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The remote Antarctic continent, distant from human industrial activity, should be one of the last places on Earth to capture Anthropogenic change. Hence, stratigraphic evidence of pollution and nuclear activity in the Antarctic provides proof of the global nature of the Anthropocene epoch. We propose an Antarctic Peninsula ice core candidate for the Global boundary Stratotype Section and Point (GSSP) to the onset of the Anthropocene. The Palmer ice core captures the first evidence of spheroidal carbonaceous fly ash particles (SCPs), resulting from high temperature combustion deposited in Antarctic ice. SCPs first appear in 1936 CE, preceding the rise in plutonium (239+240Pu) concentrations from 1945 CE onwards. GSSP 1952 CE occurs at a depth of 34.9 m, coincident with the peak in 239+240Puthe primary marker for this site.

Sea‐ice extent is predicted to decrease in a warming climate. However, despite global warming over the past century, total Antarctic sea ice remained relatively stable from 1979 until 2015, before strongly melting. Here we explore the long‐term sea ice variability by reconstructing Antarctic sea ice since 1700 CE, based on paleoclimate records and data assimilation. Our results indicate a decline in southern hemisphere sea‐ice extent over the 20th century, driven by a reduction of 0.26 million km2in the Weddell Sea that reached values at the end of the century lower than any other reconstructed period. The Ross Sea experienced an increasing sea‐ice cover trend due to a low‐pressure system located off the Amundsen Sea coast, offset by a decreasing trend in the Bellingshausen‐Amundsen Sea. Models failed to account for the Ross Sea increase, resulting in an overly uniform estimate of Antarctic sea ice loss over the 20th century. Despite global warming, the Antarctic sea‐ice extent remained relatively constant over the satellite era until 2015 before a strong reduction. Numerous studies analyzed this intriguing behavior but no consensus emerged, mainly because of the shortness of observations. Here, we reconstruct the Antarctic sea‐ice over the past 300 years based on paleoclimate observations, in particular ice‐core records, combined with the physics of climate models using a so‐called data‐assimilation technique. We show that there has been a decline in the Antarctic sea‐ice extent during the 20th century. Specifically, the Atlantic sector displays a significant sea‐ice extent reduction, reaching its lowest level at the end of our reconstruction compared to the past three centuries. Our results indicate sea‐ice extent trends over the 20th century of variable signs in the Pacific sector, with a sea ice increase in western Pacific while a decrease is noticed in western Pacific. These heterogeneous spatial changes are mainly related to changes in the low‐pressure system located off the West Antarctic coasts. According to our analysis, climate models are not able to simulate the observed sea ice expansion in the eastern Pacific, leading to a too large and too homogenous Antarctic loss during the 20th century. The Antarctic sea‐ice extent experienced a reduction over the 20th centuryThe sea‐ice extent in Weddell Sea reached its lowest level over the past 300 years at the end of the 20th centuryModels are unable to simulate the reconstructed sea ice increase in the Ross Sea, leading to an overestimation of the Antarctic sea ice loss The Antarctic sea‐ice extent experienced a reduction over the 20th century The sea‐ice extent in Weddell Sea reached its lowest level over the past 300 years at the end of the 20th century Models are unable to simulate the reconstructed sea ice increase in the Ross Sea, leading to an overestimation of the Antarctic sea ice loss