Earth’s energy imbalance across an entire glacial cycle (MIS 9–7) reconstructed from noble-gas ratios in ice cores

Earth’s energy imbalance (EEI) determines whether the planet experiences a net gain or loss of energy. The ongoing surge in atmospheric greenhouse-gas concentrations, caused by burning fossil fuels and land-use change, causes a positive EEI, which ultimately drives global warming. Today, most of this excess heat is taken up by the largest, fast-responding energy reservoir that is the surface ocean. On millennial to orbital timescales, by contrast, energy partitions between two considerably larger but slower-responding reservoirs: the global (deep, intermediate, and surface) ocean and the latent heat involved in growing and melting continental ice sheets. Ocean heat content (OHC) and global sea level, which mirrors ice sheet volume, are thus key metrics to assess the global energy balance during the Quaternary.

Past OHC can be reconstructed by analyzing noble-gas ratios in polar ice-core samples. This method makes use of the temperature-dependent and species-specific solubility of noble gases in seawater, as well as their inertness, due to which the total amount of noble gases in the ocean‐atmosphere system is conserved. Earlier studies mostly focused on the last glacial Termination and other periods of interest across the last glacial cycle. Here, we present data for an entire glacial cycle (MIS 9–7) together with data over the last four glacial terminations in millennial resolution.

By combining our OHC record with past sea-level reconstructions we obtain an EEI record spanning an entire glacial cycle. This EEI record shows the expected orbital-scale variability in response to the albedo and greenhouse gas feedback, with energy fluxes partitioning approximately equally between the ocean and ice sheet reservoirs. The EEI record also manifests strong millennial power. These millennial-scale EEI features are mirrored in the OHC record, whereas the ice sheet response is delayed and subdued, indicating that the ocean is the dominant millennial-scale energy reservoir. Millennial-scale EEI and OHC variability is closely linked with changes in AMOC strength, suggesting that ocean circulation modulates EEI and OHC across different climate states. Potential AMOC weakening under future global warming may thus add to the EEI anomaly for centuries to come.