Ocean heat content

Ocean heat content (OHC) describes the amount of thermal energy in a volume or layer of seawater. To estimate it, researchers combine observed water temperatures with information about the water's mass and properties, then integrate over a chosen depth and area. Results depend on the depth interval and reference period, so an OHC value must be interpreted with its measurement method. Observations come from ship-based instruments, autonomous Argo floats, other in-water profilers and satellite measurements; ocean reanalyses combine observations with models. NOAA calculates regular global estimates from multiple data streams, while NASA's Earth indicator includes a record for the upper 2,000 metres.

Water has a high heat capacity: it can take up substantial energy with a comparatively modest temperature rise. Greenhouse gases strengthen the climate system's energy imbalance by limiting the rate at which infrared energy escapes to space. Most of the excess energy accumulating in the climate system is taken up by the ocean. Currents, mixing and circulation move heat laterally and downward, meaning that surface temperature alone does not capture the full amount of energy the planet is gaining.

The Intergovernmental Panel on Climate Change assessed that global ocean heat content has increased since at least 1970 and that ocean warming accounted for about 91% of the observed increase in Earth's energy inventory over 1971–2018. The same assessment finds that ocean heat content is virtually certain to continue increasing through the 21st century, with deep-ocean warming persisting for a long time because of slow circulation. These findings describe long-term trends; regional and year-to-year changes can differ as circulation redistributes heat.

As seawater warms it expands, contributing to sea-level rise alongside the addition of water from melting land ice. Ocean warming also affects stratification, marine heatwaves, ice shelves, ecosystems and the distribution of heat among ocean regions. These effects are not determined by heat content alone: local impacts depend on circulation, geography and other physical and ecological processes. Tracking OHC complements measurements of atmospheric temperature and helps quantify changes in Earth's overall energy budget.