Abstract The melt season length of the Arctic sea ice is an important indicator and driver of large changes occurring in the climate system. Since 1979 the melt season has lengthened by ~40 days, driven mostly by delayed freeze onset (~ 34 days) compared to earlier melt onset (~ 7 days). However, since 2010 the melt season length has stabilized (~ 108 days), showing no consistent change over the years, instead becoming highly variable (+/− 11 days), largely driven by a loss of multi-year ice in 2000–2009 and a small change in the freeze onset (~ 2 days). There is a stark difference between the decades, where the largest changes in the melt season occurred between 2000–2009 (+ 25 days) and the smallest occurred between 2010–2023 (−2 days). This leads us to believe that, while there might be some periodicity in the processes that control the decadal variations in the melt season length, anthropogenic forcing has altered the Arctic background state and led to a new Arctic melt season that is much longer with a much thinner ice pack that is more susceptible to external forcings. Thermodynamic forcings help to explain these differences, where the summer net surface energy budget (SEB) and melt season length are highly correlated (+ 1 Wm −2 = +2.13 days longer melt season). Between 2000–2009 there was a large increase in net shortwave (SW) radiation (+ 12.6 Wm −2 ) and therefore the SEB (+ 10 W m −2 , driven by decreasing surface albedo (−0.022%) and cloud cover (−2.3%)). During 2010–2023, an increase in cloud cover (+ 7.2%) and only minimal changes in albedo (0.011%) drove a net decrease in SW (−3.14 Wm −2 ) and summer SEB (−2 W m −2 ). The increased summer net SEB drives sea ice melt (1 m of melt requires ~34 W m −2 ) with the additional energy (+ 8.5 Wm −2 ) accounting for 23 cm extra melt, thus reducing the October/November sea ice thickness. Although since 2010 the melt season length appears to have stabilized, conditions in the Arctic are ever changing with interannual variations superposed on global warming. Therefore, it is imperative to continue closely monitoring the melt season length of the Arctic sea ice to assess its resilience to the present conditions and estimate future changes.
Boisvert et al. (Thu,) studied this question.