Several recent studies have provided evidence for a warming of the world9s oceans. In their Perspective, Hegerl and Bindoff highlight the report by Barnett et al., whose study substantially strengthens the evidence that human activities are responsible for the observed ocean warming. The observed space-time pattern of ocean warming with depth can only be explained if greenhouse gas forcing is considered in simulations of ocean climate change. The study has important implications for validating model simulations and hence for projections of future warming. Further study of the world9s oceans is still needed to better understand ocean variability, to quantify the likelihood of sudden ocean change, and to predict future changes in ocean biogeochemistry and ecosystems.