Increased aerosol SAD in the lower stratosphere causes stratospheric heating, which in turn would increase cold-point tropopause temperatures, allowing more water vapour to enter the stratosphere. This water vapour acts as a greenhouse gas, reducing the overall negative radiative forcing from SAI, and also impacts ozone.
Metric
The positive radiative forcing from stratospheric water vapour increases under SAI is greater than 20% of the direct negative forcing from scattering.
Uncertainty
There is substantial variation in model representation of stratospheric heating and baseline water vapor levels (Bednarz et al., 2023), which causes a +40% to -15% range in water vapor change in the tropical lower stratosphere under SAI. However, the negative number here comes from GISS, which is behaving in an unphysical manner, and therefore it is likely that the sign is positive.
Decision relevance
Contributions of aerosol lifetime and size factors 5-6 greater than water vapor impact on RF (Lee et al., 2023). But, the RF from water vapour is non-trivial, equal to ~ 20% of the RF from sulfate for 15NS injection and 10% for 30NS injection in Bednarz\_2023\_ACP.