We analyze recordings from more than 3000 shallow earthquakes and measure S410S -- S660S differential travel times on a global scale. These differential times are obtained using a novel cross-correlation technique that eliminates effects associated with a priori assumptions of crustal thickness and velocity variations above the upper mantle transition zone. Our measurements show absolute perturbations up to 8 s from the global average (242 km), which imply transition zone thickness variations of ¿20 km on a length scale of ~2000 km. The transition zone under major subduction zones, e.g., the western Pacific and South America, is significantly thicker than the global average; this could be associated with large-scale depressions of the 660-km discontinuity due to the accumulation of cold oceanic lithosphere at the base of the upper mantle. A narrow transition zone is observed under the central Pacific and northern Atlantic Oceans. We also observe a modest anticorrelation between S410S -- S660S times and the delay times of shear waves (predicted using S12/WM13 <Su et al., 1994>) in the transition zone, which suggests considerable thermal influence on the large-scale topography of the 410- and 660-km discontinuities. The lack of a stronger anticorrelation suggests that (1) compositional heterogeneities may be present and (2) a significant fraction of thermal anomalies near the transition zone do not extend through the transition zone and influence both the 410- and the 660-km discontinuities. A regional analysis shows that the transition zone is, on average, 6--8 km thicker under continents than under oceans; this implies a temperature difference of 50--70¿C. However, we do not observe a clear correlation between thickness and the age of the crust.