Global Changes in Water Vapor 1979–2020.

Global‐scale changes in water vapor and responses to surface temperature variability since 1979 are evaluated across a range of satellite and ground‐based observations, a reanalysis (ERA5) and coupled and atmosphere‐only CMIP6 climate model simulations. Global‐mean column integrated water vapor increased by 1%/decade during 1988–2014 in observations and atmosphere‐only simulations. However, coupled simulations overestimate water vapor trends and this is partly explained by past studies showing that internal climate variability suppressed observed warming in this period. Decreases in low‐altitude tropical water vapor in ERA5 and ground‐based observations before around 1993 are considered suspect based on inconsistency with simulations and increased column integrated water vapor in microwave satellite data since 1979. Atmospheric Infra‐red Sounder satellite data does not capture the increased tropospheric water vapor since 2002 shown by other satellite, reanalysis, and model products. However, global water vapor responses to interannual surface temperature variability are consistent across data sets with increases of ∼4%–5% near the surface and 10%–15% at 300 hPa for each 1°C increase in global surface temperature. Global water vapor responses are explained by thermodynamic amplification of upper tropospheric temperature changes and the Clausius Clapeyron temperature dependence of saturation vapor pressure that are dominated by the tropical ocean responses. Upper tropospheric moistening is larger in climate model simulations with greater upper tropospheric warming. Plain Language Summary: Evaporated water becomes a gas (water vapor) in the air where it traps heat by absorbing thermal infrared radiative energy as well as sunlight. Water vapor is also the "fuel" for rain and snowfall. As the climate warms, water vapor increases in the lowest few kilometers of the atmosphere, therefore, causes greater trapping of heat but also heavier precipitation events. This study looks at how water vapor has changed since 1979 by examining satellite measurements, observations at ground level and complex computer simulations that are also used to make predictions of future climate change. We find that the total water vapor in the atmosphere is increasing by about 1% every 10 years. Changes calculated as a percentage of the initial amount are larger higher up in the atmosphere, which is consistent with simple physics. There are some differences between the observations and simulations: some simulations overestimate the observed changes and this is partly because natural fluctuations in the ocean temporarily slowed the warming over the period studied (1988–2014). It is not known for sure what other differences between observations are caused by but it seems possible that decreases in humidity in some data sets may not be real and we are confident that water vapor is increasing with warming of climate. Key Points: Tropospheric water vapor increased globally since 1979 in observations, reanalyses, and CMIP6 simulationsInternal climate variability suppressed water vapor increases during 1988–2014Observations‐based data and simulations display consistent representations of moisture variability but not long term trends [ABSTRACT FROM AUTHOR]