Mg/Ca ratios of planktonic foraminiferal tests are important tools for reconstructing past ocean temperatures at different levels of the upper water column. Yet numerous studies suggest a significant influence of calcite dissolution on Mg/Ca ratios lowering their initial signal recorded within a planktonic foraminiferal habitat. To determine the effect of dissolution, this study presents Mg/Ca ratios of eight planktonic foraminiferal species from the South China Sea sediment surface. Continuously decreasing with increasing water depth, the Mg/Ca ratios also decrease with calcite-saturation states close to and below saturation (bottom water Δ[CO 2− 3 ] <30 μmol kg −1 ) but are stable in well calcite-supersaturated bottom waters (>40 μmol kg −1 ). This preservation pattern compares well with examples of Mg/Ca dissolution from the tropical Atlantic Ocean and is independent of the foraminiferal species. Merging a global data set by separate normalization of 79 Mg/Ca data sets from the Pacific, Atlantic, and Indian Oceans, which removes thermal differences between the ocean regions and foraminiferal species, enabled us to quantify a global decrease in planktonic foraminiferal Mg/Ca ratios of 0.054 ±0.019 μmol mol −1 per μmol kg −1 below a critical threshold for dissolution of 21.3 ±6.6 μmol kg −1 . The absolute decline in Mg/Ca ratios, which is similar for all species, affects temperature estimates from (sub-)thermocline species more strongly than those from shallow dwellers. The water depth of this critical threshold in the global oceans shoals from >3.5 km in the North Atlantic to <0.5 km in the North Pacific based on calculations of the global calcite-saturation state from 6321 hydrographic stations. Above this critical threshold Mg/Ca ratios are well preserved, and paleotemperature estimates are broadly unaffected by dissolution.