The 2016–2017 Arctic sea ice growth season (October–March) exhibited one of the lowest values for end‐of‐season sea ice volume and extent of any year since 1979. An analysis of Modern‐Era Retrospective Analysis for Research and Applications, Version 2 atmospheric reanalysis data and Clouds and the Earth's Radiant Energy System radiative flux data reveals that a record warm and moist Arctic atmosphere supported the reduced sea ice growth. Numerous regional episodes of increased atmospheric temperature and moisture, transported from lower latitudes, increased the cumulative energy input from downwelling longwave surface fluxes. In those same episodes, the efficiency of the atmosphere cooling radiatively to space was reduced, increasing the amount of energy retained in the Arctic atmosphere and reradiated back toward the surface. Overall, the Arctic radiative cooling efficiency shows a decreasing trend since 2000. The results presented highlight the increasing importance of atmospheric forcing on sea ice variability demonstrating that episodic Arctic atmospheric rivers, regions of elevated poleward water vapor transport, and the subsequent surface energy budget response is a critical mechanism actively contributing to the evolution of Arctic sea ice. Plain Language Summary In the winter of 2016–2017, the Arctic experienced several days when temperatures over the region were 20°C (over 30 °F) above normal. In this article, we investigate the reasons that these warm days existed and show how this winter was unprecedented in the last 15 years. The key to the warming is periods of warm and moist air transported from lower latitudes. These warm periods reduced the rate of growth of sea ice and contributed to sea ice at the end of the 2016–2017 winter season that was among the thinnest and least expansive on record. The factors that supported the record low growth in 2016/2017 are also important for other recent winters, and these factors may become more important as Arctic sea ice becomes thinner and more vulnerable to change. Key Points The 2016/2017 freeze‐up season exhibited one of the lowest values for end‐of‐season Arctic sea ice extent and volume on record Arctic atmospheric rivers increased downwelling longwave fluxes and reduced sea ice growth rate for periods during the 2016/2017 freeze‐up season Cooling efficiency of Arctic surface by longwave fluxes in 2016/2017 was lowest since 2000, with contribution from Arctic atmospheric rivers