Hall, B.L. 2007. Late-Holocene advance of the Collins Ice Cap, King George Island, South Shetland Islands. The Holocene 17: 1253-1258.
What was done
The author presents "radiocarbon and geomorphologic data that constrain [the] late-Holocene extent of the Collins Ice Cap on Fildes Peninsula (King George Island, South Shetland Islands: 62°10'51"S, 58°54'13"W)," which, in her words, "yield information on times in the past when climate in the South Shetland Islands must have been as warm as or warmer than today," based on field mapping of moraines and glacial deposits adjacent to the ice cap, as well as radiocarbon dates of associated organic materials.
What was learned
Hall's data "indicate ice advance after ~650 cal. yr BP (AD ~1300)," which she notes is "broadly contemporaneous with the 'Little Ice Age', as defined in Europe." She also says that this was "the only advance that extended beyond the present ice margin in the last 3500 years, making the Little Ice Age in that part of the world likely the coldest period of the current interglacial. And the fact that "the present ice cap margin ... is still more extensive than it was prior to ~650 cal. yr BP" leads her to conclude that the climate prior to that time -- which would have comprised the Medieval Warm Period -- may have been "as warm as or warmer than present."
What it means
These several observations help to demonstrate the worldwide nature of the millennial-scale oscillation of climate that alternately brings the planet century-scale warm and cold periods independently of any variations in atmospheric CO2 concentration, which suggests there is no need to invoke the historical rise in the air's CO2 content as the cause of the Little Ice Age-to-Current Warm Period transition, as it is likely nothing more than the natural continuation of this age-old cycle of climate.