Many data sets exist that contain both geospatial and temporal elements, in addition to the core data that requires analysis. Within such data sets, it can be difficult to determine how the data have changed over spatial and temporal ranges. In this design study we present a system for dynamically exploring geo-temporal changes in the data. GTdiff provides a visual approach to representing differences in the data within user-defined spatial and temporal limits, illustrating when and where increases and/or decreases have occurred. The system makes extensive use of spatial and temporal filtering and binning, geo-visualization, colour encoding, and multiple coordinated views. It is highly interactive, supporting knowledge discovery through exploration and analysis of the data. A case study is presented illustrating the benefits of using GTdiff to analyze the changes in the catch data of the cod fisheries off the coast of Newfoundland, Canada from 1948 to 2006.