Underestimated accelerated Antarctic phytoplankton net primary production in winter over past decade from spaceborne...

The Southern Ocean's vital carbon sink is driven by phytoplankton Net Primary Production (NPP). Winter phytoplankton seed spring algal blooms and regulate nutrient cycling and ecosystem dynamics, yet Antarctic winter NPP remains poorly constrained due to limited in situ data and passive satellite sensor challenges in low-light, ice-covered conditions. Here, we leverage spaceborne LiDAR (CALIOP), analyzing 16 years of data (16,236 tracks spanning 2006-2023), to reveal a significant, previously underestimated rise in winter NPP, increasing by ~2.2 Tg C yr⁻¹ (P < 0.01). Enhanced coverage with CALIOP boosts ice-free sea observations from 12.5% to 80.7%, exposing pronounced NPP gains in the seasonal sea-ice zone, notably the Weddell and Ross Seas, driven by declining sea ice, greater light penetration, and nutrient mixing. These shifts, modulated by climate modes like the Southern Annular Mode and El Niño-Southern Oscillation, highlight the Southern Ocean's escalating role in global carbon dynamics. Integrating winter NPP into carbon models is essential to refine projections of polar carbon sequestration under climate change.