The paper linked is also only an estimate using a different source of data. >”Even adding in the Nimbus V single-channel Electrically Scanning Microwave Radiom

The paper linked is also only an estimate using a different source of data. >”Even adding in the Nimbus V single-channel Electrically Scanning Microwave Radiometer (ESMR) data extends the record by only another 6 years and that data is not as com- plete or consistent [2].” > “This paper presents Antarctic sea ice estimates for winter 1964 and 1966 derived from early Nimbus satellite data as an initial step towards creating Earth Science Data Records of sea ice in the 1960s.”