A new study that examines human records hundreds of years old of the freeze dates of certain bodies of water confirm what scientists fear: the Earth is showing a very steep warming curve.

Researchers recently examined the oldest inland water ice records in history — records of freeze dates of a Japanese lake in the 1400s in Japan and ice breakup dates of a Finnish river in the late 1600s — and they suggest that ice cover formation is happening later and later ever since the Industrial Revolution, according to a University of Wisconsin-Madison statement.

An international team of scientists examined the data and published their findings in Nature Scientific Reports.

Lake Suwa in Japan was monitored for 240 years, betwen 1443 and 1683, by Shinto priests. The freeze date came later and later in the year as the years went on, but that trend accelerated very quickly, eventually pushing the ice coverage date back 4.6 days per decade. And the Torne River in Finland saw a similar trend.

“These data are unique,” John Magnuson, a researcher and emeritus professor at the University of Wisconsin-Madison’s Center for Limnology, said in the statement. “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 statement adds: “In recent years, she notes, both waters have exhibited more extreme ice dates corresponding with increased warming. For Lake Suwa, that means more years where full ice cover never occurs. Before the Industrial Revolution, Lake Suwa froze over 99 percent of the time, but beginning more recently, it does so only half the time. A similar trend is seen with extremely early ice breakup on the Torne. Extreme cases once occurred in early May or later 95 percent of the time, but they are now primarily in late April and early May.”