Ocean temperature ranges from nearly –2 °C in polar and deep waters to more than 30 °C in tropical surface waters. Temperature, together with salinity, determines density and therefore drives circulation. It also controls the solubility of gases, the rates of biological reactions, and the distribution of marine species.

Surface temperature is measured by ships, buoys, and satellites; the interior is profiled by CTDs, Argo floats, and instrumented marine mammals. The upper ocean has warmed detectably over recent decades as it absorbs most of the excess heat trapped by greenhouse gases. This warming contributes to sea-level rise through thermal expansion and intensifies marine heatwaves that stress corals, kelp forests, and fisheries.

Vertical temperature structure includes the warm mixed layer, the thermocline where temperature drops rapidly, and the cold deep ocean. Changes in this structure affect stratification, nutrient supply, and the intensity of tropical cyclones. Continuous temperature monitoring is a core element of the global ocean observing system.

• Measure of the heat content of seawater
• Controls density, chemistry, and biology
• Surface and interior measured by multiple platforms
• Rising due to climate change