During the last ice age, the Barents Sea ice sheet began to grow 22 kyr ago(1), only 8 kyr before it began to disintegrate(2). This implies that the ice must have grown very rapidly from the coast to the edge of the continental shelf. Such rapid growth of a large ice sheet requires significant amounts of moisture; but the origin of this moisture has been unclear, particularly as the CLIMAP climate reconstruction suggests(4,5) that the Greenland-Iceland-Norwegian (GIN) seas were perennially ice-covered during this period. Here we present data from deep-sea sediment cores from the Fram Strait, which suggest that relatively warm water from the North Atlantic Ocean was advected into the GIN seas in two short-term events (27-22.5 and 19.5-14.5 kyr ago). We suggest that the resulting seasonally ice-free waters were an important regional moisture source for the Barents Sea ice sheet, and that the GIN seas played a much more active role in climate during the last glaciation than has previously been supposed.