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

Shapiro-Wilk test, Kruskal-Wallis test, and the Kolmogorov-Smirnov test were conducted in the ‘stats’ package in R [37]. Dunn’s test used the ‘FSA’ package in R [36,38]. 3. Results 3.1 Transition period timing Ice formation at a thickness of 2 cm occurred on day 313 (i.e., November 9th) on average dur- ing the historical reference period (1851–1880) across the Northern Hemisphere. Ice breakup occurred on approximately day 121 (i.e., May 1st). After ice formation, ice growth continued and reached a safe thickness for general ice safety (i.e., walking on ice) approximately 6 days later, during the historical reference period. This duration between the average formation of ice across the Northern Hemisphere and the increase of ice thickness to a stable ice cover for use is the transition period between unsafe and safe ice (Fig 2). This 6-day transition period stipulated that the ice column is 100% black ice, which supports more weight with a smaller thickness than does white ice [5]. During the same period, it took 19 days from ice formation (� 2 cm) to a 20 cm ice thickness, which is a required thickness for general ice use under 100% white ice conditions. Ice breakup occurred over a more compressed period, where lake ice diminished from 20 cm (i.e., 100% white ice conditions) to a complete ice breakup (< 2 cm) over only 4 days. 3.2 Quantifying increasing transition periods Both anticipated warming air temperatures and ice quality degradation interacted to prolong the duration of the transition period from unsafe to safe ice during the ice formation period

(Fig 2). The transition period during ice formation increased in duration by 1.1±2.3 (SD, n = 2829), 1.5±2.7 (SD, n = 2762), and 2.6±3.7 (SD, n = 2618) days when air temperatures warmed 1˚C, 2˚C, and 4˚C, respectively (Table 1). These increases imply that lake ice takes longer to increase in thickness from initial ice formation (� 2 cm), which is unsafe to walk on, to a safe thickness (�10 cm, �15 cm, and �20 cm, depending on assumed ice quality). Simul- taneously, lake ice formed later. The duration of the transition period increased further when considering ice quality (Fig 3). The transition period increased by 8.0±4.9 (SD, n = 2743), 8.7 ±5.7 (SD, n = 2674), and 10.7±6.3 (SD, n = 2512) days when the ice column changes to 50% white ice and air temperatures warm by an additional 1˚C, 2˚C, and 4˚C, respectively, com- pared to historical black ice conditions (Table 1). Finally, when the lake ice column was entirely composed of white ice, the transition period duration increased by 15.3±8.1 (SD, n = 2643), 16.6±8.6 (SD, n = 2580), and 19.8±8.8 (SD, n = 2379) days when air temperatures warmed 1˚C, 2˚C, and 4˚C, respectively, compared to historical black ice conditions (Table 1). The increasing duration of the transition period between unsafe and safe ice during the ice for- mation process implies that lake ice, on a Northern Hemisphere average, can be delayed from November 15th to between December 13th and January 18th. This date range incorporates both the average later ice formation and the average transition period with their relative stan-

dard deviations. This date range, then, incorporates the minimum and maximum projected change in the timing of safe ice thickness. The transition period followed a similar, though muted, pattern during the melt period when incorporating the compound effects of increasing air temperatures and ice quality. The unsafe transition period during the lake ice melt occurs when lake ice thins to �10 cm (black ice), �15 cm (50% black ice and 50% white ice), and �20 cm (white ice) until ice breakup (�2 cm) (Fig 2). This transition period extended under warming of 1˚C, 2˚C, and 4˚C by 0.81±2.4 (SD, n = 2829), 1.4±2.6 (SD, n = 2762), and 2.4±3.6 (SD, n = 2618) days. When combined with 50% white ice, the transition period increased to 3.9±3.5 (SD, PLOS ONE Changing ice conditions inhibit safe use PLOS ONE | https://doi.org/10.1371/journal.pone.0313994 December 11, 2024 6 / 19