Winter Sea Ice Boosts CO2 Absorption in the Southern Ocean

Recent research underscores the critical role winter sea ice plays in influencing the annual fluctuations in atmospheric carbon dioxide (CO₂) absorption within a segment of the Southern Ocean. The study reveals that when sea ice persists longer throughout the winter months, the Southern Ocean can absorb approximately 20% more CO₂ compared to years characterized by late sea ice formation or early melting. The correlation lies in the ability of sea ice to shield the ocean from harsh winter winds, which typically drive the mixing of the surface waters with the deeper, carbon-rich ocean layers.

Conducted by scientists at the University of East Anglia (UEA), alongside experts from the Alfred Wegener Institute (AWI, Germany), the British Antarctic Survey (BAS, UK), and the Institute of Marine Research (IMR, Norway), the study has been published in the journal Communications Earth & Environment. This collaborative effort aims to shed light on the Southern Ocean’s role in climate change mitigation, as it absorbs about 40% of the global ocean’s CO₂ uptake from human activities.

Dr. Elise Droste, the lead author from UEA’s School of Environmental Sciences, articulated the current gaps in understanding the Southern Ocean’s carbon cycle. With the future implications for atmospheric CO₂ uptake remaining uncertain, Dr. Droste emphasized the necessity for further exploration of sea ice’s impact on carbon exchanges between oceanic layers. To enhance predictive capabilities regarding climate change, more wintertime data collection in the Southern Ocean is essential.

The Southern Ocean, particularly around the west Antarctic Peninsula, is enveloped in sea ice during winter, which recedes during the warmer months. The seasonal dynamics of phytoplankton proliferation and meltwater during spring and summer facilitate significant CO₂ absorption, alleviating the global effects of anthropogenic emissions. However, during winter, as sea ice develops, the underlying ocean experiences mixing with ancient carbon-rich waters, potentially elevating surface CO₂ levels that could lead to atmospheric release.

Notably, sea ice acts as a barrier, moderating much of this CO₂ release, but it also allows for a natural discharge during specific seasonal cycles. This interplay means the net CO₂ absorption by the Southern Ocean can be heavily influenced by the balance of summer absorption against winter releases.

Dr. Droste remarked on the challenges posed by limited data availability regarding Southern Ocean dynamics, particularly during winter months when harsh conditions make observations difficult. The research utilized data from 2010 to 2020, focusing on a project coordinated by BAS, featuring comprehensive year-round monitoring along the west Antarctic Peninsula. At the Rothera research station, ocean scientists conducted direct seawater measurements and collected samples for nutrient and CO₂ analyses.

Dr. Hugh Venables from BAS noted the significance of conducting year-round studies, highlighting the efforts of numerous oceanographers who have braved the tough conditions to compile a valuable time series of oceanographic data over the past 25 years. The findings advocate for increased winter sampling efforts, harnessing both human expertise and autonomous technologies for better insights into the Southern Ocean’s dynamics.

Further contributions to the study came from the National Institute of Oceanography and Applied Geophysics (Italy) and the University of Gothenburg (Sweden), with financial backing from the UK’s Natural Environment Research Council and the European Union’s Horizon 2020 initiative. This research points toward necessary directions for understanding and predicting the Southern Ocean’s future contributions to global climate dynamics.