The Southern Ocean is an essential part of the climate system, due to its disproportionately-important role in global heat and carbon uptake. Hence, changes in its physical properties, especially those that affect water mass transformations (temperature and salinity) have global implications. In this study, we perform a multi-model attribution of the primary forcings of historically-observed Southern Ocean temperature and salinity change, from 1966-2005. Consistent with previous research, we find a robustly-detectable anthropogenic greenhouse gas (GHG) response, characterized most clearly by a warming of Sub-Antarctic Mode Waters and a freshening of Antarctic Intermediate Waters. The warming pattern is somewhat mitigated by non-GHG anthropogenic forcings (i.e. anthropogenic aerosols or stratospheric ozone depletion). We also find evidence of a detectable GHG-forced change in denser watermasses (i.e. deeper than 2000m and south of 60^oS), with a warming of Circumpolar Deep Water and warming and freshening of Antarctic Bottom Water; the latter result must be considered with caution given the poor representation of Bottom Water in climate models.