Widespread loss of safe lake ice access in response to a warming climate

shifting mean ice timings, lake morphology, local and regional climate, and climate oscilla- tions. Furthermore, this increased variability leads to increasingly unpredictable ice safety as implied by the increasing standard deviations that occur with warming temperatures and degrading ice quality. 4.3 Consequences of increasingly unsafe ice Lengthening transition periods of unsafe ice highlight the need to update and evolve safety protocols surrounding individual and community use of lake ice [50,53]. Ice conditions are shifting rapidly and ice loss has accelerated in recent decades [3]. These changes to ice condi- tions, including the phenology of lake ice, have contributed to dangerous conditions of ice during periods of human use [15]. The rapid decay of safe ice conditions suggests that the end of the ice season will offer the most dangerous conditions, which has been observed in an anal- ysis of winter drowning data from the Northern Hemisphere [15]. Our results support this analysis, showing that while the ice formation and breakup periods occur later and earlier, respectively, the warming winter and spring seasons [54] prolong the amount of time between unsafe and safe ice thicknesses, despite the continued presence of ice. Climate change not only shortens the ice season, it degrades the ice such that it is not as safe for human use. As an example, the Rideau Canal, which flows through Ottawa, Canada, is known to open annually for ice skating throughout the winter and is a site of historical and

socio-ecological interest as the world’s largest outdoor skating rink and a UNESCO World Heritage Site [55]. In the winter of 2023–2024, one of the warmest winters in recent history, the Canal opened for approximately one week before shutting down, owing to degraded and weak ice conditions, although the canal remained frozen. The weather did not reach the conse- cutive 10–14 days of -20 to -10˚C temperatures, which are typical of January in Ottawa [56]. The weather tended to vacillate around 0˚C alongside rain that made ice conditions unstable and unable to consistently reach the 30 cm required for safe operation, according to the National Capital Commission [56]. In the previous winter (2022–2023), the Rideau Canal did not open at all for the first time since the 1970–1971 winter. These adverse weather conditions have been increasing in frequency, which has shortened the duration of operation of the skate- way by -5.2 ± 2.9 days between 1972 and 2013 [57], owing to warming weather and climate oscillations [58]. Though seasons can vary strongly (35–90 days), the period of skating days (seasonal length minus in-season closures) is projected to decline to 28±13.4 days by 2090 [57]. Other projections indicate shorter outdoor skating periods of 34% by 2090 in the Toronto area [59]. Beyond recreation, lake ice in Arctic regions is a necessity for mobility. Travel for Arctic communities involves navigating hundreds of kilometers of semi-permanent “trails” located on land, water bodies, lake ice, and sea ice [60]. Delayed ice formation or insufficient ice thick-

ness or quality interrupts access to these trails. Trails are often used to access traditional foods, as hunting is seasonal, owing to migration, breeding, and feeding patterns [61]. For example, hunters may need to cross frozen lakes to access game; however, the timing of safe ice travel may conflict with seasonal hunting patterns. If alternate trails cannot be found (i.e., “trail switching”) [60], communities may then have to cope with the shorter period or lack of hunt- ing by increasing the ice fishing season, another activity that requires sufficient ice thickness and poses the risk of cold water submersion [62,63]. At the beginning of the season, the delay of sufficient ice thickness to use ice for fishing or hunting may require communities to increase fall fishing and hunting activities to cope with the inaccessibility of traditional winter hunting foods or rely on store-bought goods, though the window for those transportation periods is simultaneously shrinking [21,64]. While safe practices, such as Indigenous ice safety PLOS ONE Changing ice conditions inhibit safe use PLOS ONE | https://doi.org/10.1371/journal.pone.0313994 December 11, 2024 13 / 19