Fig. 2. Frequency of calving in Börgen Bay.

A time series of the terminus position of William Glacier digitized from Sentinel-1 synthetic aperture radar (SAR) imagery, acquired between 3 May 2015 and 28 December 2021 (see Materials and Methods). This sequence places the 21 January 2020 calving event in the longer-term context of the terminus position change, particularly in relation to the size/frequency of other calving events. In the May 2015 to December 2021 period, we observe nine other terminus retreat events of equivalent or greater magnitude (red symbols) and a total of 39 other events at least half as large (blue symbols). These terminus retreats correspond to large collapses of the ice front and input of solid ice into Börgen Bay, although the calving behavior in the period between satellite images cannot be resolved exactly. Events half as large as January 2020 occur most frequently in November, and there is at least one event larger than January 2020 in every austral summer except 2020 to 2021. Although less frequent, there are also some notable winter retreats, with events larger than January 2020 occurring in August 2017, August 2018, and June 2019. This time series also shows a trend of multiyear retreat through this period, with the terminus of William Glacier retreating by 118 ± 10 m from 29 December 2015 to 28 December 2021.