White ice is formed by the snow that has fallen on the ice surface, got soaked with water and then frozen. This happens, e.g., when thawing and freezing days alternate, when the snow first melts and then freezes. On rainy days in winter, snow imbibes water, and when it gets colder, the slush freezes up and turns into white ice. Sometimes there is so much snow on lakes that the pressure it produces causes water to squeeze out to the ice surface through cracks and soak the bottom layer of snow. During the subsequent cooling, this wet snow freezes up. Thus, late in winter, white ice on lakes is usually multi-layered. Its layers may be interbedded with layers of wet snow or water.
Scientists extracting an ice sample
– During lake surveys, we use an ice drill and a saw to extract a sample - a block of ice. Previously, it used to be a monolithic chunk mostly composed of clear ice, whereas nowadays we often fail to pull out an integral sample. On warm winters ice on Karelian lakes is now predominantly white, and we pull it out of the hole piece by piece, because it is like a "layered pancake" - several layers of white ice separated by water or wet snow, - said Galina Zdorovennova, Head of Hydrophysics Laboratory at the Northern Water Problems Institute KarRC RAS.
The duration of the ice-covered period, the thickness and structure of ice have been affected by climate change. Observational data from weather stations in southern Karelia for the period 1950-2022 indicate a steady warming of the regional climate in all seasons of the year. Winters have become milder: the number of frosty days has decreased, while thaws, on the contrary, have become more frequent. Each event of air temperature rising above zero and liquid or mixed precipitation tells on ice thickness and structure. For example, total ice thickness on Lake Vendurskoye was 60-85 cm in April 1994-2004, 50-65 cm in 2005-2018, and 40 cm at the end of March 2020. The proportion of white ice has been increasing in recent years and often exceeds 50%.
Changes in ice structure on Lake Vendyurskoye: ice samples taken in spring 2006–2007 and in 2022
In 2020, Karelian specialists joined the international group of limnologists who organized the Global Lake Ecological Observatory Network (GLEON). As part of the IceBlitz campaign, GLEON participants measured ice characteristics on 31 lakes in ten countries of the Northern Hemisphere from December 2020 to April 2021. The sample set included Russian lakes: Mozhaysk storage reservoir, seven lakes of the Kola Peninsula, and Karelian lakes Krosnozero, Vedlozero and Vendyurskoye. The results of this large-scale effort were published in Nature Communications.
Data from all countries participating in the campaign show: during the abnormally warm winter of 2020-2021, white ice was the predominant type on all lakes and its share steadily increased from the beginning to the end of the ice-covered period.
– If the trends of air temperature rise and changes in the precipitation regime persist, lake ice cover will be increasingly affected by alternating freeze-thaw cycles during the winter. As the total ice thickness decreases, there will also be changes in its structure - the proportion of white ice, which is less strong than crystalline ice, will increase. During warm winters, the load-bearing capacity of ice and its strength will decline, - the scientist remarked.
Senior Researcher at the Hydrophysics Laboratory NWPI KarRC RAS Roman Zdorovennov and Head of the Geography and Hydrology Laboratory NWPI KarRC RAS Alexey Tolstikov
This result is important not only for basic science, but it also directly concerns the safety and lives of people. Fishing, transportation, sports, recreation, and cultural traditions of millions of northerners around the world are tied to the seasonal freezing of lakes. Thus, just a couple of decades ago news of fatal accidents on ice were typical only for the end of the ice season, near the time of active thawing, while midwinter ice on lakes was considered safe. In Sweden, e.g., moving over ice in February used to be quite safe for people. In February 2021, however, ten people died after falling through the ice on Swedish lakes. This is the highest ever rate of winter drowning deaths in the country during this period.
According to scientists, the ice situation has been getting riskier in recent years because of the warm winters, signaling the need for people to adapt to the new reality. The traditionally safe conditions of past winters are no longer the same. Considering that not only the thickness, but also the structure of ice change noticeably, the authors of the article suggest revising the existing ice safety rules for the population.
– It is especially important to educate children, who don't always evaluate ice safety adequately due to lack of life experience, as well as the elderly, recreational fishermen, and skiers. They, on the contrary, are often victims of their rich life experience and their old notions of "winter" ice safety, - Galina Zdorovennova summarized.
