Scaling carbon fluxes from eddy covariance sites to globe: synthesis and evaluation of the FLUXCOM approach

Abstract

Abstract. FLUXNET comprises globally distributed eddy-covariance-based estimates of carbon fluxes between the biosphere and the atmosphere. Since eddy covariance flux towers have a relatively small footprint and are distributed unevenly across the world, upscaling the observations is necessary to obtain global-scale estimates of biosphere–atmosphere exchange. Based on cross-consistency checks with atmospheric inversions, sun-induced fluorescence (SIF) and dynamic global vegetation models (DGVMs), here we provide a systematic assessment of the latest upscaling efforts for gross primary production (GPP) and net ecosystem exchange (NEE) of the FLUXCOM initiative, where different machine learning methods, forcing data sets and sets of predictor variables were employed. Spatial patterns of mean GPP are consistent across FLUXCOM and DGVM ensembles (R2>0.94 at 1∘ spatial resolution) while the majority of DGVMs show, for 70 % of the land surface, values outside the FLUXCOM range. Global mean GPP magnitudes for 2008–2010 from FLUXCOM members vary within 106 and 130 PgC yr−1 with the largest uncertainty in the tropics. Seasonal variations in independent SIF estimates agree better with FLUXCOM GPP (mean global pixel-wise R2∼0.75) than with GPP from DGVMs (mean global pixel-wise R2∼0.6). Seasonal variations in FLUXCOM NEE show good consistency with atmospheric inversion-based net land carbon fluxes, particularly for temperate and boreal regions (R2>0.92). Interannual variability of global NEE in FLUXCOM is underestimated compared to inversions and DGVMs. The FLUXCOM version which also uses meteorological inputs shows a strong co-variation in interannual patterns with inversions (R2=0.87 for 2001–2010). Mean regional NEE from FLUXCOM shows larger uptake than inversion and DGVM-based estimates, particularly in the tropics with discrepancies of up to several hundred grammes of carbon per square metre per year. These discrepancies can only partly be reconciled by carbon loss pathways that are implicit in inversions but not captured by the flux tower measurements such as carbon emissions from fires and water bodies. We hypothesize that a combination of systematic biases in the underlying eddy covariance data, in particular in tall tropical forests, and a lack of site history effects on NEE in FLUXCOM are likely responsible for the too strong tropical carbon sink estimated by FLUXCOM. Furthermore, as FLUXCOM does not account for CO2 fertilization effects, carbon flux trends are not realistic. Overall, current FLUXCOM estimates of mean annual and seasonal cycles of GPP as well as seasonal NEE variations provide useful constraints of global carbon cycling, while interannual variability patterns from FLUXCOM are valuable but require cautious interpretation. Exploring the diversity of Earth observation data and of machine learning concepts along with improved quality and quantity of flux tower measurements will facilitate further improvements of the FLUXCOM approach overall.

Interannual variation of terrestrial carbon cycle: Issues and perspectives

“…Studies based on the eddy covariance data‐driven model (FLUXCOM with both remote sensing and climate data as forcing dataset; Jung et al, ; Tramontana et al, ) and atmospheric inversions (Table S1) ubiquitously attribute most of the global IAV to tropical land ecosystems since the 1980s (e.g., Baker et al, ; Bousquet et al, ; Jung et al, , ; Patra et al, ; Peylin et al, ; Rayner et al, ; Rödenbeck et al, , ), though the absolute magnitude of IAV in FLUXCOM data is about one order of magnitude smaller than the other approaches (Figure a; Figure S2). Studies also differ on the ecosystems from which IAV is sourced.…”

Definitions and methods to estimate regional land carbon fluxes for the second phase of the REgional Carbon Cycle Assessment and Processes Project (RECCAP-2)

“…Eddy-covariance flux tower networks measure the net CO2 flux of terrestrial ecosystems (NPP-SHR) across a global network with a typical footprint of about 1 km 2 . The networks currently consist of about 600 sites (Jung et al, 2020). Given the small footprint, flux tower sites do not adequately measure the fluxes of Fgeological, Ffires, Freduced, Frivers + lakes outgas (except for a very few towers in wetlands or flooded systems), Fcrop products and Fwood products.…”

Kernel methods and their derivatives: Concept and perspectives for the earth system sciences

“…Hence, it is key to understand the spatial and temporal dynamics of GPP at regional and global scales. Here, we consider the GPP FLUXCOM ( http://www.fluxcom.org/ ) product, computed as described in [ 75 , 76 ].…”