Because of water’s high specific heat, Earth’s oceans have the capacity to store much of the excess energy generated by humanity’s burning of fossil fuels. Indeed, the globally averaged ocean heat content has increased. But that finding was inferred from observations made in the upper 2000 m of the ocean and going back only to the 1950s. Now Laure Zanna of the University of Oxford and her colleagues have expanded the analysis to the ocean’s deep interior and back in time to 1871.

A view of the Dune of Pilat at Arcachon Bay, on the Atlantic coast of France. Photo courtesy of Laure Zanna

The method the researchers used employed Green’s function to connect sea surface temperature observations to the deeper ocean. By combining it with a time-independent ocean transport model, they could treat temperature as a passive tracer that didn’t alter the circulation as the ocean water moved to different depths and basins. The reconstruction revealed that since 1871, about 436 × 1021 J of additional heat have been stored in the ocean and that a substantial portion of that heat occupies the midlatitudes of the Atlantic and Southern Oceans. That figure is more than the world’s 2017 annual energy consumption estimated by the US Energy Information Administration. Between one-third and one-half of the stored heat in the mid-Atlantic could have been redistributed there by ocean circulation changes from 1971 to 2017. During that period the Atlantic’s mean sea level rose by 1.8 cm. According to the model, 1 cm could have come from heat imported to the Atlantic.

Zanna and her colleagues suggest that winds at the surface can induce the upper ocean to modify its flow and mix with the lower ocean to exchange heat. Changes to ocean circulation may also arise from a combination of wind and buoyancy forcing—namely, a change in the meridional overturning circulation. In the Atlantic basin, that circulation brings warm water from the equator to higher latitudes, where it interacts with deeper water. (L. Zanna et al., Proc. Natl. Acad. Sci. USA, 2019, doi:10.1073/pnas.1808838115.)