Records have been kept for many decades of the time that northern lakes become iced over and the time when these lakes thaw out after winter. This information is important for winter transportation (e.g., ice roads), fishing, recreation, economic and cultural purposes in the regions where these lakes occur.
In addition, the phenology (seasonal cycles) of lake ice is an excellent indicator of climatic variation, with the formation and loss of ice being particularly sensitive to changes in air temperature.
Recently, a team of scientists updated the ice phenology records for 60 lakes in the northern hemisphere that already have long-term data series covering between 107 and 204 years. The experts re-assessed northern hemisphere ice trends for the first time since 2004, and added 15 years of additional data.
The lakes studied included 40 that are in North America (including Lakes Michigan and Superior, Detroit Lake in Minnesota, Lakes Monona and Mendota in Wisconsin, Cazenovia and Oneida lakes in New York, and several in Ontario – Lake Simcoe, Lake Nipissing and Lake of Bays), 18 in Europe, and two in Asia – Lake Baikal in Siberia, Russia, and Lake Suwa in Japan.
The researchers found that, on average, the lakes are freezing 11 days later and thawing 6.8 days earlier, meaning that ice duration is 17.0 days shorter per century over the entire record for each lake. They found that factors including late ice-on, early ice-off, shorter periods of ice cover, no ice at all, warmer winter temperatures and more extreme weather events have contributed to the increasing rate of ice loss, especially in larger lakes and in southern and coastal regions.
Sapna Sharma is an associate professor in the Faculty of Science at York University. He led the study with Professor David Richardson at the State University of New York and climate scientist Dr. Iestyn Woolway of the European Space Agency Climate Office.
“We found that lakes are losing on average 17 days of ice cover per century. Alarmingly, what we found is that warming in the past 25 years, from 1992 to 2016, was six times faster than any other period in the last 100 years,” said Professor Sharma.
“In addition, we found that the duration of winter ice cover has decreased, particularly since 1995, to the point where some lakes are beginning to have more winters with minimal or even no ice cover. For example, some deep lakes in Switzerland and Germany, which historically used to freeze each winter, have permanently lost their ice cover in the past few decades,” said Professor Richardson.
For example, Lake Superior, the most northern of the Great Lakes which straddles the Canada/U.S. border, is one of the fastest warming lakes, losing more than two months of ice cover since ice conditions were first recorded in 1857. In Lake Suwa, Japan, ice formed close to 26 days later per century since 1897 and is now only freezing twice every decade, while Grand Traverse Bay in Lake Michigan had one of the fastest ice-off trends, melting about 16 days earlier per century.
“The findings were in line with our expectations as air temperature has increased over recent decades,” said Dr. Woolway. “Air temperature is one of the most important climatic drivers of lake ice dynamics, owing to its additive effects on various components of the lake energy budget.”
The study authors stress that a reduction in greenhouse gas emissions is a matter of priority if increases in air temperature and decreases in lake ice cover are to be mitigated. This would help to limit ecological, cultural, and socioeconomic consequences, including increased evaporation rates, warmer water temperatures, degraded water quality, and the formation of toxic algal blooms.
“Many of our lakes may be approaching a tipping point to ice-free conditions which will have vast cultural and ecological implications,” said Professor Sharma.
The research is published in the Journal of Geophysical Research: Biogeosciences.