The impact of anthropogenic chlorine emissions, stratospheric ozone change and chemical feedbacks on stratospheric water
Mixing ratios of water (H2O) in the stratosphere appear to increase due to increased input of H2O and methane from the troposphere and due to intensified oxidation of CH4 in the stratosphere, but many of the underlying mechanisms are not yet understood. Here we identify and quantify three chemical mechanisms which must have led to more efficient oxidation of CH4 in the stratosphere over the past several decades: 1) The increase in stratospheric chlorine levels due to anthropogenic CFC emissions, 2) the thinning of the stratospheric ozone column and 3) enhanced OH levels in the stratosphere due to increasing H2O levels themselves. In combination with the increase in tropospheric CH4 mixing ratios and with solar cycle related variations of upper stratospheric ozone, these effects can explain about 50% of the additional conversion of CH4 to H2O as observed throughout the stratosphere. The relative contributions from the individual processes have varied over the past decades.