Vegetation growth due to CO2 fertilization is threatened by increasing vapor pressure deficit

Numerous studies found that the CO2 fertilization effect can enhance vegetation growth, however, some recent studies showed that the increase of vapor pressure deficit (VPD) could reduce vegetation growth due to an increase in surface resistance. It remains unclear to what extent VPD increases can offset the CO2 fertilization effect. Here, we examined the long-term trends of terrestrial gross primary productivity (GPP) at the global scale using six products derived from satellite observations, machine learning algorithms, and dynamic vegetation model simulations. While we found significant increases (p less than 0.05) in GPP in most of the world, we also found significant decreases in GPP over the Amazon basin, western North America, eastern Europe and central Asia. Our attribution analysis showed that although the elevated CO2 concentration dominated the long-term trends of GPP, VPD also played an important role. The increasing VPD could explain the decreasing GPP over the arid and tropical regions. The negative contribution of VPD to GPP trends appeared to become amplified with time, leading to suppressed global vegetation growth in the last two decades. The amplified contribution of VPD to GPP trends was directly related to the decrease in soil moisture, indicating the soil moisture-induced land–atmosphere coupling (LAC) and the vegetation growth stagnation since the year 2000. Our results provide insight into the negative contribution of VPD to long-term GPP trends, which can partly offset 68.21 % of the CO2 fertilization effect and even stagnate the vegetation growth with time. The possible mechanisms behind the effect of soil moisture-VPD coupling on the vegetation dynamics at the global scale needs further investigation.