- Due to lower sea levels during the Last Glacial Maximum (LGM), tidal energy dissipation was shifted from the shallow margins into the deep ocean. Here using a high-resolution tide model, we estimate that global energy fluxes below 200 m depth were almost quadrupled during the LGM. Applying the energy fluxes to a consistent tidal mixing parameterization of a global climate model results in a large intensification of mixing. Global mean vertical diffusivity increases by more than a factor of 3, and consequently, the simulated meridional overturning circulation accelerates by ~21–46%. In the model, these effects are at least as important as those from changes in surface boundary conditions. Our findings contrast with the prevailing view that the abyssal LGM circulation was more sluggish. We conclude that changes in tidal mixing are an important mechanism that may have strongly increased the glacial deep ocean circulation and should no longer be neglected in paleoclimate simulations.