Towards critical white ice conditions in lakes under global warming

Many studies report on a shrinking cryosphere with a rapid lake ice cover loss in a warmer world, but very little information is available on ice quality changes. Here we show for the first time spatial and seasonal variation in the thickness and proportion of white ice on lakes across the Northern Hemisphere during one of the warmest winters since 1880. Based on 167 field observations during 2020/2021, we found white ice conditions in 77 % of the sampling occasions with a white ice layer varying between 0 and 44 cm, contributing between 0 and 100 % to the total ice thickness. We noted that white ice built up over the winter season, being thickest and taking up the largest proportion towards the end of the ice cover season when fatal winter drownings occur most often and light limits the growth and reproduction of primary producers. We attribute the dominance of white ice before ice-off to air temperatures varying around the freezing point, causing the upper ice layer and snow on ice to melt and refreeze to a white ice layer. During warmer winters, the seasonal cycle of freezing air temperatures flattens, resulting in an increased number of days when air temperatures vary around the freezing point. Thus, under continued global warming, the prevalence of white ice is likely to increase during the critical period before ice-off, for which we adjusted commonly used equations for ice safety and light transmittance through ice