What was done
"This paper," in the words of the authors, "investigates the effects of climate variability on the termination of the ice season in the Baltic region," focusing on Lake Kallavesi, Lake Nasijarvi, River Tornionjoki (all of Finland) and several Estonian rivers.

What was learned
Similar non-linear trends were observed at all sites over the last few centuries, which led the authors to conclude that "these trends have been induced by a common climatic forcing acting in the spring at the synoptic and global scales."  Specifically, they report that "a dramatic change in the dates of ice break-up (towards earlier thaw) has been observed between the end of the 19th and the beginning of the 20th century."  Describing these changes in more detail, Yoo and D'Odorico note that "the strongest long-term climatic changes nowadays observable in the Finnish cryophenological records started in the second half of the 19th century," which is also when the temperature record of Esper et al. (2002) shows the demise of the Little Ice Age to have begun in earnest.  In addition, they report that "the shift in the ice break-up dates terminated before [our italics] 1950," in harmony with our view of the temperature history of the globe, i.e., that there has been little net warming since the 1930s.

What it means
Although the authors suggest that the observed changes in the dates of ice break-up may be due to CO2-induced warming, it is clear that that attribution does not follow from what is claimed by the world's climate alarmists with respect to the temperature history of the Northern Hemisphere: no warming until abut 1910 or 15 and "unprecedented" warming thereafter, particularly over the last two decades of the 20th century (Mann et al., 1999).  Hence, the data depicted in this treatise bear strong testimony against the temperature history of the earth that has been used by climate alarmists to implicate the ongoing rise in the air's CO2 content as a threat to life on earth.  In fact, we could probably say they refute it.

References
Esper, J., Cook, E.R. and Schweingruber, F.H.  2002.  Low-frequency signals in long tree-ring chronologies for reconstructing past temperature variability.  Science 295: 2250-2253.

Mann, M.E., Bradley, R.S. and Hughes, M.K.  1999.  Northern Hemisphere temperatures during the past millennium: Inferences, uncertainties, and limitations.  Geophysical Research Letters 26: 759-762.