Unveiling the Role of Sea‐Ice Loss in Early‐20th‐Century Arctic Warming

Abstract Recent Arctic warming and melting sea ice are iconic features of global warming. Yet, it is unlikely that anthropogenic forcing is solely responsible for these changes. The Early‐20th‐Century Arctic Warming (ETCAW), comparable to the recent one, provides a benchmark for natural climate variability but remains poorly understood. Sparse sea‐ice observations are a major issue—limiting also past modeling studies. Here, we use a new physically based sea‐ice reconstruction and atmospheric model experiments to replicate, for the first time, the rapid ETCAW. We find that two thirds of the strong winter warming is driven by increased ocean heat release, amplified further by the lapse‐rate feedback. This response is linked to extensive sea‐ice loss present in the reconstruction and to strengthened poleward Atlantic heat transport. These results clarify the role of sea‐ice loss in the ETCAW and provide new insight into natural variability's influence on future Arctic climate change.