OK, I'll bite. Did you look at the paper? I greatly appreciate that the article linked it. Check out Figure 1. It's the basis for their claim that "the trend in the freeze date in the earlier time period (1443–1683) was 0.19 days per decade and increased to 4.6 days per decade in the more recent time period (1923–2014; Fig. 1A)." The fit for the the recent period looks extremely suspect. The slope they fit is an artifact of considering the "did not freeze" dates as equivalent to Day 62. Change the day, and you get a different slope. Why did they choose 62? Because that was the latest that the lake ever was observed to freeze. Thus the precision of the exact slope conveys nothing other than "did not freeze" was more common over than last few decades than earlier.

OK, so probably the lake has been less likely to freeze in recent decades because the climate is warmer, caused by increased CO2 emissions over the last century. Or is there anything else that could change the lake's likelihood of freezing? There is that mention of "hot springs". Luckily, as mentioned in the linked National Geographic article, "It turns out that local factors, such as changes in land use, human population growth and even the installation of a floodgate and development of hot springs on Lake Suwa, had little influence on the patterns of change in ice freezing dates for the lake or ice break-up dates for the Torne River."

The river in Finland is even luckier. From the paper: "Torne is one of the largest unregulated rivers in Scandinavia, but is influenced by several local anthropogenic factors including urban development, bridge construction, hydropower development on a tributary of Torne, and demolition of constructed dams. For example, the population of central Tornio has increased from 1000 inhabitants in 1880 to 10,000 in 200919. In addition, several bridges were constructed including the railway bridge in 1910 and a highway bridge in the 1930s and 1970s. Three small hydroelectric power stations have been developed on one of Torne’s tributary rivers (Tengelionjoki) in 1954 (2.5 MW), 1955 (0.5 MW), and 1987 (10.5 MW). In the 20th century, 162 log-driving dams were constructed on Torne, but later demolished in the 1970s that may have sped up the runoff. Despite the number of anthropogenic factors on Torne, the human influence on Torne is small relative to the size of the river and no statistically significant consequences of local anthropogenic activities on ice breakup dates in Torne are apparent."[1]

Good thing these have no "statistically significant consequences", although it might be worth noting that "not statistically significant" absolutely does not mean "statistically non-significant". And it also doesn't consider that cloudiness and coal dust changing the albedo of the snow may have greater effect on the timing of spring melt than air temperature.

Anyway, that's probably the angle I'd take in discrediting this paper. Well, that and asking how the analysis would look if they didn't throw away all the 250 years of data in the middle. What's that? Well, for Lake Suwa, "We eliminated all data from 1682–1923 from the analyses to reduce the uncertainty in dates of ice freeze". What uncertainty? "Unfortunately, there are missing data in the midyears of the time series (1505–1515) and more importantly, data from 1682–83 to 1922–23 are considered unreliable for analysis ice cover freeze dates". I see. You had a couple years that were considered unreliable, so you did the obvious thing by throwing away all 250 years between those dates. I know it's probably just coincidence, and for all we know the case for catastrophic climate change would be even stronger if the unreliable data was included, but still, that seems odd.

For the record, I'm strongly environmentalist, believe that protecting the earth is more important than building the economy, believe that humans are affecting the climate, believe that the world is currently warming, and really hate bad science. And although I've only glanced at it, I'm pretty confident in saying that this is bad science.

[1] For an interesting contrast, where hydroelectric projects appear to have a large effect on climate, "People of a Feather" is a recent documentary centered around the effect of dams in Northern Canada on ice formation in Hudson Bay: http://www.theglobeandmail.com/arts/film/people-of-a-feather...