General Circulation Models (GCMs) used to distinguish anthropogenic forcing of the Earth's past climate from its natural variability need to be validated by observations. The GCM ECHO-g was used to produce three millennial simulations of the Earth's climate. Two simulations include changes in anthropogenic and natural external forcing factors through the last millennium, differing only in their initial conditions, and a control run with constant external forcing representing internal variability. Since the ground contains a record of long-term trends in SAT, we use borehole temperatures in Canada, grouped into regions, as a record of past climate. The regional average SATs from ECHO-g were used to solve the forward subsurface thermal profile, and compared with the underground temperature anomalies observed at each region. In all cases simulated subsurface anomalies from the forced simulations are in better agreement with observations than those from the control simulation.