Abstract
Abstract. The terrestrial carbon fluxes show the largest variability among the components of the global carbon cycle and drive most of the temporal variations in the growth rate of atmospheric CO2. Understanding the environmental controls and trends of the terrestrial carbon budget is therefore essential to predict the future trajectories of the CO2 airborne fraction and atmospheric concentrations. In the present work, patterns and controls of the inter-annual variability (IAV) of carbon net ecosystem exchange (NEE) have been analysed using three different data streams: ecosystem-level observations from the FLUXNET database (La Thuile and 2015 releases), the MPI-MTE (model tree ensemble) bottom–up product resulting from the global upscaling of site-level fluxes, and the Jena CarboScope Inversion, a top–down estimate of surface fluxes obtained from observed CO2 concentrations and an atmospheric transport model. Consistencies and discrepancies in the temporal and spatial patterns and in the climatic and physiological controls of IAV were investigated between the three data sources. Results show that the global average of IAV at FLUXNET sites, quantified as the standard deviation of annual NEE, peaks in arid ecosystems and amounts to ∼ 120 gC m−2 y−1, almost 6 times more than the values calculated from the two global products (15 and 20 gC m−2 y−1 for MPI-MTE and the Jena Inversion, respectively). Most of the temporal variability observed in the last three decades of the MPI-MTE and Jena Inversion products is due to yearly anomalies, whereas the temporal trends explain only about 15 and 20 % of the variability, respectively. Both at the site level and on a global scale, the IAV of NEE is driven by the gross primary productivity and in particular by the cumulative carbon flux during the months when land acts as a sink. Altogether these results offer a broad view on the magnitude, spatial patterns and environmental drivers of IAV from a variety of data sources that can be instrumental to improve our understanding of the terrestrial carbon budget and to validate the predictions of land surface models.
Highlights
Atmospheric CO2 concentration has been constantly increasing since the Industrial Revolution, and has caused a corresponding rise of 0.85 ◦ C in the global air temperature from 1880 to 2012 (IPCC, 2013)
The growth rate of atmospheric CO2 concentration is characterized by a large inter-annual variability (IAV), which mostly results from the variability in the CO2 net ecosystem exchange (NEE) on land (Bousquet et al, 2000; Le Quéré et al, 2009; Yuan et al, 2009)
The observed range of IAV is similar for the two gridded products and substantially lower than that observed at the site level, probably due to the spatial averaging of the land fluxes that dampens the temporal variability
Summary
Atmospheric CO2 concentration has been constantly increasing since the Industrial Revolution, and has caused a corresponding rise of 0.85 ◦ C in the global air temperature from 1880 to 2012 (IPCC, 2013). The growth rate of atmospheric CO2 concentration is characterized by a large inter-annual variability (IAV), which mostly results from the variability in the CO2 net ecosystem exchange (NEE) on land (Bousquet et al, 2000; Le Quéré et al, 2009; Yuan et al, 2009). Multisite synthesis confirms that a large inter-annual variability in NEE is a common feature at all flux sites around the world (Baldocchi, 2008; Baldocchi et al, 2001). The reason why the IAV is so large is that NEE results from the small imbalance between two larger fluxes: the photosynthetic uptake of CO2 (gross primary production, GPP) and the respiratory release of CO2 (total ecosystem respiration, TER).
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