Global changes in water vapor 1979-2020
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• Richard Allan,
• Kate Willett,
• Viju John,
• Tim Trent
Richard Allan

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Kate Willett
Met Office
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Viju John
EUMETSAT
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Tim Trent
University of Leicester
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## Abstract

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 but coupled simulations overestimate trends because 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 inconsistent with simulations and increased column integrated water vapor in a satellite dataset since 1987. AIRS satellite data does not capture the increased tropospheric water vapor since 2002 in other satellite, reanalysis and model products. However, global water vapor responses to interannual temperature variability is consistent across datasets with increases of $\sim$4-5\% per K near the surface and 10-15\%/K at 300 hPa. 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.