My impression is that the volcanic coolings — even after major erruptions — are not only underestimated by tree rings but also by ice cores (d18o or melt layers) where the actual volcanic signals (sulfates, ashes etc) were recorded at the beginning. (realclimate.org)
All reconstructions show a highly significant volcanic signal, and all but Moberg et al. (which ends in 1925) show a detectable greenhouse gas signal at the 5 % significance level. (ipcc.ch)
It's straightforward for the volcanic signal, since many simulations of the 20th century done in support of the IPCC report included volcanic forcing. (realclimate.org)