Sea Ice Loss leads to regime shifts in the arctic biological pump.

The Arctic Ocean has undergone accelerated warming and a marked decline in sea ice over recent decades. Yet, the response of the biological pump-a critical mechanism for atmospheric carbon sequestration-remains poorly understood. Here, we develop a satellite-derived dataset (2003-2022) to identify a regime shift in the Arctic biological pump. Between 2003 and 2012, the strength and efficiency of the biological pump increased rapidly, primarily driven by sea ice decline. However, from 2013 to 2022, this trend plateaued, coinciding with stabilized sea ice conditions and increased phytoplankton biomass. Earth system model simulations (1850-2100) support the observed link between biological pump enhancement and sea ice loss, and project a future regime shift towards a weakened biological pump under nearly ice-free conditions, associated with shifts in phytoplankton community structure. These findings underscore the Arctic's vulnerability to climate-driven changes, with far-reaching implications for Arctic carbon sequestration and ecosystem stability.