What was done
Noting that "the extent of the spring ice cover reflects the net effect of winter dynamics," the author documents changes in this aspect of Arctic climate by documenting changes in the April extent of ice in the Nordic Seas -- the Greenland, Iceland, Norwegian, Barents and Western Kara Seas, bounded by 30°W, 70°E, and 80°N -- over the period 1864-1998, augmenting these findings with the results of some of his earlier studies that look back in time a full 400 years.

What was learned
It is reported that "the extent of ice in the Nordic Seas measured in April has decreased by 33% over the past 135 years."  However, the author says that "nearly half of this reduction is observed over the period 1860-1900."  Hence, as he goes on to note, "global warming is accordingly concurrent with a decelerating [our italics] seasonal freezing rate over the past 100 years."

We additionally note, in this regard, that the first half of the total Nordic sea-ice decline occurred over a period of time when the atmosphere's CO2 concentration rose by only 7 ppm, whereas the second half of the sea-ice decline occurred over a period of time when the air's CO2 concentration rose by more than 70 ppm.  This observation suggests that if the historical rise in the air's CO2 content has been responsible for the historical decrease in sea-ice extent, its impact over the last century has declined to less than a tenth of its impact over the preceding four decades, which in turn suggests that the rise in the air's CO2 content over the past 135 years has likely had nothing to do with the same-period decline in sea-ice cover.

That this is indeed the case is born out by the author's earlier observation (Vinje, 1999) that "annual melt-backs of the magnitude observed after about 1930 have not been observed in the Barents Sea since the eighteenth century temperature optimum," which was followed by "a fall in the NHMT [Northern Hemisphere mean annual temperature] of about 0.6°C over the last few decades of the eighteenth century," which temperature decline has just now been finally erased by "a rise of about 0.7°C over the period 1800-2000."  Consequently, the Northern Hemisphere would appear to be not much warmer now (and the extent of Barents sea-ice cover not much less now) than it was sometime during the 1700s, when the air's CO2 concentration was on the order of 90-100 ppm less than it is now.

What it means
The data presented in this paper, together with the known history of the atmosphere's CO2 concentration, argue mightily against the climate-alarmist claim that the decline in Nordic sea-ice extent over the past 135 years is a direct result of the concomitant rise in the air's CO2 content.

Reference
Vinje, T.  1999.  Barents Sea ice edge variation over the past 400 years.  Extended Abstracts, Workshop on Sea-Ice Charts of the Arctic, Seattle, WA, USA. World Meteorological Organization, WMO/TD No. 949, pp. 4-6.