The stratosphere is strongly sensitive to radiative perturbations, as caused for example by the observed decrease of stratospheric ozone and increases in well-mixed greenhouse gases (GHG) and water vapour (H2O) concentrations (e.g., Ramaswamy et al., 2001; Langematz et al., 2003). Changes in stratospheric composition lead to a radiative forcing, defined as the net change in radiative fluxes at the tropopause, and can be related to a forcing of climate (e.g., Chapter 4 in WMO, 2011). The concentrations of the stratospheric climate forcing agents are controlled by atmospheric chemistry. Additionally, changes in tropospheric composition alter the stratospheric composition by the vertical transport into the stratosphere. Changes in climate impact significantly and in a non-linear fashion on the atmospheric chemistry, and hence change atmospheric composition, significantly.
Satellite and radiosonde measurements reveal an overall cooling trend in the global-mean lower stratosphere of up to 1 K/decade over the period 1979-2005 (Randel et al., 2009). Future increases of GHG concentrations will contribute to a continued cooling in the stratosphere. As the majority of reaction rate coefficients in the atmosphere are dependent on temperature and on phase changes, the concentrations of chemical substances such as ozone are very sensitive to temperature changes. For example, GHG-induced temperature and circulation changes are predicted by models currently to accelerate the global increase of ozonein the next decades, which might lead to a “super-recovery” of theozonelayer towards the end of this century, i.e. larger total ozonecolumns than observed around 1980 (e.g., Chapter 5 in WMO, 2007). Conversely this also has potential negative impact on human health reducing vitamin D production.