Recent slowdown in global land gross carbon uptake: evidence from MF-CW, a new FLUXNET upscaled product

Terrestrial gross primary production (GPP), the largest C flux between the terrestrial biosphere and the atmosphere, is a key component of the terrestrial C cycle. It is widely believed that the rapid rise of atmospheric carbon dioxide (CO2) concentration (Ca) has increased the net assimilation of carbon through photosynthesis. On the other hand, recent studies indicated that rising vapor pressure deficit reduced plant growth. However, the combined and interactive effects of CO2 fertilization, warmer temperatures, and rising VPD on global land GPP remain uncertain. We used 24 machine learning (ML) methods to upscale in-situ GPP data from FLUXNET sites to the globe with satellite-derived observations and climate reanalysis data. With the upscaled product (MF-CW), we examined how climate change and elevated Ca interactively affected the trends and underlying processes of GPP from 1982 to 2016. The increasing rate of GPP slowed down after 2000 by a factor of 2.14 (2.96±0.55 g C m-2 yr-2 during 1982–2000 vs. 1.38±1.15 g C m-2 yr-2 during 2001–2016). The enhancement effect of CO2 fertilization and the net negative effect of climate change jointly influence ecosystem CO2 uptake over global terrestrial ecosystems. The negative effect of rising VPD largely offset the CO2 fertilization effect. The slowdown in the increase in GPP does not appear to be the result of changes in vegetation regrowth and land cover, and nutrient constraints on photosynthesis due to N and P changes, or their disequilibrium. We also looked at the trends in global land evapotranspiration (ET). GPP exhibited a larger slowdown than ET, leading to the global saturation of water use efficiency (WUE). This indicates that human reliance on nature-based climate sinks to achieve carbon neutrality may be undermined by the adverse effects of climate warming.