Response of the stratospheric temperatures and ozone to past and future increases in stratospheric humidity

Balloonsonde and aircraft in situ and remote sensing measurements in the Northern Hemisphere midlatitudes show that stratospheric humidity has been increasing since 1954. Satellite data, albeit only available over a much shorter period (1992 through 1999), also suggest that water vapor has been increasing globally over much of the stratosphere. Implications of these trends for stratospheric temperatures and ozone are explored using a two‐dimensional interactive radiative‐chemical‐dynamical model. It is shown that the increase in humidity cools the model stratosphere, in agreement with previous studies. In the midlatitude lower stratosphere, increasing water vapor induces a temperature trend of about −0.25 to −0.35 K per decade, which is ∼30–50% of the observed cooling in this region. According to our results, such trends in water vapor have intensified Northern Hemisphere midlatitude ozone trends over the last few decades, mainly via the enhancement of ozone loss in HO x catalytic cycles and changes in chlorine partitioning (in spite of some buffering by concomitant increases in the O 3 production in the methane oxidation cycle). Increasing humidity in the model accounts for additional depletion of midlatitude column ozone at a rate of −0.3% per decade and up to −0.7% (−1%) per decade enhancement of local ozone trends in the lower (upper) stratospheres, accordingly. It is important to investigate what will be the likely consequences for stratospheric climate and ozone if the water vapor trend continues. Our simulations suggest that smaller effects on ozone are to be expected over the 2000–2050 period as compared to 1979–1996 because of the projected reduction in the stratospheric chlorine (and bromine) loading. However, our model simulations assuming World Meteorological Organization future emission scenarios for halogenated compounds, CH 4 , N 2 O, and CO 2 , show that increasing humidity will induce further cooling of the stratosphere and result in about a decade of delay in the ozone recovery, all other factors being equal.