Using 150 core top samples, we developed a conservative but fairly robust new measure to reconstruct past changes in (maximum) seasonal sea ice distribution in the northern North Atlantic, hitherto a major unknown. The proxy is based on Similarity Maximum Modern-Analog Technique (SIMMAX) estimates of threshold temperatures near the sea surface (SST). Today, almost 100% of all sites with SST >2.5¿C during summer, >0.4¿ for winters 1978--1987, and >0.75¿C for Little Ice Age winters lie seaward of the sea ice margin. When applied to >60 sediment records of the Last Glacial Maximum, this proxy shows that peak glacial sea ice was far more restricted than in the classic CLIMAP Project Members <1981> reconstruction. During glacial summer, sea ice only covered the Arctic Ocean and western Fram Strait. The northern North Atlantic and Nordic Seas were largely ice-free and thus formed a high-latitude moisture source for the continued buildup of continental ice sheets. In contrast, sea ice spread far south across the Iceland Faeroe Ridge during glacial winter, with an inferred patch of sea ice also in the central east Atlantic, near the center of the Azores High. A broad ice-free channel extended from 50¿ to 60¿N, forming an ideal site for large-scale convection of glacial upper North Atlantic Deep Water. The extreme seasonality in glacial sea ice formation and melt in the Nordic Seas implies a major consumption of the regional energy income.