It is not only the thickness of the ice that determines whether it is safe for ice skating or winter fishing. Ice made of frozen slush or snow is much weaker than core ice, and researchers from the Universities of Gothenburg and Uppsala have observed that this type of fragile ice has become more common on Swedish lakes over the past 50 years.
Scandinavian lake ice has become both thinner and structurally weaker in recent years. Results from 21 Swedish lakes show that the thickness of the ice has decreased by an average of 0.22 centimetres per year over the past five decades. But the quality and thus the load-bearing capacity of the ice cover has also deteriorated. This is the result of a new study published in the scientific journal Ambio.
"The structure of lake ice is changing rapidly in a warmer world. By 2023, nine of the lakes surveyed had become unsafe for ice skaters, winter fishermen and others who depend on frozen lakes. This is mainly because the air temperature changes more during the winter than before," says Mark Peternell, Senior Lecturer in Structural Geology at the University of Gothenburg.
Refrozen ice is weaker
Long sustained periods of sub-zero air temperatures are occurring more seldom, and this is true in large parts of Sweden. Climate change has contributed to more frequent freeze-thaw cycles. The study showed that there is a strong correlation between the number of days with air temperatures above freezing and the decrease in the thickness of core ice, the strongest and safest type of ice that forms when lake water freezes into ice during sustained cold.
Warm days mean that the formation of core ice is disrupted and weaker white ice, containing more air bubbles and pollutants, becomes more common. Snowfall further exacerbates the problem by insulating the ice, and also forming slush, which later refreezes into white ice. This change results in ice that is significantly weaker and more prone to breakage when loaded. This is true even if it is quite thick.
Ice cover varies across a lake
"It is not enough to simply measure the thickness. For anyone travelling on lake ice, it is crucial to assess the quality of the ice. Core ice is strong and dark, while white ice is cloudy and much weaker. In addition, the ice is not the same over an entire lake, so you need to examine the thickness and quality of the ice again and again," says Mark Peternell.
A rule of thumb from the Ice Safety Council and the Swedish Snowmobile Owners' Association is that safe core ice for walking or skating should be at least 10 cm thick, and at least 15 cm for snowmobiles. Before heading out on the ice, it's important to check the latest temperature trends, as warm days can quickly weaken the ice. Cracking sounds or visible slush indicate instability. In addition, it is important to have ice prods and not to be alone on the ice.
"To avoid an increase in fatal drownings in winter, researchers recommend that organisations such as the Ice Safety Council regularly monitor ice structure, develop better warning systems, review ice safety guidelines and prepare society for major changes in ice conditions."
Scientific article in Ambio: Rapid lake ice structure changes across Swedish lakes puts public ice safety at risk
