This study isolates the influence of sea ice mean state on; pre-industrial climate and transient 1850-2100 climate change within a; fully coupled global model: The Community Earth System Model version 2; (CESM2). The CESM2 sea ice model physics is modified to increase surface; albedo, reduce surface sea ice melt, and increase Arctic sea ice; thickness and late summer cover. Importantly, increased Arctic sea ice; in the modified model reduces a present-day late-summer ice cover bias.; Of interest to coupled model development, this bias reduction is; realized without degrading the global simulation including; top-of-atmosphere energy imbalance, surface temperature, surface; precipitation, and major modes of climate variability. The influence of; these sea ice physics changes on transient 1850-2100 climate change is; compared within a large initial condition ensemble framework. Despite; similar global warming, the modified model with thicker Arctic sea ice; than CESM2 has a delayed and more realistic transition to a seasonally; ice free Arctic Ocean. Differences in transient climate change between; the modified model and CESM2 are challenging to detect due to large; internally generated climate variability. In particular, two common sea; ice benchmarks - sea ice sensitivity and sea ice trends - are of limited; value for comparing models with similar global warming. More broadly,; these results show the importance of a reasonable Arctic sea ice mean; state when simulating the transition to an ice-free Arctic Ocean in a; warming world. Additionally, this work highlights the importance of; large initial condition ensembles for credible model-to-model and; observation-model comparisons.
