Mixing ratios of water (H<sub>2</sub>O) in the stratosphere appear to increase due to increased input of H<sub>2</sub>O and methane from the troposphere and due to intensified oxidation of CH<sub>4</sub> 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 CH<sub>4</sub> 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 H<sub>2</sub>O levels themselves. In combination with the increase in tropospheric CH<sub>4</sub> mixing ratios and with solar cycle related variations of upper stratospheric ozone, these effects can explain about 50% of the additional conversion of CH<sub>4</sub> to H<sub>2</sub>O as observed throughout the stratosphere. The relative contributions from the individual processes have varied over the past decades.