Most of the excess energy that accumulates in the Earth system due to increasing concentrations of greenhouse gases is taken up by the ocean. The added energy warms the ocean, and the consequent thermal expansion of the water leads to sea level rise, which is added to by melting land ice.
IPCC AR6 Working Group 1 concluded that it is unequivocal that human influence has warmed the atmosphere, ocean and land, and that it is extremely likely that human influence was the main driver of the ocean heat increase observed since the 1970.
Ocean heat content (OHC) is an ideal variable to monitor changing climate as it is calculated using the entire water column, so ocean warming can be documented and compared between particular regions, ocean basins, and depths. The upper 2000m depth of the ocean continued to warm in 2019 reaching a new record high, and it is expected that it will continue to warm in the future (WMO 2021).
Credit: Daniel Poloha Underwater / Alamy
OHC anomaly
Human-emitted greenhouse gases (GHGs) have resulted in a long-term and unequivocal warming of the planet (IPCC, 2019). More than 90% of the excess heat is stored within the world’s oceans, where it accumulates and causes increases in ocean temperature. Because the oceans are the main repository of the Earth’s energy imbalance, measuring ocean heat content (OHC) is one of the best way to quantify the rate of global warming. The data reveal that the world’s oceans (especially at upper 2000 m) in 2019 were the warmest in recorded human history. Source: Cheng et al., 2020. The same trend continues in 2020 and 2021.
Ocean temperature change
Ocean 0-2000m averaged temperature change since 1940, with 95% confidence interval shown in shading. Source: Cheng et al., 2017.