Atmospheric constraints on gross primary productivity and net ecosystem productivity: Results from a carbon‐cycle data assimilation system

Abstract

[1] This paper combines an atmospheric transport model and a terrestrial ecosystem model to estimate gross primary productivity (GPP) and net ecosystem productivity (NEP) of the land biosphere. Using atmospheric CO 2 observations in a Carbon Cycle Data Assimilation System (CCDAS) we estimate a terrestrial global GPP of 146 AE 19 GtC/yr. However, the current observing network cannot distinguish this best estimate from a different assimilation experiment yielding a terrestrial global GPP of 117 GtC/yr. Spatial estimates of GPP agree with data-driven estimates in the extratropics but are overestimated in the poorly observed tropics. The uncertainty analysis of previous studies was extended by using two atmospheric transport models and different CO 2 observing networks. We find that estimates of GPP and NEP are less sensitive to these choices than the form of the prior probability for model parameters. NEP is also found to be significantly sensitive to the transport model and this sensitivity is not greatly reduced compared to direct atmospheric transport inversions, which optimize NEP directly.

Constraining a land-surface model with multiple observations by application of the MPI-Carbon Cycle Data Assimilation System V1.0

“…As in our study, their large GPP range was not reflected in large differences of the net land carbon flux. Our work thus supports earlier findings Scholze et al, 2007;Koffi et al, 2012) that despite some constraint on northern extra-tropical GPP, the global land GPP cannot be well constrained with atmospheric CO 2 alone.…”

“…The reduction of GPP in the northern extra-tropics is likely associated with the overestimation of the seasonal cycle of atmospheric CO 2 by the prior model, which was successfully reduced primarily by reducing northern extra-tropical productivity, in particular in temperate and boreal grasslands. Nevertheless, our study supports earlier findings that despite some constraint on northern extra-tropical production, the constraint of observed atmospheric CO 2 on global production is small (Koffi et al, 2012). A detailed comparison of the simulated vegetation and soil carbon stocks is beyond the scope of this paper, partly because the simplifications of the spin-up procedure entail biases in predicted vegetation and soil carbon stocks, as transient land-use changes, forest management, and forest-age structure are ignored.…”

“…A striking difference to the results of Koffi et al (2012) occurred in the tropics, where BETHY-CCDAS overestimated GPP compared to data-driven estimates, whereas the MPI-CCDAS underestimated GPP. As will be discussed below (Sect.…”

“…Our study showed considerable differences in the GPP estimates, which were not reflected in the net carbon fluxes for the CO2alone and JOINT cases, as the net flux is more directly constrained by the atmospheric CO 2 observations. Using a variant of the BETHY-CCDAS, Koffi et al (2012) also found large differences in their posterior GPP estimates ranging from 109 to 164 PgC yr −1 resulting from the use of alternative transport models, atmospheric station densities, and prior uncertainties. As in our study, their large GPP range was not reflected in large differences of the net land carbon flux.…”

The carbon cycle in the Australian Community Climate and Earth System Simulator (ACCESS-ESM1) – Part 2: Historical simulations

“…For example, Welp et al (2011) provided a best guess of 150-175 PgC yr −1 and Koffi et al (2012) an estimate of 146 ± 19 PgC yr −1 . However, the estimate by Jung et al (2011) is based on the largest set of observations and also provides a spatial distribution of GPP.…”