Uncertainties in 45+ years of Sea-Ice Area and Sea-Ice Area trend observations

Abstract. In this study, we assess and quantify uncertainties associated with observation-based estimates of Arctic and Antarctic Sea-Ice Area (SIA). In particular, we examine uncertainties inherent in SIA estimates from a single product, based on the EUMETSAT Ocean and Sea Ice Satellite Application Facility (OSI SAF) sea ice concentration record, and compare these with the uncertainty in SIA as estimated from the spread across several satellite products. We provide results of a refined uncertainty estimation method that propagates local sea-ice concentration (SIC) uncertainties to hemispheric SIA estimates, accounting for spatial and temporal error correlations. While SIA itself shows notable seasonal and long-term variability, the uncertainties associated with these measurements have remained relatively stable over time. The seasonal cycle of the uncertainties is not directly following the SIA seasonal cycle, but is linked largely to the distribution of the ice. In the growing seasons, the SIC fields are more compact with a rather sharp boundary between high and low sea ice concentrations compared to the more diffuse boundary in the melting season. This seasonal evolution of the so called Marginal Ice Zone (MIZ) leads to relatively large SIA uncertainties in the melting season and smaller uncertainties in the growing season.

We also show that the differences between SIA estimates from different SIC products is characterized by seasonally varying biases. After accounting for these biases, the remaining differences are consistent with our new single-product SIA uncertainty. The presented uncertainty product complements the inter-product approach by providing dynamic daily and monthly estimates which do not rely on the selection and availability of other products. It represents the non-systematic (bias-free) component of the uncertainties, which, among other things, determines the significance of new SIA extremes.

We estimate that non-systematic uncertainties in SIA trend estimates are 11 · 10³ km²/dec. (Arctic) and 14 · 10³ km²/dec. (Antarctic). We further infer the presence of additional systematic uncertainties in these trend estimates, which indicates that a longer time-series will not be sufficient to remove trend uncertainties. These systematic uncertainties are related to inherent differences between the processing chains of the different products, which reveals an unfortunate influence of methodological choices in the SIC product development on SIA trends.