Two decades ago, researchers at Suwa started noticing that dissolved oxygen levels five meters below the lake’s surface were frequently dropping well below three milligrams per litre in the summer, an environment uninhabitable for most fish, just as Fujimori had long predicted.

After a decades-long effort by the prefecture to divert wastewater and remove pesky weeds and algae, Suwa’s waters are noticeably cleaner now. But the natural circulation of the lake has also been disrupted by rising temperatures and shorter winters. Warm water is less dense and naturally sits above colder water, which sinks to the bottom of the lake. In the past, when summers were less hot and winters reliably cold, water at the lake’s surface and its depths would settle at similar temperatures, aiding circulation. This would ensure that oxygen would mingle and saturate the entire lake.

“This isn’t a place where you can play,” he says, explaining that even in summer, locals avoid swimming in the water because it was contaminated for decades by wastewater and later overrun by algae and weeds. “The lake is more of a place you look at from a distance.”

The only time his team avoids the lake is in the midst of winter. One of the first stories Miyabara heard when he arrived in Suwa was how a promising young researcher had died after falling through the ice decades earlier.

Less than a kilometer away, Yuichi Miyabara sits in his concrete office building overlooking the lake. The Shinshu University professor arrived here in 2001 to study Lake Suwa and the disruptions to its natural rhythms. His team takes regular samples from the lake and analyzes them to closely track fluctuations in the water’s temperature and oxygen levels.

Recently, the prefectural government has tested a project that funnels “nano-bubbles” of compressed oxygen into the lake through a plastic hose. Similar tests in the past have been unsuccessful.

An official in the prefectural division in charge of the project said it is still awaiting results from the August study, but conceded that continuing the project would require a “considerable” budget and had to be weighed carefully.

The contraption is a neat idea, Miyabara says, but it’s hard to imagine how many machines it would take to pump enough oxygen into the lake.

“We’re not talking about a small body of water. It’s not like we can artificially mix the entire lake,” he says.

More than 20 years ago, John Magnuson, a longtime researcher of inland waters at the University of Wisconsin-Madison, was scouring the world for climate observations taken before the 1840s when he remembered Suwa. Magnuson flew to meet Miyasaka, the Shinto priest, and worked with a local researcher to trawl through the historic lake-ice data.

Magnuson found that since the advent of industrialization, ice began to freeze later in winter at Lake Suwa. In a 2016 paper published in Nature, Magnuson and his colleagues wrote that extreme warm weather had become more common in Suwa and attributed such changes to the increase in atmospheric carbon dioxide that has led to a rapid rise in local temperatures.

Annual air temperatures in Suwa have warmed at a rate of 2.4 degrees Celsius over the past century, double the national figure, according to the Japan Meteorological Agency.

Between 1950 and 2014, Lake Suwa failed to freeze 17 times. In comparison, between 1443 to 1700, there were only three instances in which the lake didn’t freeze over completely. And when it comes to the omiwatari, the absences have grown more common: In the 1990s, it once disappeared for six consecutive years.

“There is something different about a human being looking at the lake, saying it iced over or broke up, that resonates more than complicated paleoclimate research,” says Magnuson, mentioning data taken from ice cores and tree rings that scientists can use to understand climate conditions from millions of years ago.

Human-made data has obvious shortfalls, Magnuson says, with sometimes damaged or partial recordings making it hard for researchers to draw large conclusions. “But it’s something people can relate to more easily, it’s something that makes more sense to them,” he says.

Scientists have since discovered even older data, like those taken since the 9th century at the Bodensee, a lake that straddles the Swiss, German and Austrian border. Churches on opposite banks of the lake used to carry a bust of St. John the Evangelist across the ice every winter.

But the Bodensee, also known as Lake Constance, stopped freezing in 1963, interrupting a centuries-old tradition. The religious relic is now permanently stored on the Swiss side of the water.

Magnuson predicts a similar fate for the Suwa omiwatari ritual. “The future generation may not see ice on Suwa,” he says.

Pausing on the phone, Magnuson asks after Miyasaka, and wonders if he has responded to a question about how he feels about the disappearance of the ice.

“I am curious how he feels about it, about the fact that he may be among the last to see the crossing,” he says.