TY - JOUR
T1 - Scaling carbon fluxes from eddy covariance sites to globe: Synthesis and evaluation of the FLUXCOM approach
AU - Jung, Martin
AU - Schwalm, Christopher
AU - Migliavacca, Mirco
AU - Walther, Sophia
AU - Camps-Valls, Gustau
AU - Koirala, Sujan
AU - Anthoni, Peter
AU - Besnard, Simon
AU - Bodesheim, Paul
AU - Carvalhais, Nuno
AU - Chevallier, Frederic
AU - Gans, Fabian
AU - Goll, Daniel S.
AU - Haverd, Vanessa
AU - Köhler, Philipp
AU - Ichii, Kazuhito
AU - Jain, Atul K.
AU - Liu, Junzhi
AU - Lombardozzi, Danica
AU - Nabel, Julia E. M. S.
AU - Nelson, Jacob A.
AU - O'Sullivan, Michael
AU - Pallandt, Martijn
AU - Papale, Dario
AU - Peters, Wouter
AU - Pongratz, Julia
AU - Rödenbeck, Christian
AU - Sitch, Stephen
AU - Tramontana, Gianluca
AU - Walker, Anthony
AU - Weber, Ulrich
AU - Reichstein, Markus
N1 - info:eu-repo/grantAgreement/EC/H2020/640176/EU#
info:eu-repo/grantAgreement/EC/H2020/776810/EU#
info:eu-repo/grantAgreement/EC/H2020/820852/EU#
info:eu-repo/grantAgreement/EC/H2020/647423/EU#
info:eu-repo/grantAgreement/EC/H2020/776186/EU#
This research has been supported by the European Space Agency (grant no. ESRIN/4000123002/18/I-NB), the National Science Foundation under cooperative agreement no. 1852977 and the U.S. Department of Energy (no. DE-SC0016323)
PY - 2020/3/16
Y1 - 2020/3/16
N2 - 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 ( 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 class with the largest uncertainty in the tropics. Seasonal variations in independent SIF estimates agree better with FLUXCOM GPP (mean global pixel-wise) than with GPP from DGVMs (mean global pixel-wise). Seasonal variations in FLUXCOM NEE show good consistency with atmospheric inversion-based net land carbon fluxes, particularly for temperate and boreal regions. 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 (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 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.
AB - 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 ( 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 class with the largest uncertainty in the tropics. Seasonal variations in independent SIF estimates agree better with FLUXCOM GPP (mean global pixel-wise) than with GPP from DGVMs (mean global pixel-wise). Seasonal variations in FLUXCOM NEE show good consistency with atmospheric inversion-based net land carbon fluxes, particularly for temperate and boreal regions. 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 (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 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.
UR - http://www.scopus.com/inward/record.url?scp=85082322779&partnerID=8YFLogxK
U2 - 10.5194/bg-17-1343-2020
DO - 10.5194/bg-17-1343-2020
M3 - Article
AN - SCOPUS:85082322779
SN - 1726-4170
VL - 17
SP - 1343
EP - 1365
JO - Biogeosciences
JF - Biogeosciences
IS - 5
ER -