Last day of navigation from the Barnett Ice Severity Index (blue), freeze-up as calculated here (red), freeze-up based on direct observations at Barrow by local observer J.C. George (green), date of first persistent ice noted by local observers J.C. George and J.M. Leavitt (black), and first shore ice from Radarsat SAR (white squares). Blue and red lines are linear fits to the data.