What was done
The authors combined 36 nine-thousand-year-long pollen-based July and annual mean temperature reconstructions for the portion of Europe stretching from the Norwegian Atlantic coast to 26°E in Estonia and Finland and from 57°N in Southern Fennoscandia to 70°N, the latter 5000 years of which temperature reconstruction they compared to a stacked chironomid-based July mean temperature record based on data obtained from seven Fennoscandinavian sites.

What was learned
Seppa et al. report that "the stacked records show that the 'Holocene Thermal Maximum' in the region dates to 8000 to 4800 cal yr BP and that the '8.2 event' and the 'Little Ice Age' at 500-100 cal yr BP are the clearest cold episodes during the Holocene," while the graphical representations of their data clearly indicate that the Little Ice Age was the colder of the two major cold episodes

What it means
Yet again, we have another major analysis of multiple paleotemperature records that reveal the Little Ice Age to have been the coldest interval of the entire Holocene. And with 20th-century global warming starting from the current interglacial's coldest point in time, it is only natural to expect that the ensuing warming would be rather substantial, irrespective of what the air's CO2 content might have been doing concurrently. And so it was. But does the fact that the atmosphere's CO2 concentration rose substantially over the 20th century while air temperature also rose substantially (albeit more haltingly) not suggest that the CO2 increase was driving the temperature increase? Not at all; because over the prior seven thousand years, when Seppa et al.'s data show Northern European temperatures to have been steadily falling, earth's atmospheric CO2 concentration was slowly but surely steadily rising. And you can't get much more out of phase than that.