What was done
The authors derived a record of early Holocene atmospheric CO2 concentration based upon an analysis of the stomatal frequency of birch tree leaves buried in peat deposits near Denekamp, The Netherlands.

What was learned
Atmospheric CO2 concentrations 10,000 years ago were determined to be between 260 and 265 ppm.  Thereafter, they rose to a value near 330 ppm over the course of a century.  Concentrations remained in the 330 ppm range over the next 300 years, whereupon they declined to about 300 ppm.  A second sharp increase in atmospheric CO2 concentration to a maximum value of 348 ppm followed, with concentrations hovering between 333 and 347 ppm for the duration of the record.

What it means
The results of this study challenge the notion that atmospheric CO2 concentrations have been relatively stable throughout the Holocene, as the authors' data clearly show there have been large CO2 shifts on a century time scale.  Their results also suggest that the atmospheric CO2 concentrations of today might not be unprecedented in the current interglacial, as some of the values they report approach 350 ppm in the mean, and possibly 365 ppm when potential errors are considered.  Thus, since atmospheric CO2 concentrations may have risen by as much as 90 ppm over the course of a few centuries 10,000 years ago in the absence of human influence, it is possible that the rise in atmospheric CO2 since 1800, which is on the order of 95 ppm, may contain a large non-anthropogenic component.