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

that used CESM2-LE lake ice thickness data in other capacities [16,27] and those that used other methods to describe patterns in lake ice [1,43]. Areas that experience longer transition periods are in the minority; however, these regions can experiencing cooling temperatures [6]. Finally, changes in transition variability will impact the predictability of safe ice conditions. The increasing standard deviation in the transition period when temperatures increase and ice quality degrades indicates that lakes may have increasingly unpredictable safety conditions during ice cover periods as durations decrease and ice quality degrades [5,44]. Lake ice is typi- cally safe in temperate Northern Hemisphere lakes in February when many people use lakes for recreation; however, accidents have occurred as lake ice becomes less predictable [5,15,40]. Lakes are projected to enter no-analogue scenarios as early as 2050 where average metrics such as lake temperature exceed current variability [27]. Regions in southern latitudes of North American, Europe and Asia have experienced intermittent ice cover between 2000–2009 and higher latitudes are expected to experience intermittent ice cover between 2090–2099 [16]. Additionally, lake ice duration has become more variable, especially when ice duration has sig- nificantly decreased [44]. 4.2 Ice quality conditions predominantly dictate safe ice conditions The transition period during which ice is present on a lake’s surface, but unsafe for use, increases due to warming air temperatures; however, our findings show that changes to ice

quality dramatically increase the length of the unsafe transition period when compared to an assumed black ice historical condition. Rising winter and spring air temperatures [39,41] increase proportions of white ice that compose the ice column [5], enhancing the variability of transition periods. For example, the average transition period of unsafe ice increases by 5.8 and 11 times longer because of ice quality degradation from 100% black ice to 50% white ice and 100% white ice, respectively, compared to 1.7 times longer for black ice conditions between 2˚C and 4˚C warming. Our results, then, suggest that ice quality is the primary con- cern for lake ice safety in the future. Warming air temperatures are the root cause of lake ice quality degradation [5]; however, the diminished density [10] and reduced flexural strength [9,10,45] that result from increasing proportions of white ice will lead to the most dangerous conditions when ice is present but unsafe, as noted in [5]. Increasingly degraded ice quality results in significantly longer mean transition period val- ues and right-skewed transition period distributions, implying much longer maximum transi- tion periods when ice quality degrades to greater proportions of white ice (Fig 4). We found that the distributions of transition periods across the Northern Hemisphere become increas- ingly skewed when accounting for the interdependence of warming and ice quality. While the Northern Hemisphere as a whole will tend toward longer average transition periods (see

Table 1), lakes may have extremely long transition periods (i.e., longer than 1 month), shorten- ing the viable ice cover period for activities more than the predicted ice duration loss of 38 ± 11 days [16], which used CESM2-LE ice thickness data. The additional loss of usable ice cover that the unsafe transition periods represent demonstrates that ice phenology metrics do not fully capture the overall loss of ice cover. While the presence of ice is important for ecosys- tems and biogeochemical processes [46–49], ice cover must be of sufficient thickness to be used for recreation and subsistence [15,21,50]. Climate warming is already enhancing variation in phenological events [51] and increasing the frequency of extreme events through significant shifts in mean ice timings [52], particu- larly in large lakes and warmer regions [3,52]. Lake ice cover tends to have increased variability in conjunction with climate oscillations, such as the North Atlantic Oscillation and the El Niño Southern Oscillation [44]. Therefore, longer transition periods may also respond to PLOS ONE Changing ice conditions inhibit safe use PLOS ONE | https://doi.org/10.1371/journal.pone.0313994 December 11, 2024 12 / 19