The increase in the length of the ice-free season in the Arctic

The length of the ice-free season and a variable designated by inverse sea ice index are used to quantify the rapid decline of the Arctic sea ice of the last decades. It is argued that for this purpose they are at least as suitable as the other variables more often found in the literature, namely the sea ice area and extent at the times of annual minimum. Daily sea ice concentrations obtained from satellite passive microwave imagery are used to calculate the length of the ice-free season and the inverse sea ice index in each point of the Arctic for each year between 1979 and 2008. Time-series for these two quantities are constructed and analysed to investigate how they have been varying in the last 30 years in 85 disjoint regions of the northern hemisphere. It is shown that between 1979 and 2006 the spatially averaged ice-free season in the Arctic increased at a steady rate of 1.1 days/year and that the growth was considerably faster (5.5 days/year), and monotonic, in the 2001–2007 period. In 2007 and 2008 the average ice-free seasons in the Arctic were 168 and 158 days long, respectively, making them the longest and the fourth longest since the beginning of the systematic satellite monitoring of the Arctic.