Estimating the uncertainty of sea-ice area and sea-ice extent from satellite retrievals.

The net Arctic sea-ice area (SIA) can be estimated from the sea-ice concentration (SIC) by passive microwave measurements from satellites. To be a truly useful metric, for example of the sensitivity of the Arctic sea-ice cover to global warming, we need, however, reliable estimates of its uncertainty. Here we retrieve this uncertainty by taking into account the spatial and temporal error correlations of the underlying local sea-ice concentration products. As 1 example year, we find that in 2015 the average observational uncertainties of the SIA are 306 000 km 2 for daily estimates, 275 000 km 2 for weekly estimates, and 164 000 km 2 for monthly estimates. The sea-ice extent (SIE) uncertainty for that year is slightly smaller, with 296 000 km 2 for daily estimates, 261 000 km 2 for weekly estimates, and 156 000 km 2 for monthly estimates. These daily uncertainties correspond to about 7 % of the 2015 sea-ice minimum and are about half of the spread in estimated SIA and SIE from different passive microwave SIC products. This shows that random SIC errors play a role in SIA uncertainties comparable to inter-SIC-product biases. We further show that the September SIA, which is traditionally the month with the least amount of Arctic sea ice, declined by 105000±9000 km 2 a -1 for the period from 2002 to 2017. This is the first estimate of a SIA trend with an explicit representation of temporal error correlations. [ABSTRACT FROM AUTHOR]