What was done
Based on a wealth of historical data, the author reconstructed the severity of winter seasons in the northern Baltic Sea on the basis of observed times of ice break-up between 1529 and 1990 at the port of Riga, Latvia.

What was learned
The long date-of-ice-break-up time series was best described by a fifth-order polynomial, which identified four distinct periods of climatic transition: (1) 1530-1640, warming with a tendency toward early break-up of 9 days/century, (2) 1640-1770, cooling with a tendency toward late break-up of 5 days/century, (3) 1770-1920, warming with a tendency toward early break-up of 15 days/century, and (4) 1920-1990, cooling with a tendency toward late break-up of 12 days/century.

What it means
Over the 390 years from 1530 to 1920 that comprise the sum of the first three climate change periods identified in this analysis, the atmosphere's CO2 concentration rose by approximately 16 ppm; while over the 70 years of the last period it rose by about 54 ppm.  Hence, with respect to potential impacts of the historical rise in the air's CO2 content, the last of the four periods would be the one most likely to depict what climate alarmists want us to believe, i.e., that rising CO2 levels are bringing unprecedented warming to the planet, especially in high-latitude regions.  As this study clearly shows, however, this latter period exhibits the greatest rate of warming of the entire record, which is not exactly what the climate alarmists are predicting should have happened.  In fact, it is quite the opposite, with the last year of the time series posting the longest-lasting presence of ice of the entire 460-year period.