found to commonly be on the order of magnitude of the mean fluxes. None of the models consistently reproduce observed interannual variability within measurement uncertainty. Underrepresentation of variability in spring phenology, soil thaw and snowpack melting, and difficulties in reproducing the lagged response to extreme climatic events are identified as systematic errors, common to all models included in this study.

Biotic and climatic controls on interannual variability in carbon fluxes across terrestrial ecosystems

“…However, current state-of-the-art C-cycle models failed to generate the interannual pattern of NEE (Keenan et al, 2012). According to our results, a lack of BE in the model might be one of the causes because these models usually use constant parameters and varied climate to simulate C fluxes.…”

Temperate and boreal forest tree phenology: from organ-scale processes to terrestrial ecosystem models

“…This class of models encompasses various degrees of details in the mechanistic description of tree and ecosystem functioning (e.g., Keenan et al 2012) and is applied from the forest stand to the global scale. TEMs pay considerable attention to the simulation of carbon fluxes between the ecosystem and the atmosphere and, accordingly, to the representation of photosynthesis, with a strong influence of leaf phenology.…”

Transient dynamics of terrestrial carbon storage: mathematical foundation and its applications

“…For example, incorporating an increasing number of processes that influence the C cycle may represent the real-world phenomena more realistically but makes the models more complex and less tractable. MIPs have effectively revealed the extent of the differences between model predictions (Schwalm et al, 2010;Keenan et al, 2012;De Kauwe et al, 2013) but provide limited insights into sources of model differences (see Medlyn et al, 2015). The physical emulators make data assimilation computationally feasible for global C cycle models (Hararuk et al, , 2015 and thus offer the possibility to generate independent yet constrained estimates of global land C sequestration to be compared with the indirect estimate from the airborne fraction of C emission and ocean uptake.…”

“…Photosynthetic C input is usually simulated according to carboxylation and electron transport rates (Farquhar et al, 1980). Ecosystem C influx varies with time and space mainly due to variations in leaf photosynthetic capacity, leaf area index of canopy, and a suite of environmental factors such as temperature, radiation, and relative humidity (or other water-related variables) (Potter et al, 1993;Sellers et al, 1996;Keenan et al, 2012;Walker et al, 2014;Parolari and Porporato, 2016).…”