Trends in stratospheric humidity and the sensitivity of ozone to these trends

Measurements of stratospheric water vapor and methane from the Halogen Occultation Experiment (HALOE) mounted on the Upper Atmosphere Research Satellite (UARS) are used to investigate changes in stratospheric water vapor over the period 1992-1996 inclusive. An increase in water vapor mixing ratio is found at levels between 30 km and 65 km across the globe which fit, to first order, a linear trend varying with altitude from 40 parts per billion by volume per year (ppbv yr(-1)) to a maximum of 90 ppbv yr(-1) at 45 km. These trends appear to be greater than that expected due to the growth in tropospheric methane over the past several decades, and possible mechanisms accounting for this are discussed. The trend of the combined budget of 2 x CH4 + H2O is approximately constant with altitude with a global mean value of 61 +/- 4 ppbv yr(-1). On the basis of these estimates, sensitivity studies have been performed using a two-dimensional (2-D) radiative-chemical-dynamical model to assess the impact on concentrations of stratospheric ozone of this degree of change in stratospheric water vapor over timescales consistent with doubling CO2 scenarios. We find that the impact of increased stratospheric water vapor is to enhance the ozone increase in the midstratosphere by similar to 1 - 2% compared to the response due to a doubling of CO2 itself of similar to 5 - 10%. In the upper stratosphere the destruction of ozone is enhanced and the changeover from production to loss is lowered to similar to 50 km (from similar to 70 km). A chemical mechanism for these processes involving enhanced OH and NO2 is identified.