What was done
Using annual maximum ice extent data obtained from both observational and proxy sources, the authors examined trends in sea ice extent for the Baltic Sea over the period 1720-1997.

What was learned
The data revealed a statistically significant change from a colder to a warmer climatic regime centered at about 1877, with mean maximum ice extent decreasing by 44,000 km².  The reason for the climate change, according to the authors, "cannot be explained in the present study," although they did suggest it might be due to changes in atmospheric circulation.  A second important feature of the data was the observation that the colder 1720-1877 period experienced greater variability in ice extent than did the warmer 1878-1997 period.

What it means
The step-like change in climate that occurred in the Baltic Sea around 1877 cannot possibly be construed as evidence of CO2-induced global warming.  Rather, it most likely is an expression of natural climate variation; for ice core data reveal that the atmospheric CO2 concentration in 1877 was relatively constant around 288 ppm, which was well within its range of natural variability prior to the age of industrialization (see Carbon Dioxide History: The Last 1,000 Years).  Additionally, the data demonstrate a principle we have consistently discussed on our web site, i.e., that colder climates exhibit greater variability than warmer climates.