Abstract
Climate warming has resulted in extensive sea ice loss across the Arctic. Polar bears ( Ursus maritimus ) rely on sea ice for hunting, resting, travelling and in some parts of the Arctic also maternity denning. In the European Arctic, polar bears rely on snow drifts on land to den and give birth. Consequently, timely arrival of sea ice around land masses during autumn is important for pregnant females to reach their denning habitat from their sea ice hunting grounds. We defined denning habitat as landforms necessary to accumulate snow to a depth sufficient for dens. We quantified availability of terrestrial denning habitat across the three European Arctic archipelagos throughout the last four (1979–2020) and the next eight decades (until 2100) using arrival of autumn sea ice around these islands. Across the study area, a clear trend was visible towards later sea ice arrival, varying up to 102 days. Female polar bears in the European Arctic now have 33% denning habitat available compared to the 1980's as many areas became inaccessible in time to start maternity denning. By the 2090's, all areas were projected to be inaccessible to pregnant bears. This decline was unequally distributed, with most reduction in Svalbard and Novaya Zemlya until 2020, whilst denning habitat availability in Franz Josef Land remained unchanged until 2020 but is predicted to become inaccessible by the end of the century. This work emphasizes the importance of the temporal dimension of sea ice dynamics for the persistence of polar bear populations.