Sea ice extent, concentration, and duration [in “State of the Climate in 2015”]

Net sea ice areal extent was well above average during the first few months of 2015 (Fig. 6.8a). Monthly record extents were observed in January (7.46 × 106 km²), April (9.06 × 106 km²), and May (12.1 × 106 km²). The January extent marked the highest departure from average for any month since records began in 1979, at 2.39 × 106 km² above the 1981-2010 mean of 5.07 × 106 km², or nearly 50% greater. These early season records follow on from the record high extent and late retreat of sea ice in 2014 (Reid et al. 2015). During the first half of 2015, there were 65 individual days of record daily sea ice extent, the last occurring on 11 July, and 46 record-breaking days of sea ice area within the first half of the year. However, the expansion of sea ice slowed so dramatically midyear that although sea ice area was at a record high level in May, it was at a record low level in August, just 83 days later (Fig. 6.8a). Close-to-average net sea ice extent levels were then observed in the latter half of 2015.