The Arctic Ocean's Wandel Sea is the easternmost sector of the Last Ice
Area, where thick, old sea ice is expected to endure longer than
elsewhere. Nevertheless, in August 2020 the area experienced record-low
sea ice concentration. Here we use satellite data and sea ice model
experiments to determine what caused this record sea ice minimum. In our
simulations there was a multi-year sea-ice thinning trend due to climate
change. Natural climate variability expressed as wind-forced ice
advection and subsequent melt added to this trend. In spring 2020, the
Wandel Sea had a mixture of both thin and---unusual for recent
years---thick ice, but this thick ice was not sufficiently widespread to
prevent the summer sea ice concentration minimum. With continued
thinning, more frequent low summer sea ice events are expected. We
suggest that the Last Ice Area, an important refuge for ice-dependent
species, is less resilient to warming than previously thought.