The CarbonTracker Data Assimilation System for CO<sub>2</sub> and <i>δ</i><sup>13</sup>C (CTDAS-C13 v1.0): retrieving information on land–atmosphere exchange processes

Abstract

Abstract. To improve our understanding of the global carbon balance and its representation in terrestrial biosphere models, we present here a first dual-species application of the CarbonTracker Data Assimilation System (CTDAS). The system's modular design allows for assimilating multiple atmospheric trace gases simultaneously to infer exchange fluxes at the Earth surface. In the prototype discussed here, we interpret signals recorded in observed carbon dioxide (CO 2 ) along with observed ratios of its stable isotopologues 13 CO 2 / 12 CO 2 (δ 13 C). The latter is in particular a valuable tracer to untangle CO 2 exchange from land and oceans. Potentially, it can also be used as a proxy for continentwide drought stress in plants, largely because the ratio of 13 CO 2 and 12 CO 2 molecules removed from the atmosphere by plants is dependent on moisture conditions. The dual-species CTDAS system varies the net exchange fluxes of both 13 CO 2 and CO 2 in ocean and terrestrial biosphere models to create an ensemble of 13 CO 2 and CO 2 fluxes that propagates through an atmospheric transport model. Based on differences between observed and simulated 13 CO 2 and CO 2 mole fractions (and thus δ 13 C) our Bayesian minimization approach solves for weekly adjustments to both net fluxes and isotopic terrestrial discrimination that minimizes the difference between observed and estimated mole fractions.With this system, we are able to estimate changes in terrestrial δ 13 C exchange on seasonal and continental scales in the Northern Hemisphere where the observational network is most dense. Our results indicate a decrease in stomatal conductance on a continent-wide scale during a severe drought. These changes could only be detected after applying combined atmospheric CO 2 and δ 13 C constraints as done in this work. The additional constraints on surface CO 2 exchange from δ 13 C observations neither affected the estimated carbon fluxes nor compromised our ability to match observed CO 2 variations. The prototype presented here can be of great benefit not only to study the global carbon balance but also to potentially function as a data-driven diagnostic to assess multiple leaf-level exchange parameterizations in carbon-climate models that influence the CO 2 , water, isotope, and energy balance.

Increased water-use efficiency and reduced CO2 uptake by plants during droughts at a continental scale

“…S2 for a map of sampling locations). Our method relies on a separately published data assimilation technique20 to simultaneously interpret these measurements, yielding spatiotemporal patterns of Net Ecosystem Exchange (NEE, defined here as negative when CO 2 is taken up from the atmosphere) and Δ.…”

Moisture Dependent Diffusion and Shrinkage in Yam during Drying

“…This correlated response in the land biosphere is not found with a traditional NEE estimate based on atmospheric CO₂ observations alone, and neither is it found if δa observations are used as in previous studies to estimate terrestrial NEE but not discrimination (see Table 3 in [20] and Suppl. Info S4).…”

“…The optimized parameters combined with the a-priori NEE and Δ constitute an 11-year reanalysis of surface fluxes and discrimination used in Figs 1 and 2. In [20] and in the Suppl. Info, we included investigations of the robustness, linearity, and signal-to-noise of our results across the mentioned set of six inversions.…”

“…S2 for a map of sampling locations). Our method relies on a separately published data assimilation technique 20 to simultaneously interpret these measurements, yielding spatiotemporal patterns of Net Ecosystem Exchange (NEE, defined here as negative when CO₂ is taken up from the atmosphere) and Δ.…”

“…The measurements are assimilated into a new framework for joint CO₂-δ¹³Ca data assimilation presented separately in [20]. Briefly, it uses both atmospheric CO₂ and δ¹³Ca observations (N=99,300) to simultaneously estimate biospheric NEE, biospheric Δ, and oceanic net carbon fluxes; fossil fuel and biomass burning emissions are fixed.…”