Caspian sea ice concentration from satellite passive microwave data and surface daily air temperatures are analysed from 1978 to 2009. Relationships between mean winter air temperatures, cumulative freezing degree days (CFDD) and the sum of daily ice area (cumulative ice area) are found. These show that mean monthly air temperature of less than 5.5–9.5 °C, and a minimum CFDD of 3.6 ± 11.2 °C, are required for ice formation in the northern Caspian. Examination of climate projections from multi-model ensembles of monthly mean air temperatures suggest that the northern Caspian may be largely ice-free by 2100 for the highest emission scenario. An ocean–ice–atmosphere model of the Caspian shows weak sensitivities of the minimum CFDD to varied sea ice albedo and ice compressive strength. Sea level decline is found to reduce the minimum CFDD as well as promote the formation of higher concentration or ‘closed ice’.