Reference
McKay, J.L., de Vernal, A., Hillaire-Marcel, C., Not, C., Polyak, L. and Darby, D. 2008. Holocene fluctuations in Arctic sea-ice cover: dinocyst-based reconstructions for the eastern Chuckchi Sea. Canadian Journal of Earth Sciences 45: 1377-1397.
Background
Writing about the Arctic Ocean, the authors say that over the past thirty years "there has been a rapid decline in the extent and thickness of sea-ice in summer and more recently in winter as well," but they state there is "debate on the relative influence of natural versus anthropogenic forcing on these recent changes." Hence, they decided "to investigate the natural variability of sea-ice cover in the western Arctic during the Holocene and thus provide a baseline to which recent changes can be compared," in order to help resolve the issue.
What was done
McKay et al. analyzed sediment cores obtained from a site on the Alaskan margin in the eastern Chukchi Sea for their "geochemical (organic carbon, δ13Corg, Corg/N, and CaCO3) and palynological (dinocyst, pollen, and spores) content to document oceanographic changes during the Holocene," while "the chronology of the cores was established from 210Pb dating of near-surface sediments and 14C dating of bivalve shells."
What was learned
Since the early Holocene, according to the findings of the six scientists, sea-ice cover in the eastern Chuckchi Sea appears to have exhibited a general decreasing trend, in contrast to the eastern Arctic, where sea-ice cover was substantially reduced during the early to mid-Holocene and has increased over the last 3000 years. Superimposed on both of these long-term changes, however, are what they describe as "millennial-scale variations that appear to be quasi-cyclic." And they write that "it is important to note that the amplitude of these millennial-scale changes in sea-surface conditions far exceed [our italics] those observed at the end of the 20th century."
What it means
Since the change in sea-ice cover observed at the end of the 20th century (which climate alarmists claim to be unnatural) was far exceeded by changes observed multiple times over the past several thousand years of relatively stable atmospheric CO2 concentrations (when values never strayed much below 250 ppm or much above 275 ppm), there is no compelling reason to believe that the increase in the air's CO2 content that has occurred since the start of the Industrial Revolution has had anything at all to do with the declining sea-ice cover of the recent past; for at a current concentration of 385 ppm, the recent rise in the air's CO2 content should have led to a decrease in sea-ice cover that far exceeds what has occurred multiple times in the past without any significant change in CO2.