Ice fishing, transportation and skating - the frozen period of a year is crucial for the culture of millions of people in the world. Because of the immediate and direct response to global warming, lake ice has captured growing attention not only for those living in cold geographical regions but also for others whose life is not directly affected by lake ice. Despite this general interest, there is surprisingly little information available on the quality of lake ice. It has simply no tradition to go out sampling during cold and dark winter months, especially not if sampling is expected to be very time-consuming. So why should we care about the quality of lake ice?

Importance of lake ice quality. Of most interest is the formation and spatial distribution of white ice. Compared to black ice, white ice is considered being rather unstable with a low bearing capacity, making it rather risky to use the ice for transportation, recreation or other purposes. In addition to ice safety issues, white ice is also known to substantially reduce the amount of sunlight penetrating through ice. If less sunlight reaches the water below ice, the growth and reproduction of all organisms which are able to photosynthesize are endangered and the entire food web can become disturbed.

IceBlitz sampling campaign. Since white ice conditions play such a crucial role for ice safety and under-ice ecology we initiated an IceBlitz sampling campaign across the Northern Hemisphere within the Global Lake Ecological Observatory Network (https://gleon.org/) during the winter 2020/2021 to understand where, when and why white ice conditions occur. It turned out that the sampling winter was one of the warmest ones in the Northern Hemisphere since 1880, giving us a hint what kind of ice conditions we can expect when winters become warmer. Altogether we took 167 ice cores in ten different countries. The first samples were taken in December. During that month, we could hardly find any white ice on lakes. In January, we were a bit luckier - most lakes had a thin white ice layer which then gradually grew over the entire ice cover season with a maximum just before ice-off in March or April. During the time before ice-off, white ice was the dominant ice type for most lakes which we had sampled.

White ice formation. White ice is commonly formed when snow accumulates on ice, melts, and refreezes. White ice also forms when rain falls on the snow layer to form slush, which subsequently can freeze and turn into white ice or when the snow load is sufficient to force lake water to the ice surface through cracks in the ice matrix. Melting and refreezing of ice, slush and snow as well as rainfall are conditions that typically occur when air temperatures vary around the freezing point, i.e. usually during the time before ice-off. We attribute the ice layer composition change towards spring to changes in the seasonality of winter air temperatures. When winters become warmer, the seasonal cycle of air temperatures below the freezing point flattens, similar to the flattening that occurs towards warmer geographical regions. A decrease in the winter air temperature amplitude implies that the number of days with air temperatures varying around the freezing point increases, prolonging the formation of white ice and increasing its proportion. This lake ice quality response to warmer winter air temperatures is increasingly pronounced towards warmer geographical regions. Under continued global warming, lakes located in those warmer geographical regions are most likely increasingly exposed to air temperatures varying around the freezing point, resulting in an increased prevalence of white ice, in particular during the critical time before ice-off.

Lake ice quality and ice stability. At school we usually learn that it is okay to walk on ice if the ice thickness is 10 cm or more. We estimated that 10 cm of ice allows for a load of 1,753 kg under black ice conditions while it only allows for a load of 175 kg under white ice conditions. Thus, the variation in ice stability is very large, depending on ice quality. Since the bearing strength of white ice is so low, an increase in the proportion of white ice can jeopardize the use of seasonally ice-covered lakes for subsistence, recreation, transportation and other purposes. Under complete white ice conditions, groups of people are at high risk to fall through ice, despite an ice thickness of 10 cm. Fatal winter drownings do occur. While sampling, the newspapers were full of warnings, and this had a reason. In February 2021, ten people died by falling through ice which represented the highest winter drowning death rate in Sweden during the month of February since the records began in 2000. The ice situation in February 2021 demonstrates that behavioral adaptations to a warmer world are needed. Ice conditions that traditionally have been safe during winters of the past will become unsafe in the future. There is an urgent need to revise ice safety guidelines. To keep the update of the ice safety guidelines as simple as possible a rule of a thumb could be to double the presently used ice thickness guideline.

Light transmittance through white ice and ecological effects. White ice has a severalfold higher reflectance than black ice, thus only small amounts of light penetrate through white ice. Low light conditions in spring caused by a white ice layer and/or snow on ice are critical for the development of primary producers and consumers as their growth and reproduction is dependent on light. Although organisms have developed a variety of survival strategies, changes to the photoautotroph community by snow and white ice conditions cascade through the food web, with substantial consequences for microbial, zooplankton, and fish populations. 

Get prepared. Under expected future global warming, many lakes in the world will become exposed to prolonged freeze-thaw cycles, bringing more and more white ice conditions. It is time to wake up. Lake ice might no longer be safe to use for various activities. Ice safety guidelines need to be renewed and the thickness of white ice needs to be considered as an important regulator of physical, chemical, and biological processes in lakes.

A ten minute scientific presentation about this topic is available here.