Although state-of-the-art reconstructions based on proxies and (semi-) empirical models converge in terms of the total solar irradiance, they still significantly differ in terms of SSI with respect to the mean spectral distribution of energy input and temporal variability. This study aims at quantifying the related uncertainties for the Earth’s climate by forcing two complex chemistry-climate models with four different SSI reconstructions and the reference spectrum RSSV1-ATLAS3, derived from observations. This study represents the work performed within the ROMIC-SOLIC project which contributes directly to the CMIP6 solar forcing recommendation by the SPARC/SOLARIS-HEPPA initiative.

We use two state-of-the-art chemistry climate models - CESM1(WACCM) and EMAC - to conduct a number of timeslice experiments. External forcings and boundary conditions are fixed and identical for all experiments, except for the solar forcing. A first set of simulations employing absolute solar minimum forcing according to the four SSI-reconstructions is analyzed to assess the impact of differing spectral energy distribution. The second set of simulations for analyzing effects of differing solar cycle variability consists of one solar minimum simulation, employing the reference spectrum RSSV1-ATLAS3 and four solar maximum experiments. The latter are a result of adding the 11 year solar cycle amplitude according to the four reconstructions to RSSV1-ATLAS3.

The results of this study show significant differences in temperature and ozone concentrations for the stratosphere and partly for the troposphere originating from a combination of photolytic and radiative effects. The impact of the differences in spectral energy distribution is significantly larger than that of solar cycle uncertainty.