Quantifying sea ice trends in the Southern Ocean: is extent or area the better measure?

Abstract

This report presents an assessment of the relationship between sea ice area and extent measurements in the Southern Ocean, in order to scrutinise the significance of the reported trend of increasing Antarctic sea ice over the past 40 years. Two key research questions are addressed: How are Antarctic sea ice extent and area values calculated and what information (and to what accuracy) are they actually telling us about sea ice mass balance? How do measurements of sea ice extent and area compare between that derived from low resolution and high-resolution data? The methods undertaken include a close examination of the NSIDC sea ice concentration, area and extent data trends from 1978-2016, and a case study analysis in the Weddell Sea that compares sea ice concentration, extent and area data derived from low resolution passive microwave radiometers (SSM/IS and AMS2) and higher resolution SAR. The findings reveal that the average trend conceals a large amount of spatial and seasonal variability and that there are several extreme months throughout the record where extent anomalies significantly exceed area anomalies. It is suggested that this could be a reflection of either physical processes (e.g. wind behaviour) that may vary between regions, or instrumental errors. However, the fact that measures of sea ice thickness are not incorporated into record of sea ice cover, points to the conclusion that the reported rising trend in Antarctic sea ice cover in the past few decades is highly incomplete and cannot be used to interpret sea ice mass balance changes.