Abstract This study provides the first estimates of the global stratosphere‐troposphere exchange (STE) of water vapor (H2O) using the lowermost stratosphere mass budget approach. Observationally derived H2O fluxes across the isentropic surface fitted to the tropical tropopause are −1.16 ± 0.20 Pg/yr in the Northern Hemisphere extratropics, −0.94 ± 0.18 Pg/yr in the Southern Hemisphere extratropics, and 2.20 ± 0.36 Pg/yr in the tropics, resulting in a small net global flux of 0.10 ± 0.04 Pg/yr into the stratosphere. In contrast, MERRA2 and ERA5 yield global fluxes of −1.84 and −0.27 Pg/yr, respectively, suggesting a net H2O source in the stratospheric overworld from reanalyses. Large discrepancies in the seasonal cycle of H2O net fluxes at the tropopause between reanalyses and observations are also identified. These differences in the H2O STEs are mainly caused by biases in reanalysis H2O concentrations at the isentropic surface and in the lowermost stratosphere.
Observations from the Little Bunker
Last updated: December 19, 2025 03:16 AM
Atmospheric CO₂
427.2 ppm
+17.7
ppm/year
Atmospheric CH₄
1930.95 ppb
+-3.5
ppb/year
Global Temperature
+1.48 °C
Peak: +1.73°C
World Population
8.1 billion
+67
million/year