Comparison of long-term hemispheric and regional sea ice extent trends from passive microwave sea ice climate records

Passive microwave records of sea ice concentration and extent represent one of the longest continuous satellite-derived climate records, and the substantial decline in Arctic sea ice is an iconic indicator of climate change. There are several passive microwave sea ice climate records, stitched together from a series of multi-channel radiometers from 1979 to the present. Here we compare trends from several of these products, including NASA Goddard NASA Team and Bootstrap algorithm products, the EUMETSAT Ocean and Sea Ice Satellite Application Facility (OSI SAF), and a new version of the NOAA/NSIDC Sea Ice Concentration Climate Data Record. This new version was released in June 2021 and includes substantial enhancements over the previous version. The different algorithms, different processing approaches, and in some cases, different sensors yield variation in the retrieved concentration and extent estimates. These independently developed approaches provide an ensemble view of sea ice change and inform the uncertainty of long-term trends and variation. We also examine specific regional trends, via newly-developed regional maps, to further highlight uncertainties and provide improved regional assessments of sea ice change. The results are in line with previous analyses showing that the different products have biases between them, often varying by season, but that long-term trends are generally consistent. Regional trends do show some discrepancies, particularly across sensor transitions due to different approaches to inter-sensor calibration. The results suggest that an ensemble approach could be useful in conveying long-term trends and variability of sea ice concentration and extent.