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

knowledge and map-making (i.e., Sikumik Qaujimajjuti), can mitigate human risk on ice [65] and even mitigate impacts to trail access under climate change [60], changes to lake ice phenol- ogy and ice quality have strong impacts on the individuals and communities who rely on ice as a resource for transportation and food access. The loss of safe lake ice conditions is a distinct reality for regions of the Northern Hemi- sphere. Our results indicate that lake ice will no longer be safe in more southern latitudes where lake ice may no longer grow thick enough for safe use in a warming world, especially when lake ice quality degrades and requires a thicker ice column to support human weight [5]. Lake ice is likely to become less consistent with warming air temperatures throughout the 21st Century, leading to intermittent ice cover in southern latitudes [66]. Moreover, as many as 5,700 lakes may also permanently lose ice cover with warming throughout the century [67]. Research using multiple model means, including the Community Land Model 4.5 (CLIM4.5), projects that lake ice thickness will decline by 3.3 cm per degree Celsius increase in air temper- ature [1]. An average loss of approximately 13.2 cm of ice thickness will make lakes at southern latitudes and coastal areas more likely to become unsafe, as their ice thickness is generally lower than northern latitudes [4]. These regions, therefore, should be particularly sensitive to the effects that changing ice thickness and quality have on human safety as the world warms. 5. Conclusion

The phenology of lake ice describes its first appearance and subsequent breakup; however, ice phenology does not adequately describe the use of lake ice. Where phenology in its broadest definition (i.e., ice on date and ice off date) is useful to describe the response of lakes to climate warming [40], the transition periods that we highlight in this study emphasize the role that timing has on the ecosystem services that communities derive from ice presence on lakes. The conjunction of warming and changing ice quality toward less stable white ice might lead to prolonged unsafe periods of 19.8±8.8 and 8.7±6.6 days for the ice formation and melt periods, respectively. Rapidly degrading ice conditions call for heightened vigilance during the ice breakup period [68]. A greater ratio of white ice earlier in the season will lead to unsafe ice conditions during periods when safe ice is anticipated by communities, given prior experience with safe ice timing [5]. This asynchronous relationship between actual ice safety and pre- sumed ice safety from communities might lead to more frequent ice-related drownings [15]. Where ice presence is evident from a visual inspection, ice quality may require further scrutiny by drilling into the ice. Adaptation, then, is key in the face of rapidly shifting ice conditions [60], and improved safety protocols must be included for winter ice use during recreation and ice use broadly [65] to prevent winter drownings. Supporting information S1 Fig. Observed and modeled maximum ice thickness. A comparison of modeled and

observed data for the maximum ice thickness for each of the 71 validation lakes. Each point represents an individual lake. The line is the regression line with the R2 and line equation in the upper left of the plot. (TIF) S2 Fig. Lake ice variability from climate forces. The anomaly of ice thickness comapred to a historical condition (1851–1880) (colored points) when considering the forcing of air temper- ature anomalies (y-axis) and snow depth anomalies (x-axis). Each point represents the mean of an individual grid cell at 4˚C warming. (TIF) PLOS ONE Changing ice conditions inhibit safe use PLOS ONE | https://doi.org/10.1371/journal.pone.0313994 December 11, 2024 14 / 19