Cecilia M. Bitz

Arctic albedo has declined steadily over the satellite era, driven by a self-reinforcing cycle in which retreating sea ice exposes darker ocean water and the surviving ice itself becomes less reflective. This sea ice albedo feedback is a cornerstone of Arctic climate dynamics with broader implications for global climate sensitivity, yet its magnitude remains poorly constrained. Previous estimates have focused almost exclusively on the loss of sea ice area, neglecting changes in reflectivity of the remaining ice. Here we show that the progressive darkening of surviving sea ice constitutes approximately one third of the total sea ice albedo feedback. Extending satellite records across four decades and validating them against field measurements, we find that leading climate models largely underestimate sea ice albedo feedback due to both sea ice darkening and area loss. These results reveal a critical, underestimated amplifier of Arctic warming with implications for sea ice loss and global climate sensitivity.

Over the satellite period, Arctic and Antarctic sea ice extent seemed to follow opposite pathways. Arctic sea ice showed a strong and prolonged decrease until 2007 and then stalled in its rate of decline. Antarctic sea ice extent, on the other hand, displayed a small but significant increase until 2015. After 2015, Antarctic sea ice extent experienced a sharp decline, and has since been characterized by a lower mean state with enhanced variability, suggested as a new sea-ice regime. Arctic sea ice has continued transitioning towards a regime of substantially lower summertime coverage with reduced multiyear ice, increasingly resembling Antarctic sea ice in conditions and seasonality. We suggest that the parallel changes in Arctic and Antarctic sea-ice behavior share a common mechanism: the breakdown of ocean stratification sustaining sea-ice decline, whilst episodic atmospheric forcing increasingly influences high-frequency sea-ice variability. In both the Antarctic and the Eurasian Basin of the Arctic, positive sea surface salinity anomalies have caused a weakening of the ocean halocline. The associated erosion of stratification makes sea ice more vulnerable to subsurface heat, and appears to underpin recent sea-ice loss and elevated variability. The convergence of the two polar regions toward a seasonally dominated sea-ice regime leads us to call for enhanced knowledge transfer and coordination between polar communities to accelerate understanding of recent sea-ice changes. A better understanding of the mechanisms driving changes in the coupled atmosphere--ocean--sea-ice system is needed to improve projections of future sea-ice loss and assess the cascading risks it poses to global climate and regional ecosystems.

Photosynthetic eukaryotic algae survived the Neoproterozoic Snowball Earth events, indicating that liquid‐water refugia existed somewhere on the surface. We examine the potential for refugia at the coldest time of a snowball event, before CO ₂ had risen and with high‐albedo ice on the frozen ocean, before it became darkened by dust deposition. We use the Community Earth System Model to simulate a “modern” Snowball Earth (i.e., with continents in their current configuration) in which the ocean surface has frozen to the equator as “sea glaciers”, hundreds of meters thick, flowing like ice shelves. Despite global mean surface temperatures below −60°C, some areas of the land surface reach above‐freezing temperatures seasonally because they are darker than the ice‐covered ocean. With low CO ₂ (10 ppm) and land‐surface albedo characteristic of bright sand‐deserts (0.4), 0.1 percent of the land surface could host liquid water seasonally; this increases to 12 percent for darker land of albedo characteristic of polar deserts (0.2). Given a water source to these locations, such as a narrow bay intruding from the ocean like the modern Red Sea, or summer precipitation or melt of nearby glaciers, we argue that photosynthetic life would have the potential to survive on or near warm, bare land. The abundance of warm land and consequently potential refugia increases more strongly in response to reducing the land albedo than to higher CO ₂ , for the same global radiative forcing, due to the concentration of the forcing over land.

We describe and present results from a 2024/2025 field campaign that is the first to test and observe the impact of flooding and meltwater draining on Arctic sea ice over the winter growth and spring melt seasons. The campaign was conducted in Cambridge Bay, Nunavut, Canada. A 1 by 1 km fieldwork site was used, comprising three control areas, which were never flooded, and eight test areas. In these, flooding treatments were carried out by pumping seawater onto the sea ice. Some test areas were flooded once (in December or January), while others were flooded twice (in December and February, or January and February). The total area flooded was 0.25 . Additionally, one control area was used for a melt pond drainage experiment during spring. By mid May, prior to melt, flooded test areas were up to 32 cm thicker than control areas, with snow cover that was 1–13 cm thinner. Areas flooded twice exhibited greater thickening than those flooded once. During the melt period, sea ice in the flooded areas appeared brighter and showed slower melt rates, remaining thicker than that in the control areas. The drained melt pond site also brightened markedly within 1 week of borehole drilling. Comparison with a historical sea ice thickness record from Cambridge Bay indicates that a 30 cm increase corresponds to roughly the magnitude of long‐term thinning observed over the past 50 years.

We describe a fieldwork campaign designed to assess whether artificial flooding results in thicker Arctic sea ice. The campaign was conducted in Cambridge Bay, Nunavut, Canada, over the 2024/2025 sea ice season. The fieldwork site comprised three control areas, which were never flooded, and eight test areas. In these, flooding treatments were carried out in three periods – in late November/early December, January, and February – by pumping seawater onto the sea ice. Some test areas were flooded once (in December or January), while others were flooded twice (in December and February, or January and February). Additionally, one control area was used for a melt pond drainage experiment during spring. Measurements of sea ice thickness, snow depth, and ice core salinity were collected throughout the ice-growth season in all test and control areas. Four thermistor strings provided continuous temperature records of the air, ice, and upper ocean. By early May, flooded test areas were up to 30 cm thicker than control areas, with slightly thinner snow cover. Sites flooded twice exhibited greater thickening than those flooded once. During the melt period, ice in the flooded areas appeared brighter and showed slower melt rates, remaining thicker than that in the control areas. The drained melt pond site also brightened markedly within one week of borehole drilling. Comparison with a historical sea ice thickness record from Cambridge Bay indicates that a 30 cm increase corresponds to roughly the magnitude of long-term thinning observed over the past 50 years.

Photosynthetic eukaryotic algae survived the Neoproterozoic Snowball Earth events, indicating that liquid-water refugia existed somewhere on the surface. We examine the potential for refugia at the coldest time of a snowball event, before CO ₂ had risen and with high-albedo ice on the frozen ocean, before it became darkened by dust deposition. We use the Community Earth System Model to simulate a “modern” Snowball Earth (i.e., with continents in their current configuration), in which the ocean surface has frozen to the equator as “sea glaciers”, hundreds of meters thick, flowing like ice shelves. Despite global mean surface temperatures below -60°C, some areas of the land surface reach above-freezing temperatures because they are darker than the ice-covered ocean. With low CO ₂ (10 ppm) and land-surface albedo characteristic of bright sand-deserts (0.4), 0.1 percent of the land surface could host liquid water seasonally; this increases to 12 percent for darker land of albedo characteristic of polar deserts (0.2). Given a water source to these locations, such as a narrow bay intruding from the ocean like the modern Red Sea, photosynthetic life could survive on warm, bare land. The abundance of habitable land and consequently potential refugia increases more strongly in response to reducing the land albedo than to higher CO _2, for the same global radiative forcing.

In January 2022, the strongest Arctic cyclone on record resulted in a record weekly loss in sea ice cover in the Barents‐Kara‐Laptev seas. While ECMWF operational forecasts skillfully predicted the cyclone, the loss in sea ice was poorly predicted. We explore the ocean's response to the cyclone using observations from an Argo float that was profiling in the region, and investigate model biases in simulating the observed sea ice loss in a fully coupled GCM. The observations showed changes over the whole ocean column in the Barents Sea after the passage of the storm, cooling and mixing with enough implied heat release to melt roughly 1 m of sea ice. We replicate the observed cyclone in the GCM by nudging the model's winds to observations above the boundary layer. In these simulations, the associated loss of sea ice is only about 10%–15% of the observed loss, and the ocean exhibits very small changes in response to the cyclone. With the use of a simple 1‐D ice‐ocean model, we find that the overly strong ocean stratification in the GCM may be a significant source of model bias in its simulated response to the cyclone. However, even initialized with observed stratification profiles, the 1‐D model also underestimated mixing and sea ice melt relative to the observations.

Since 2016, Antarctic sea ice area (SIA) has set three record summertime minimums occurring in 2017, 2022, and 2023. These recent extremes culminated in a record low SIA anomaly in austral winter 2023, when SIA was over 2 million km^2 below climatology, resulting from negative sea ice concentration (SIC) anomalies centered in the Ross, eastern Weddell, and East Antarctic Seas. We show that a fully-coupled Earth System Model run with historic and anthropogenic forcing and nudged to observed winds over 1950-2023 reproduces the observed austral winter 2023 SIC anomalies. In a sensitivity test that had the impact of 2023 ENSO conditions removed via regression, we show that the 2023 transition from La Niña to El Niño had a negligible impact. Next, using an ensemble initialized on January 1st 2023 nudged to past years' winds, we demonstrate that ~70% of the total Antarctic SIA anomaly was predictable six months in advance and driven by warm Southern Ocean sea surface temperatures that developed prior to 2023, with the remaining ~30% attributable to 2023 atmospheric circulation. Finally, an ensemble forecast suggests that Antarctic SIA is likely to remain significantly below climatology in austral winter 2024 due to continuing warm Southern Ocean conditions.