Solar irradiance (or the flux of the Sun’s output directed toward Earth) provides virtually 100% of the energy received and absorbed by the Earth system. So it shouldn’t be any surprise that if you open up an elementary climate change textbook and search for suitable candidates that can actually cause the global climate to vary in time, changes in sunlight will be on the list (along with volcanic eruptions, land use changes, anthropogenic greenhouse gas emissions, etc).
When one actually targets specific past climate change “events,” however, changes in sunlight rarely enter the picture in any appreciable way. It is not self-evident that this should be the case, but we seem to be rather fortunate in orbiting a well-behaved star that only exhibits changes in output on the order of a tenth of a percent or so on decadal-to-centennial timescales. See the record below, for instance. Various choices for solar forcing used by groups in the recent (CMIP5/PMIP3) generation of climate simulations for the last millennium can be found in Figure 8 of Schmidt et al., 2011. Note the vertical scales. We’ll come back to some of the features in the plot below.