What was done
The authors documented glacier and vegetation changes at high elevations in the upper Bowser River basin in the northern Coast Mountains of British Columbia via studies of the distributions of glacial moraines and trimlines, tree-ring data, cores from two small lakes that were sampled for a variety of analyses (magnetic susceptibility, pollen, diatoms, chironomids, carbon and nitrogen content, ²¹⁰Pb, ¹³⁷Cs, ¹⁴C), similar analyses of materials obtained from pits and cores from a nearby fen, and by accelerator mass spectrometry radiocarbon dating of plant fossils, including wood fragments, tree bark, twigs and conifer needles and cones.

What was learned
Clague et al. report finding evidence of a glacial advance that began about 3000 cal yr ago and "may have lasted for hundreds of years," which would have placed it within the unnamed cold period that preceded the Roman Warm Period.  They then describe evidence for a second "minor phase of activity [that] began about 1300 cal yr ago but was of short duration," which would have placed it within the Dark Ages Cold Period.  Then they describe the "third and most extensive Neoglacial interval [that] began shortly after AD 1200 [following the Medieval Warm Period] and ended in the late 1800s," which was, of course, the Little Ice Age, during which they say that "glaciers achieved their greatest extent of the past 3000 years and probably the last 10,000 years."  Thereafter, they report that the "climate warmed about 1-2°C during the 20th century, accompanied by a rise in treeline, an increase in coniferous tree cover in the subalpine zone, and an increase in the temperature and biological productivity of ponds."

What it means
Once again we see the regular alternation between non-CO2-forcecd multi-century cold and warm periods that is the trademark of the millennial-scale oscillation of climate that reverberates throughout glacial and interglacial periods alike.  That a significant, but by no means unprecedented, warming followed the most recent cold phase of this cycle is in no way unusual, as this "Little Ice Age" was perhaps the coldest multi-century period of the last 10,000 years.  The significant warming of the 20th century would have occurred within the same timeframe and been just as strong if there had never been an Industrial Revolution and the atmosphere's CO2 content had remained constant at pre-industrial levels.  The historical increase in the air's carbon dioxide concentration had absolutely nothing to do with it.  The warming was simply the next expected phase of this ever-recurring natural climatic cycle.