Employing a model democracy in which each model is equally weighted may lead to a poor estimation of the true uncertainty in climate projections from phase 6 of the Coupled Model Intercomparison Project (CMIP6). The improvement and increase in number of CMIP6 models compared with previous phases of CMIP indicate that both model skill and independence need to be considered to provide convincing projections. In this study, we use a weighting scheme, which weights both the skill and independence of multi‐model simulations, to efficiently constrain the large uncertainty in the projection of the timing of an ice‐free Arctic. The uncertainty‐constrained projections of CMIP6 show that the multi‐model spread of the projected first year of an ice‐free Arctic can be reduced by about 29 years under the Shared Socioeconomic Pathway (SSP)3–7.0 scenario (a regional rivalry scenario), indicating a faster tendency to an ice‐free Arctic summer than projections based solely on model democracy. A fossil‐fuel‐based development scenario (i.e., SSP5‐8.5) leads to an ice‐free Arctic before the 2070s (ranging from 2038 to 2071), while an ice‐free Arctic occurs slightly later (by ∼10 years) under the SSP2‐4.5 scenario (i.e., intermediate scenario) and the SSP3‐7.0 scenario, but is inevitable this century. The sustainable development scenario (i.e., SSP1‐2.6) is likely to prevent the occurrence of an ice‐free Arctic. Internal variability strongly affects the projection estimated by the equally weighted ensemble; however, it has a negligible impact on the results obtained by the weighting scheme, thereby indicating that the results of this study are robust and convincing.

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 is a major issue—limiting also past modelling 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.

The Arctic has warmed at more than twice the global mean rate, with winter anomalies over the Barents‐Kara Seas (BKS) linked to remote tropical forcing. While El Niño Southern Oscillation is often emphasized, the role of the Atlantic Niño remains poorly understood. Using reanalysis data and CESM2 large‐ensemble simulations, we show that summer Atlantic Niño sea surface temperature anomalies persist into autumn and induce cross‐basin atmospheric circulation adjustments that extend their influence to the Arctic. These changes trigger wave activity that propagates into the stratosphere, where the signal is stored and released downward in winter, producing a negative NAO‐like circulation. This response reinforces a dipole in Arctic surface air temperature, with cooling over the BKS and Eurasia and warming over Greenland and northeastern Canada. These results identify the Atlantic Niño as a predictable driver of Arctic variability, with implications for seasonal prediction.