Increased future ocean heat uptake constrained by Antarctic sea ice extent

The ocean is the major sink of excess heat from anthropogenic climate change, and has so far prevented global warming from already surpassing the limits set by the Paris Agreement. This warming of the ocean impacts metabolic processes in marine species and causes sea level rise, more frequent extreme events, and ocean deoxygenation. The current generation of Earth system models has large uncertainties in projections of historical and future ocean heat uptake. Reducing this uncertainty is paramount for informing climate mitigation and adaptation measures.Here we demonstrate that the amount of future global ocean heat uptake is strongly linked to present day Antarctic sea ice extent, so that satellite observations of sea ice can be used to reduce the uncertainty of future ocean heat uptake. Antarctic sea ice extent serves as an indicator of the baseline climate state of the Southern Ocean, and is linked to ocean heat uptake through hemispheric-scale cloud feedbacks. Climate models typically simulate insufficient Antarctic sea ice, a warm bias in Southern Ocean surface temperatures and insufficient Southern Hemisphere low cloud concentrations, negatively biasing future ocean heat uptake. Using present day Antarctic sea-ice extent observations as an emergent constraint allows to reassess the cumulative ocean heat uptake from 2024 to 2100 under a high-emissions scenario, yielding an increased estimate with reduced uncertainty of 2596 ± 216 ZJ.Our findings indicate that ocean heat uptake and its associated impacts will likely be greater than previously estimated, and underline the climatic significance of recent observed changes in Antarctic sea ice, which may foreshadow changes in oceanic and atmospheric warming rates.