The Little Ice Age was a period of global cooling. Scholars are unable to give precise beginning and ending dates for such time segments, but it seems to have include a century or two both before and after the year 1600.
The Medieval Warm Period was also global in scope, and represented an extreme warming trend. Beginning shortly after the reign of Charlemagne, this era lasted several centuries.
Because both of these events - the term ‘singularity’ is justified - occurred prior to the recording of daily temperatures around the globe, they are documented by evidence such as the advance and retreat of glaciers, tree ring measurements, written observations of snowfall, and other data.
These two time spans are indeed outliers, as the Intergovernmental Panel on Climate Change (IPCC) notes in its report:
There is high confidence for droughts during the last millennium of greater magnitude and longer duration than those observed since the beginning of the 20th century in many regions. There is medium confidence that more megadroughts occurred in monsoon Asia and wetter conditions prevailed in arid Central Asia and the South American monsoon region during the Little Ice Age (1450–1850) compared to the Medieval Climate Anomaly (950–1250).
The Little Ice Age, then, represents an extreme condition, an outlier well beyond any data points which have been observed in the last two or three centuries.
It also represents a condition with significant duration.
Both the Medieval Warm Period - which the IPCC calls the ‘Medieval Climate Anomaly’ - and the Little Ice Age occurred prior to industrialization and prior to the use of coal and other fossil fuels in significant quantity. There is, then, no possibility of either time segment being anthropogenic.
If these two outliers - one much warmer, and one much cooler, than anything observed in recent centuries - are clearly not anthropogenic, then lesser variations in temperature can also be naturally occurring.
