I was quite astonished when I saw the blazen way this “research” was wholly distorted to falsely suggest a recent massive change in the ice- freeze day over a massive 570 years.
So, let’s start with the data before the troughers added their trend:
Now I’m just a simple physicist but I cannot see a trend. So, I drew in by eye the best fit line to the data below.
I’m not quite sure, but it is just possible that there is a small upward trend of less than a day a century. In other words, even if there was a trend, it hasn’t changed recently.
But, now have a look at what was actually produced:

It is just incredible!

However, there is one small problem with this “research” supposed to show recent warming. Lake Suwa is surrounded by hot springs:

Suwa has recently developed hot springs surrounded by four resort towns with a population of 91,000 inhabitants. (paper)
The thermal activity under the lake also means that therapeutic hot springs (onsen) can be found along the shores of the lake in the Kami-Suwa area. The town has long been known for its “Kami-Suwa Onsen” and is one of the most prominent hot spring towns in Japan. (see below)

And it has a highway and is built up:

If you are driving to Lake Suwa don’t forget to stop by the cool Starbucks located at the Lake Suwa service area off the Chuo Expressway. There are amazing views of the lake and it’s a great spot for a photo or two.
http://blog.gaijinpot.com/lake-suwa-gods-crossing/

So any change could easily be due to a shift in geology and a change in hot spring flow rate or just the change in traffic on the highway or urban heating. Indeed, it would not surprise me to find there is a power station using the lake water for cooling.
The “research” may also look at another site which does appear to show modest warming since 1700 , but to be frank, when you’ve got people who can produce such an incredible interpretation where there obviously is no such massive trend, one has to question whether anything they produced could be credible. (URL:  http://www.nature.com/articles/srep25061)