What Is the Observed Sensitivity of Arctic Sea Ice?

The observed sensitivity of the Arctic summer sea‐ice area (SIA) to changes in the external forcing is a key quantity for understanding and projecting sea‐ice loss. Here, we estimate this sensitivity and provide a robust analysis of its uncertainty considering the impacts of internal variability, the memory of the system and observational uncertainty. We do so by developing and using a novel linear autoregressive AR(1) emulator. We find an observed sensitivity of 2.3 ± 0.5 m 2 SIA loss per ton of anthropogenic CO 2 emissions, and of 3.3 ± 1 million km 2 per °C of global warming, offering an independent benchmark for climate models. By extrapolating the emulator's projections into the future, we confirm that the Arctic Ocean will become ice free in summer for the first time within the next few decades, namely for an additional 350 to 1,150 Gt of anthropogenic CO 2 emissions after 2025.