The Antarctic mantle and lithosphere are known to have large lateral contrasts in seismic
velocity and tectonic history. These contrasts suggest differences in the response time
scale of mantle flow across the continent, similar to those documented between the northeastern
and southwestern upper mantle of North America. Glacial isostatic adjustment
and geodynamical modeling rely on independent estimates of lateral variability in effective
viscosity. Recent improvements in imaging techniques and the distribution of seismic
stations now allow resolution of both lateral and vertical variability of seismic velocity,
making detailed inferences about lateral viscosity variations possible. Geodetic
and paleo sea-level investigations of Antarctica provide quantitative ways of independently
assessing the three-dimensional mantle viscosity structure. While observational and causal
connections between inferred lateral viscosity variability and seismic velocity changes are
qualitatively reconciled, signi cant improvements in the quantitative relations between
effective viscosity anomalies and those imaged by P- and S-wave tomography have remained
elusive. Here we describe several methods for estimating effective viscosity from
S-wave velocity. We then present and compare maps of the viscosity variability beneath
Antarctica based on the recent S-wave velocity model ANT-20 using three different approaches.