Antarctic sea ice extent has been persistently low since late 2016, possibly owing to changes in atmospheric and oceanic conditions. However, the relative contributions of the ocean, the atmosphere and the underlying mechanisms by which they have affected sea ice remain uncertain. To investigate possible causes for this sea-ice decrease, we establish a seasonal timeline of sea ice changes following 2016, using remote sensing observations. Anomalies in the timing of sea ice retreat and advance are examined along with their spatial and interannual relations with various indicators of seasonal sea ice and oceanic changes. They include anomalies in winter ice thickness, spring ice removal rate due to ice melt and transport, and summer sea surface temperature. We find that the ice season has shortened at unprecedented rate and magnitude, due to earlier retreat and later advance. We attribute this shortening to a winter ice thinning, in line with the ice-albedo feedback, with ice transport playing a more minor role. Reduced ice thickness has accelerated spring ice area removal as thinner sea ice requires less time to melt. The consequent earlier sea ice retreat has in turn increased ocean solar heat uptake in summer, ultimately delaying sea ice advance. We speculate that the observed winter sea ice thinning is consistent with previous evidence of subsurface warming of the Southern Ocean.

Both Arctic and Antarctic sea ice are being affected by climate change. While Arctic sea ice has been declining for several decades, the behavior of Antarctic sea ice was stable before an abrupt increase in its extent from 2012 to 2015, followed by a steep fall in 2016. Sea ice changes need to be quantified to estimate feedback on ocean, atmosphere and ecosystems as well as for arctic communities that are evolving in the most impacted region. Quantifying volumes changes at both basin scales rely on sea ice concentration and thickness observations. While long-term changes in sea ice area are well described, long-term sea ice thickness estimations remain too sparse in space and time to capture the global evolution of sea ice volume. Here we combine sea ice thickness estimates from four satellite radar altimeters to derive the longest time series of homogeneous sea ice thickness for both hemispheres over 30 years (1994-2023). The record supports the rapid loss of sea ice in the Arctic for each month, and the regional sea ice thickness changes heterogeneity for the Antarctic. The study enhances that most of the variability in volume is attributed to variability in thickness, holding for the two hemispheres. This first sea ice thickness time series offers a new insight for models, and the possibility to improve sea ice forecasting, especially in Antarctica where models thickness appear to be low biased.