This is a project jointly funded by the National Science Foundation’s Directorate for Geosciences (NSF/GEO) and the National Environment Research Council (NERC) of the United Kingdom (UK) via the NSF/GEO-NERC Lead Agency Opportunity. This Lead Agency Opportunity allows a single joint US/UK proposal to be submitted and peer-reviewed by the Agency whose investigator has the largest proportion of the budget. Upon successful joint determination of an award recommendation, each Agency funds the proportion of the budget that supports scientists at institutions in their respective countries. Sea ice plays a critical role in the exchange of heat and gases between the ocean and atmosphere and provides essential habitat for globally important wildlife. Antarctic sea ice has become a new focus of scientific interest due record-breaking low ice extents in recent years, which has only been possible to determine from the 50-year record of sea ice concentration (SIC) derived from satellite remote sensing. Passive microwave (PM) derived datasets of sea ice provide unprecedented insight into decadal scale trends in sea ice conditions, but these products are inadequate because they do not provide uncertainty associated with this record. For example, when PM data calculates that a locality has a SIC of 50% (corresponding to 50% of the ocean’s surface being covered by ice), the uncertainties and error associated with this measurement remain undefined. In this proposal, a comprehensive assessment