The contributions of stratospheric water vapour and ozone, volcanic eruptions, and organic and black carbon are small.» (skepticalscience.com)
Many models have large biases in lower stratospheric water vapour (Gettelman et al., 2010), which could have implications for surface temperature change (Solomon et al., 2010). (drtimball.com)
However, ozone reponses to climate changes are quite sensitive to the initial base state (how much stratospheric water vapour was there? (realclimate.org)