An Extreme Antarctic Event; 2025 Was Record Low Seasonal Sea Ice and Record High Iceberg Scouring

Antarctica is an extreme region in many ways and long term marine ice monitoring shows 2025 was the most of those extremes. Seasonal sea ice duration was observed daily at Rothera Research Station, Antarctica since 1985 and seabed markers were monitored for iceberg scour hits since 2002. Two decades of data showed that sea ice duration showed a strong inverse relationship with iceberg scouring. In 2025 sea ice duration was zero days, a record low and 68% of seabed markers were hit by icebergs, a record high. Long term monitoring data showed that ice scour responses to benthos were most significant after a two year lag period. Sustained seasonal sea ice losses in a warming world suggest these extremes may become normal in both levels in the coming decades and assemblages may take decades to recover and became pioneer dominated. A profound change in the polar oceans has been the drastic loss of seasonal sea ice. Such marine ice has been recorded daily at Ryder Bay, Adelaide Island (Antarctica) since 1985, and iceberg impacts on the seabed monitored by SCUBA divers annually since 2002. These showed an extreme event of low seasonal sea ice duration in 2007/8 (of just 9 days). However, in 2025, for the first time, zero days seasonal sea ice duration was recorded. Coincident with the 2007/8 extreme low seasonal sea ice event, an extreme high of iceberg scouring of the seabed was also recorded (53% of seabed at 5–25 m hit). As two decades before, the 2025 extreme low in seasonal sea ice was coincident with a record high of seabed iceberg impact; 68.4% of the seabed at 5–25 m depth was scoured. This was the most extreme of both marine measures and the biggest outlier recorded in the significant (r2 = 57, F = 31.5, p < 0.001) relationship between seasonal sea ice duration versus seabed iceberg scour (anomalies, see Figure 1a). Assessment of iceberg scouring has occurred annually since 2002 (Barnes et al. 2024), but as there are decades more seasonal sea ice duration than ice-scour data, we hindcast the relationship to generate likely past seasonal ice versus ice-scouring data span (grey circles in Figure 1a). Raw data of ice-scour impact frequency with time shows context of the ‘background’ variability measured in the last two decades with which to compare the 2025 data. This (2025) event was the second-most severe at 10 m depth and the most severe at both 5 m and 25 m (Figure 1b). Many icebergs with a wide range of keel sizes were involved. The effects of this extreme scouring event on local assemblages, ecology and biodiversity may not be calculable for many years. Effects of iceberg scouring on mortality of model species (the bryozoan Fenestrulina rugula) in years leading up to this event had a strong relationship between iceberg scour frequency and annual bryozoan mortality (r2 = 83.3, F = 70.6, p < 0.001, see also Barnes and Souster 2011). Likewise, monitoring of macro and mega-species richness (data extended from Zwerschke et al. 2021), showed a strong inverse relationship with ice-scour levels up to the extreme event timing (r2 = 55.8, F = 19.9, p = 0.005). However, to meaningfully assess how such extreme events impact local biodiversity after the event, we calculated when and how long monitoring must continue to account for lag time and/or cumulative impact. The significant relation between species richness (loss) and iceberg scour measured the same year explained 37.8% of data variability (r2). Model fit was higher the year after, and highest (60.2%) 2 years after the scouring, before declining (though still significant up to 6 years after). On that basis, it seems likely that only in late 2027 may reassessment of local biodiversity reveal the maximal impact of this extreme physical event. The strong inverse relationship between seasonal sea ice duration and iceberg scouring of the shallow seabed (figure 1a and Smale et al. 2008) is driven by the frozen sea surface hindering iceberg movement, reduces scouring. Thus, there is a continuous link from climate warming through to disturbance of seabed biodiversity. However, monitoring is very challenging because of site remoteness and the need for long-term data. Extreme events can span wide spatial and temporal scales, as is perhaps most obviously embodied by mass extinctions and their causes. The societal inescapability of many extreme events, such as meteorite impacts, volcanism and tsunamis, has been an important driver of research on their past impact and potential for prediction (Piotrowski et al. 2017). However, even rare events can occur often enough that more than one can happen at similar times to become multipliers (Rabinovich et al. 2025). The frequency and severity of extreme events are increasingly receiving societal and scientific attention and concern because of hazards they present to human welfare globally (Abatzoglou et al. 2025). Without such a direct human health link, and although the polar regions constitute a fifth of Earth, we have been much less aware of, or monitoring for extreme events there. It has taken multidecade monitoring to detect the increasing frequency of Arctic extreme events (Aalto et al. 2026) and a decade of diving the shallow Antarctic seabed to reveal a first-seen mass recruitment of polar invertebrates (Dayton 1989). Understandably, focus on sea ice losses has been on the impact on pagophilic vertebrates that depend on its surface for pups and chicks (e.g., Fretwell et al. 2023). Sea-ice and seabed ice-scour only monitored at two Antarctic research stations; Argentina's Carlini and UK's Rothera along the West Antarctic Peninsula. From the decades of data recorded at Rothera reported here, it is clear that 2025 seasonal sea ice data and ice scour are not only each the most extreme ever measured, but also the most extreme deviation from the normal relationship between these variables. Sean McLoughlin: methodology, investigation, writing – review and editing, data curation. Ali Hammond: data curation, writing – review and editing, methodology, investigation. David K. A. Barnes: conceptualization, investigation, writing – original draft, methodology, writing – review and editing, data curation, supervision, formal analysis. The authors thank Rothera Research Station marine team past and present for support in collecting this time series. The authors declare no conflicts of interest. The data that supports the findings of this study are available in the Supporting Information of this article. Data S1: gcb70938-sup-0001-Supinfo.ods. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.