Researchers who studied hundreds of years of data on two bodies of water have found that since the beginning of the Industrial Revolution there's been a trend toward an earlier spring thaw and later ice-cover formation.
This is significant, as ice seasonality is strongly tied to climate.
The world's oldest inland water ice records come from sources on two different continents. The researchers from York University and the University of Wisconsin analysed data dating back to 1442 on the freeze dates of Lake Suwa, Japan, recorded by Shinto priests for religious reasons, as well as data on the ice breakup dates of Finland's Torne River, beginning in 1693 and recorded by merchants who used the waters for trade and transportation.
From 1443 to 1683, Lake Suwa froze 0.19 days later in the year each decade. However, between 1923 and 2014, the freeze date moved back at a rate of 4.6 days per decade — a 24-fold increase. The breakup date of the Torne River has been moving earlier each year, with the rate of the trend toward earlier thaw dates between 1867 and 2013 double the pre-1867 rate.
More extremes in ice patterns
Both bodies of water have experienced more extremes in ice patterns at an increased rate as time goes on. Lake Suwa failed to freeze fully only three times between 1443 and 1700. Between 1950 and 2004, there were 12 years where it didn't freeze. From 2005 to 2014 alone, it didn't freeze five times.
Similarly, the Torne River experienced extremely warm years (defined as years where the ice breaks up before early May) only 10 times between 1693 and 1899, compared to nine times between 2000 and 2013.
The study's authors cite increasing air temperatures and soaring atmospheric carbon dioxide concentrations as explanatory factors, finding a strong correlation between these variables and the ice seasonality of both bodies of water since the Industrial Revolution.
This is the first climate change study to look at such ancient human-recorded data.
"These data are unique," York University biologist and study author Sapna Sharma said in a press release. "They were collected by humans viewing and recording the ice event year after year for centuries, well before climate change was even a topic of discussion."
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