Precipitation and carbon-water coupling jointly control the interannual variability of global land gross primary production

Overview & Synopsis

Carbon uptake by terrestrial ecosystems is increasing along with the rising of atmospheric CO 2 concentration. Embedded in this trend, recent studies suggested that the interannual variability (IAV) of global carbon fluxes may be dominated by semi-arid ecosystems, but the underlying mechanisms of this high variability in these specific regions are not well known. Here we derive an ensemble of gross primary production (GPP) estimates using the average of three data-driven models and eleven process-based models. These models are weighted by their spatial representativeness of the satellite-based solar-induced chlorophyll fluorescence (SIF). We then use this weighted GPP ensemble to investigate the GPP variability for different aridity regimes. We show that semi-arid regions contribute to 57% of the detrended IAV of global GPP. Moreover, in regions with higher GPP variability, GPP fluctuations are mostly controlled by precipitation and strongly coupled with evapotranspiration (ET). This higher GPP IAV in semi-arid regions is co-limited by supply (precipitation)-induced ET variability and GPP-ET coupling strength. Our results demonstrate the importance of semi-arid regions to the global terrestrial carbon cycle and posit that there will be larger GPP and ET variations in the future with changes in precipitation patterns and dryland expansion. Carbon uptake through photosynthesis by terrestrial ecosystems is the largest flux in the global carbon cycle 1. This flux, also known as gross primary production (GPP), drives not only ecosystem functioning, but also terrestrial carbon sequestration, which currently offsets more than one third of anthropogenic CO 2 emissions 2. Over the past 50 years, an enhanced seasonal exchange of CO 2 has been observed in the Northern Hemisphere 3 , which was interpreted as increasing GPP in northern ecosystems induced by CO 2 fertilization, extended growing seasons , and nitrogen deposition 3-5. However, the GPP fluctuations superimposed on this increasing trend are less studied. With the increasing frequency of extreme climate events 6 , the interannual variability (IAV) of GPP is also projected to increase 7 , and will cause significant impacts on the global terrestrial carbon cycle 8. The IAV of carbon uptake was shown to have distinctive spatial patterns, with hotspots on semi-arid or grassland ecosystems 9-11. Although these spatial patterns are often explained by the interannual variation of water availability in semi-arid ecosystems, specifically, the precipitation variability, biotic meristem growth potential and their interactions 11 ,