Weakened Increase in Global Near-Surface Water Vapor Pressure During the Last 20 Years

It is well known that global warming increases the atmospheric water vapor content, which results in substantial changes in the hydrological cycle. Using five observational data sets, the results show that an increasing trend of near-surface water vapor pressure (AVP) over land and ocean was significant from 1975 to 1998, while such an increasing trend in AVP subsequently weakened from 1999 to 2019. This phenomenon is associated with decreased oceanic evaporation and land surface evapotranspiration in response to recent climate variations. One consequence of such a phenomenon is a large increase in near-surface vapor pressure deficit (VPD), which in turn increases atmospheric demand for water vapor and thus aridity and drought over land. This result emphasizes the importance of water vapor change under global warming. As one of the key components of the atmosphere, water vapor plays a crucial role in regulating the processes in the climate system. It has long been established that water vapor generally increases with global rising mean air temperature as dictated by the Clausius-Clapeyron theorem if relative humidity changes little. Here, we use five observational data sets to study the trend changes in near-surface actual water vapor pressure (AVP) from 1975 to 2019. Our results show an increasing trend of global land and ocean AVP from 1975 to 1998, but the increasing trend has weakened since the late 1990s. This phenomenon in water vapor is associated with decreased oceanic evaporation and land surface evapotranspiration. This phenomenon further enhances the atmospheric vapor pressure deficit (VPD), which dominates the water and carbon cycling in the terrestrial ecosystem by regulating vegetation stomatal conductance. The increasing trend of near-surface actual water vapor pressure (AVP) over land and ocean has weakened since the end of the 1990sThe weakened water vapor pressure increase is associated with decreased oceanic evaporation and land surface evapotranspirationOne consequence of the weakened water vapor pressure increase is a large increase in near-surface vapor pressure deficit