What was done
For parts or all of the period AD 500-2000, the authors derived continuous decadal-scale records of a number of climate-relevant parameters related to precipitation/evaporation balance - and, hence, glacier activity - from sediment cores extracted from Laguna Blanca (8°20'N, 71°47'W) and Laguna Mucubaji (8°47'N, 70°50'W), while data they obtained from the nearby Piedras Blancas peat bog yielded "pollen histories that chronicle vegetation change in response to climate."

What was learned
Although Polissar et al. concentrate on what their data reveal about the Little Ice Age, concluding that "temperature declines of 3.2 ± 1.4°C and precipitation increases of ~20% are required to produce the observed glacial responses," we focus on what their data reveal about the prior Medieval Warm Period (MWP) compared to the Current Warm Period (CWP). In doing so, we note that only three of their eight climate-relevant parameters span the full 1500-year period from AD 500 to 2000 and are thus suitable for this purpose. All three of them, however, suggest that the MWP was warmer than the CWP. In the case of Laguna Blanca magnetic susceptibility, the MWP's greater warmth extends from before the start of the record (sometime prior to AD 500) to approximately AD 1300. In the case of the abundance of sedge pollen from the Piedras Blancas peat bog, it extends from about AD 550 to 1020; and in the case of altitudinal shifts in ecological zones derived from the Piedras Blancas data, it extends from sometime before the start of the record to about AD 1000. Consequently, for the period of time over which all three data sets indicate the MWP was warmer than the CWP, i.e., AD 550-1000, we can be fairly confident that such was truly the case.

Another important aspect of the Polissar et al. study is that it clearly implicates solar variability as the cause of the climatic variations they observed. They note, for example, that "four glacial advances occurred between AD 1250 and 1810, coincident with solar-activity minima," and they state that the data they present "suggest that solar activity has exerted a strong influence on century-scale tropical climate variability during the late Holocene, modulating both precipitation and temperature" and demonstrating the "considerable sensitivity of tropical climate to small changes in radiative forcing from solar irradiance variability."

What it means
Since Polissar et al.'s data suggest that the Little Ice Age in the tropical Andes of South America likely owed its existence to decreased solar activity, it follows that the prior Medieval Warm Period was likely sustained by greater solar activity. In addition, since the Current Warm Period in this region has not yet achieved the high temperature level experienced by the Medieval Warm Period, when there was something on the order of 100 ppm less CO2 in the air than there is today, there is no compelling reason to believe that earth's current warmth is sustained to any degree by its extra 100 ppm of CO2. It's the enhanced activity of the sun that is likely responsible for it.