In this work, we present a three-dimensional ab-initio informed phase-field-dislocation dynamics model combined with Langevin dynamics to investigate glide mechanisms of edge and screw dislocations at finite temperatures in Nb. Over a temperature range of 150 K to 600 K, calculations reveal that the screw dislocation changes its mode of glide at two distinct temperatures. These two temperatures coincide with temperatures at which yield strengths in Nb are observed to become thermally insensitive and athermal, respectively.