Since 1979, ERA5 reanalysis has tracked sea ice cover across both poles, one of the most visible fingerprints of a changing climate. This animation shows the linear trend in annual mean sea ice concentration (%/decade). Land is grey; the vast majority of the ocean, which has never had sea ice and never will, is left pale and blank on purpose. That's not missing data, it's a genuine trend of zero.

Sea ice can only exist in a narrow band around each pole, so almost the entire signal here is compressed into a thin ring hugging the current ice edge, the marginal ice zone, where retreat (or, in a few places, expansion) is actively happening.

In the Arctic, the decline is close to uniform across basins, and the Barents Sea stands out as the single fastest ice-losing region on the planet, part of what's known as the "Atlantification" of that Arctic gateway. Antarctica tells a messier story: for decades, the Ross Sea saw ice slightly expand while the Bellingshausen and Amundsen Seas lost it, a striking regional dipole, before an abrupt, still not fully understood collapse hit nearly the whole continent after 2016. The Weddell Sea sits right at the boundary of these competing signals.

Data: ERA5 (Copernicus Climate Change Service)