Changes in Antarctic sea-ice extent from direct historical observations and whaling records

Changes in the extent of Antarctic sea-ice are difficult to quantify for the pre-satellite era. The available direct data are sparse. A substantially larger set of proxy records based on whaling positions indicated a large shift in whaling positions between the 1930s to 1950s compared with whaling positions in the 1970s to mid 1980s. However, these findings have been questioned. Further analyses here using historic ice charts, direct sea-ice observations and whaling positions agree that a substantial southward shift in the ice-edge did occur. The analyses indicate the average change is around 1.89A degrees to 2.80A degrees of latitude with a reasonable mid-range estimate of 2.41A degrees. Regional analyses show that the largest changes occurred in the South Atlantic, but change is also detected across the Indian Ocean to the Ross Sea; a 220A degrees span of longitude. A recently published proposition that the shift in ice-edges is an artefact caused by bias in the satellite derived records is not supported.