Towards critical white ice conditions in lakes under global warming

The quality of lake ice is of uppermost importance for ice safety and under-ice ecology, but its temporal and spatial variability is largely unknown. Here we conducted a coordinated lake ice quality sampling campaign across the Northern Hemisphere during one of the warmest winters since 1880 and show that lake ice during 2020/2021 commonly consisted of unstable white ice, at times contributing up to 100% to the total ice thickness. We observed that white ice increased over the winter season, becoming thickest and constituting the largest proportion of the ice layer 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, a condition which occurs more frequently during warmer winters. Thus, under continued global warming, the prevalence of white ice is likely to substantially increase during the critical period before ice-off, for which we adjusted commonly used equations for human ice safety and light transmittance through ice. Under continued global warming, lakes will increasingly be covered by white ice, in particular 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.

Automated summary

The quality and characteristics of lake ice play a crucial role in ice safety and the ecology beneath the ice. This study conducted a sampling campaign across the Northern Hemisphere during a warm winter and found that the lake ice commonly consists of unstable white ice, which increases in thickness towards the end of the ice cover season. The dominance of white ice is attributed to air temperatures fluctuating around the freezing point, which occurs more frequently during warmer winters. Under continued global warming, lakes will be increasingly covered by white ice, affecting ice safety and the growth of primary producers.