AbstractThe oxygen isotope composition of speleothems is a widely used proxy for past climate change. Robust use of this proxy depends on understanding the relationship between precipitation and cave drip water δ¹⁸O. Here, we present the first global analysis, based on data from 163 drip sites, from 39 caves on five continents, showing that drip water δ¹⁸O is most similar to the amount-weighted precipitation δ¹⁸O where mean annual temperature (MAT) is < 10 °C. By contrast, for seasonal climates with MAT > 10 °C and < 16 °C, drip water δ¹⁸O records the recharge-weighted δ¹⁸O. This implies that the δ¹⁸O of speleothems (formed in near isotopic equilibrium) are most likely to directly reflect meteoric precipitation in cool climates only. In warmer and drier environments, speleothems will have a seasonal bias toward the precipitation δ¹⁸O of recharge periods and, in some cases, the extent of evaporative fractionation of stored karst water.