The ocean takes up over 90 % of the excess heat stored in the Earth system as a result of anthropogenic climate change, which has led to sea level rise and an intensification of marine extreme events. However, despite their importance for informing climate policy, future ocean heat uptake (OHU) projections still strongly differ between climate models. Here, we provide improved global OHU projections by identifying a relationship between present-day Antarctic sea ice extent (SIE) and future OHU across an ensemble of 28 state-of-the-art climate models. Models with more sea ice at present also simulate a colder Southern Hemisphere climate state in general, allowing a larger shift in atmospheric and ocean warming. This regional change affects global warming and heat uptake via a northward-propagating cloud feedback. Combining this relationship between historical Antarctic sea ice extent and future global OHU with satellite observations of Antarctic sea ice reduces the uncertainty of OHU projections under future emission scenarios by 12 %–33 %. Moreover, we show that an underestimation of present-day Antarctic sea ice in the latest generation of climate models results in an underestimation of future OHU by 3 %–14 %, an underestimation of global cloud feedback by 19 %–32 %, and an underestimation of global atmospheric warming by 6 %–7 %. This emergent constraint is based on a strong coupling between Antarctic sea ice, deep-ocean temperatures, and Southern Hemisphere sea surface temperatures and cloud cover in climate models. Our study reveals how the present-day Southern Ocean state impacts future climate change and suggests that previous constraints based on warming trends over recent decades have underestimated future warming and ocean heat uptake.
