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North American lakes are freezing later, thawing earlier

For centuries, the beautiful lakes of Madison, Wisconsin (especially Lake Monona and Lake Mendota) have been important winter attractions for a large number of locals and tourists interested in outdoor pursuits such as ice fishing, ice skating, ice boating, or simply strolling along the lakes’ frozen surfaces. However, while it was once common for the lakes to freeze in mid-December and hold ice until April, climate change is recently driving later freeze dates and earlier thaws across the Northern Hemisphere.

Scientists have been keeping track of these lakes’ freezing and thawing dates for over 170 years – long enough to see trends in “lake ice seasons.” The data shows that, over this period, the number of days the lakes spend covered in ice each winter has shrunk by a month.

According to the Wisconsin State Climatology Office’s Madison Lakes Ice Summary, this year both Lake Monona and Lake Mendota froze only in January. While these dates (January 3rd and January 7th) don’t rank in the top ten latest freeze dates on record, they are more than two weeks beyond the median freeze date of December 20th. 

Since record keeping was initiated in 1852, the median freeze date of the Madison lakes is five days before Christmas. However, while in the first several decades, the majority of freeze dates were earlier than December 20th, the last few decades reveal a highly different picture of what a “normal” ice season looks like.

In the last thirty years, half of Lake Mendota’s freeze dates have been in January, and over the last decade, eight of them have been after the December 20th median, and seven of those were in January. Considering the fact that, before 2010, there was no single decade in the lake ice dataset when a majority of freeze dates were in January, the current situation is worrisome. 

As larger studies suggest, climate change is driving loss of lake ice all around the Northern Hemisphere, with 14,800 lakes currently experiencing intermittent winter ice cover. 

Scientists estimate that, within the next generation, an even more extensive loss of lake ice will occur, a phenomenon which highlights the importance of climate change mitigation strategies in order to preserve ecosystem structure and function, as well as local winter cultural heritage and traditions, such as those associated with the Madison Lakes for over two centuries.

By Andrei Ionescu, Staff Writer

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