Japan's 600-year-old record on global warming

You used to be able to drive a tank on lake Suwa.

Now winters in this part of the Japanese Alps are rarely cold enough for it to freeze over.

Old photographs from the 1920's - show Japanese military aircraft parked on the frozen lake.

Through the 50s there's photos of people figure skating.

And - the most sacred of all - are documents of the Omiwatari.

Giant sheets of ice cracked and buckled over each other into a mini mountain range.

But with air temperatures raising over 2 degrees Celsius every year in this region, this unique weather phenomenon is becoming a rare sight.

For priests at the local Yatsurugi Shrine, it's a huge loss to the area's spirit.

(SOUNDBITE) (Japanese) PRIEST, KIYOSHI MIYASAKA, SAYING:

"With the ice thinning and not freezing over, fishermen can't make a living. And people avoid going on the lake since it's dangerous, which has weakened the connection, the relationship between the people and the lake."

The ice ridge is known as the crossing of the gods.

And the priests have been recording its sighting for nearly 600 years.

That makes it one of the world's oldest measurements of climate change.

(SOUNDBITE) (Japanese) PRIEST AT TENAGA SHRINE, KIYOSHI MIYASAKA, SAYING:

"If you look at old records, the hundred years in the 1600s, […] there was only one year where the lake didn't freeze over and create the omiwatari - that was once in 100 years. […] But from 1965 onwards…maybe it's an effect of global warming, the omiwatari has disappeared and the lake doesn't even freeze over."

The records represent a stark warning.

And locals here worry rising global temperatures will confine the omiwatari to history books.

Along with a once thriving life on the lake.