Faster decline and higher variability in the sea ice thickness of the marginal Arctic seas

Abstract. Mean sea ice thickness is a sensitive indicator of Arctic climate change and in long-term decline despite significant interannual variability. Current thickness estimations from satellite radar altimeters employ a snow climatology for converting range measurements to sea ice thickness, but this introduces unrealistically low interannual variability and trends. When the sea ice thickness in the period 2002–2018 is calculated using new snow data with more realistic variability and trends, we find mean sea ice thickness in three of the seven marginal seas to be declining between 70–100 % faster than when calculated with the conventional method. When analysed as an aggregate, the mean ice thickness in the marginal seas is now in statistically significant decline for four of seven winter months. This is observed despite a 58% increase in interannual variability between the methods in the same time period. On a seasonal timescale we find that snow data exerts an increasingly strong control on thickness variability over the growth season, contributing only 20 % in October but 72 % by April. Higher variability and faster decline in the sea ice thickness of the marginal seas has wide implications for our understanding of the polar climate system and our predictions for its change, as well as for stakeholders involved in Arctic shipping and natural resource extraction.