What was done
The authors present new proxy temperature records for the past three centuries that were extracted from borehole temperature-depth logs obtained at ten sites scattered throughout southern Saskatchewan, Canada.  Data from the latter potion of the record were compared to observed surface temperature measurements over the last 100 years.

What was learned
The temperature proxies indicate the existence of a relatively cool period throughout most of the 18th and 19th centuries.  Then, from about 1820 to present, temperatures rose between 2.5 and 3.0°C, suggesting to the authors that "the last major warming event [which is still going on] began in the 18th-19th century."

What it means
According to the authors, "the significance of this record is that it suggests almost half of the warming occurred prior to 1900, before the dramatic buildup of atmospheric greenhouse gases."  Hence, if something other than greenhouse gases caused the first half of the most recent global warming event (in which we are still imbedded), something other than greenhouse gases may be responsible for the second half of the warming as well.