Dear Bruce,
Many thanks for your query. You can see from the Hersbach et al. 2020 paper ‘The ERA5 global reanalysis’, that the sea ice concentration input changes from HadISST2.0.0.0 to OSI-SAF in 1979, not 2007 (Table 7). In September 2007, we change the SST product from HadISST2.1.1.0 to OSTIA, and although we continue to use OSI-SAF for sea ice concentration, we start to use the operational version.
You can see from the newsletter article that you shared, that we will use ORAS6 as the lower boundary conditions for ERA6, and we aim to use OSI-SAF sea ice concentration data in ORAS6 from around 1979 once again.
Many thanks,
Alison