Closure of Ocean Heat Budget Reveals Delayed Acceleration of Earth’s Energy Imbalance

Variations in Earth’s energy imbalance are recorded predominantly by changes in ocean heat content, but in-situ temperature data below 2000 m remains sparse, leaving the ocean heat budget open. Here, we provide the first global reconstruction of historical changes in top-to-bottom ocean heat content from interpolated subsurface temperature data using an autoregressive artificial neural network, closing the ocean heat budget for the period 1946-2019. We find that from 1950-1990, cooling of the deep ocean counteracted a small heat gain in the upper ocean, leading to no robust trend in global ocean warming during this period, and implying a roughly balanced Earth energy budget within -0.16 to 0.06 W m^-2. However, the past three decades have seen a rapid acceleration in ocean warming, with the entire ocean warming from top to bottom at a rate that implies a global energy imbalance of 0.61±0.14 W m^-2. These results suggest that the Earth's energy imbalance may not be as large as previously estimated during the 20th century but is accelerating rapidly into the 21st century.