Almost every winter, after Lake Suwa in the Japanese Alps freezes, the male Shinto god Takeminakata crosses the ice to visit the female god Yasakatome at her shrine, causing a ridge known as the omiwatari to form. At least, that’s what the priests living on the shores of the lake believed. When the water froze, they would conduct a purification ritual and celebration in honor of the ridge, using its direction and starting location to forecast the harvest and rainfall for the coming year.
The priests kept records of the event beginning in 1443, inadvertently creating a massive data set of climate conditions. Now Sapna Sharma, a York University biologist, and John J. Magnuson, a University of Wisconsin limnologist, paired that data with records of the spring ice breakup of Finland's Torne River to understand the effects of climate change on inland waters.
“These data are unique,” Sharma says 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.”
The study, published today in Scientific Reports, shows that the annual freeze date of Lake Suwa changed very slowly—roughly 0.19 days earlier per decade. But once the Industrial Revolution started, the change of freeze date began drastically jumping and shifted about 4.6 days per decade.
Before the industrial revolution began in the late 18th century, the lake froze 99 percent of the time—only failing to freeze three times in the three centuries before 1800. Now, Lake Suwa completely freezes only half the time. In the last decade, the lake failed to freeze five times, writes Lisa Borre at National Geographic
In 1693, Finnish merchant Olof Ahlbom began keeping records of the date and time the ice broke up on the Torne River, part of the border between Sweden and Finland that flows from the Arctic to the Baltic Sea. A war interrupted his record keeping between 1715 and 1721, but otherwise the record has been maintained by observers ever since.
The Torne data is similar to that from Lake Suwa. While between 1693 and 1799 there were only four extremely warm years, causing the ice to break up in April, in the past decade there have been five. “Even though the two waters are half a world apart and differ greatly from one another, the general patterns of ice seasonality are similar for both systems,” Magnuson says in the press release
According to Borre, the findings fit in with other studies that have identified changes in climate cycles like the North Atlantic Oscillation and El Niño Southern Oscillation, which impact ice cover on lakes and rivers around the world. But the biggest find of the study is that there could be more high-quality data about climate change in unusual records.
“What is so interesting to me is that we were able to include in our analysis the longest ice records in the world based on direct human observation,” Magnuson tells Borre. “To have such long records from two very different freshwater systems in different parts of the world show the same general pattern is one of our key findings…and are entirely consistent with other research that shows a pattern of change after the Industrial Revolution.”