Nature Reviews Earth & Environment, Published online: 30 September 2025; doi:10.1038/s43017-025-00715-5Ocean stratification — density-related layering of seawater — influences oceanographic and climatic processes. This Review outlines observed and projected changes in stratification, noting a 0.8% dec−1 increase in 0–2,000 m stratification from 1960–2024, and a further 1.4% dec−1 increase by 2100 under SSP2-4.5.