How predictable is the timing of a summer ice‐free Arctic?

Climate model simulations give a large range of over 100 years for predictions of when the Arctic could first become ice free in the summer, and many studies have attempted to narrow this uncertainty range. However, given the chaotic nature of the climate system, what amount of spread in the prediction of an ice‐free summer Arctic is inevitable? Based on results from large ensemble simulations with the Community Earth System Model, we show that internal variability alone leads to a prediction uncertainty of about two decades, while scenario uncertainty between the strong (Representative Concentration Pathway (RCP) 8.5) and medium (RCP4.5) forcing scenarios adds at least another 5 years. Common metrics of the past and present mean sea ice state (such as ice extent, volume, and thickness) as well as global mean temperatures do not allow a reduction of the prediction uncertainty from internal variability. Key Points The first occurrence of an ice‐free Arctic is found to have a prediction uncertainty of two decades due to internal variability Common metrics of the past and present mean sea ice state do not allow a reduction of the prediction uncertainty due to internal variability For characterizing internal variability, the benefit per additional ensemble members is reduced after the first 10‐15 ensemble members