From the article:
In 1981 researchers removed a long tube of ice from the center of a glacier in southern Greenland at a site known as Dye 3. More than a mile (two kilometers) long, the deep end of the core sample had been crushed by the pressure of the ice above it and sullied by contact with rock and soil. By destroying the pattern of annual layers, this contamination seemingly made it impossible to assess the region's ancient climate. But DNA extracted from the previously ignored dirty bottom has revealed that Greenland was not only green, it boasted boreal forests like those found in Canada and Scandinavia today.
According to the article, that was between 400,000 and 800,000 years ago, not all that long in geologic time frames."