Ocean stratification in a warming climate

The ocean is highly stratified. Warm, fresh water sits on top of cold, salty water, influencing vertical oceanic exchange of heat, carbon, oxygen and nutrients. In this Review, we examine observed and projected stratification shifts and their impacts. Changes in ocean temperature and salinity have altered the ocean density field, leading to a 0.8 ± 0.1% dec⁻¹ (90% confidence interval) increase in stratification in the global upper 2,000 m since the 1960s. These increases are most pronounced in the tropics and are primarily temperature driven. Model simulations project ongoing stratification increases in the future, with global 0–2,000 m stratification increasing 0.7 [0.3,1.1; 13–87% confidence interval], 1.4 [0.9,1.8] and 2.9 [2.1,3.8]% dec⁻¹ by 2090–2100 relative to 2010–2020 under Shared Socioeconomic Pathways SSP1-2.6, SSP2-4.5 and SSP5-8.5, respectively; regional patterns of projected stratification changes generally follow observed trends. These observed and projected ocean stratification changes have important climate and ecological consequences, including alterations in ocean heat uptake, ocean currents, vertical mixing, tropical cyclone intensity, marine ecosystems and elevation of marine extremes. Further research should better quantify stratification change at critical layers and understand their drivers and impacts.