Assuming that the change is all solar would lead to a doubling of the solar impact, thus including long term volcanic effects is important. (realclimate.org)
This needs an assumption about forcing, for which they mostly use GHGs and volcanic effects saying other things like aerosols were neglected. (judithcurry.com)
Additionally, we see that the lag before ENSO is reflected in surface temperature is 2 to 4 months, 5 to 7 months for volcanic effects, and 1 month for changes in solar activity. (skepticalscience.com)