New insights into the Southern Ocean’s crucial role in the global carbon cycle have emerged from recently published research, revealing the significant influence that winter sea ice exerts on the ocean’s ability to absorb atmospheric carbon dioxide (CO₂). This pioneering study uncovers how the seasonal extent and duration of sea ice in the Southern Ocean regulate the complex interplay between surface waters and the ocean’s deep carbon reservoirs, thereby affecting year-to-year variability in CO₂ uptake. By dissecting these mechanisms, scientists aim to sharpen predictions of climate change trajectories linked to oceanic carbon storage.
The Southern Ocean is widely recognized as a major global carbon sink, responsible for nearly 40% of the ocean’s total absorption of anthropogenic CO₂ emissions. However, this carbon uptake varies widely from year to year, a phenomenon that has long puzzled researchers. The new findings pinpoint winter sea ice formation as a pivotal factor moderating this variability. When sea ice endures longer through the winter months, the Southern Ocean sequesters approximately 20% more CO₂ than in years marked by abbreviated or delayed sea ice cover. This marked difference underscores the sensitivity of carbon cycling processes to sea ice dynamics.
At the heart of this process is the role of sea ice as a physical barrier that shields the ocean surface from intense winter winds. These powerful winds typically induce vigorous mixing of ocean waters, bringing carbon-enriched deep waters to the surface. In the absence of sea ice protection, this mixing liberates centuries-old carbon from the deep ocean, potentially reversing the Southern Ocean’s role from a carbon sink to a source during winter months. Sea ice thus acts as a regulator, limiting these “outgassing” events and preserving the ocean’s net CO₂ uptake capacity.
The study focused on an extensive decade-long dataset (2010 to 2020) collected along the west Antarctic Peninsula, a region undergoing rapid environmental change. These data were acquired through coordinated efforts involving the British Antarctic Survey (BAS) and researchers from the University of East Anglia (UEA), the Alfred Wegener Institute (AWI) in Germany, and Norway’s Institute of Marine Research (IMR). Observations at the UK’s Rothera Research Station provided a unique window into year-round oceanographic conditions, capturing physical, chemical, and biological measurements critical for understanding carbon flux dynamics beneath and around the sea ice.
Data collected during the winter months are particularly rare and technically challenging to obtain due to the harsh Southern Ocean conditions and extensive ice cover. According to Dr. Elise Droste, lead author and environmental scientist at UEA, the scarcity of wintertime observations has severely handicapped efforts to fully comprehend how seasonal processes influence CO₂ exchange. The current study represents an important breakthrough, combining winter and summer measurements to create a holistic picture of the region’s carbon cycle.
This comprehensive approach revealed that, in the warmer months, biological activity such as phytoplankton blooms—and the influx of meltwater—drive down surface CO₂ levels, enabling the Southern Ocean to absorb significant amounts of carbon dioxide from the atmosphere. Conversely, the onset of sea ice formation in autumn and winter triggers a series of physical changes that limit atmospheric exchange. The underlying waters, rich in naturally occurring dissolved inorganic carbon accumulated over centuries, can be transported upward through mixing, boosting CO₂ concentrations at the surface. However, as the study illustrates, the presence of an extensive ice cover substantially curtails this mixing and the associated CO₂ release.
Understanding this seasonal seesaw is essential to accurately modeling the annual net carbon uptake by this climatically vital ocean region. If winter sea ice forms early and persists longer, it restricts outgassing more effectively, tipping the annual balance toward enhanced CO₂ absorption. Conversely, years with limited or late-forming sea ice provoke stronger surface-deep water interactions, potentially reducing the Southern Ocean’s carbon sink capacity. These findings raise important questions about how ongoing climate-induced shifts in sea ice patterns will affect future carbon cycle feedbacks.
To deepen understanding of these processes, the researchers employed an integrative analysis of physical oceanography, chemical assays, and biological indicators. Measurements conducted at Rothera encompassed seawater temperature, salinity, nutrient levels, and dissolved CO₂ specifications. Coupled with remote sensing data and advanced modeling, this multidisciplinary dataset allowed the team to isolate the mechanistic controls behind observed annual variations in CO₂ flux, distinguishing between biological and physical drivers.
Dr. Hugh Venables of BAS emphasizes the value of sustained scientific presence in extreme polar environments to build long-term datasets. He notes that the commitment of oceanographers working through perilous winter conditions—navigating ice floes by boat or sledge—has yielded an unparalleled timeseries that is instrumental to this breakthrough. The study highlights the urgent need to expand year-round sampling networks and incorporate autonomous technologies to capture essential winter data, which remain underrepresented but crucial.
Beyond regional significance, these findings resonate globally because the Southern Ocean’s carbon sink moderates atmospheric CO₂ concentrations and thus influences the pace of climate change worldwide. Professor Dorothee Bakker from UEA stresses that unraveling the physical and biological coupling mechanisms governing carbon cycling in such a dynamic environment has broad implications for Earth system models. Improved representation of winter processes and sea ice impacts in models should tighten predictions of the ocean’s buffering capacity against ongoing anthropogenic emissions.
The study also reflects extensive international collaboration, involving Italian and Swedish oceanographers alongside the core UK and German teams. Funding support from the UK’s Natural Environment Research Council and the EU Horizon 2020 programme helped sustain this ambitious project. The published results in Communications Earth & Environment on June 18, 2025, mark a critical milestone in understanding polar biogeochemical feedbacks amidst a changing climate.
As future climate scenarios forecast altered sea ice extents—driven by rising global temperatures—the delicate balance controlling ocean-atmosphere CO₂ exchange in the Southern Ocean may shift substantially. The research underscores that sea ice duration is not simply a passive indicator of climate change but an active modulator of the ocean’s role in carbon sequestration. This makes it imperative to prioritize monitoring efforts in the polar winter to anticipate and mitigate potential perturbations in the Earth’s carbon budget.
In sum, these findings offer an urgent call to action for the scientific community to intensify observational campaigns and refine biogeochemical models, leveraging advances in autonomous sensing to overcome the formidable challenge of winter data scarcity. By unlocking the secrets of wintertime stratification and sea ice dynamics, researchers move closer to revealing the true nature of the Southern Ocean’s pivotal function in the global climate system.
Subject of Research: Not applicable
Article Title: Sea ice controls net ocean uptake of carbon dioxide by regulating wintertime stratification
News Publication Date: 18-Jun-2025
Web References:
- https://www.nature.com/articles/s43247-025-02395-x
- https://www.nature.com/commsenv/
References: Droste, E., et al. (2025). “Sea ice controls net ocean uptake of carbon dioxide by regulating wintertime stratification.” Communications Earth & Environment.
Image Credits: Elise Droste (University of East Anglia)
Keywords: Southern Ocean, sea ice, carbon dioxide uptake, winter stratification, Antarctic Peninsula, ocean mixing, carbon cycle, climate change mitigation
