These measurements are based on tree-rings, the recorded advance and retreat of glaciers, written records of rainfall and of snowfall, written records about when rivers and lakes froze over, etc.
For its purposes, the Intergovernmental Panel on Climate Change (IPCC) defines the ‘Little Ice Age’ as lasting from 1450 to 1850, while the ‘Medieval Climate Anomaly’ runs from 950 to 1250.
These two eras both occurred before significant industrialization and before the use of fossil fuels in significant quantities, and can therefore be said to be non-anthropogenic.
The IPCC notes that “most glaciers around the globe have been shrinking since the end of the Little Ice Age.” If the Little Ice Age lasted approximately 400 years, and we are now a little more than a century since its end, current glacial retreat patterns may be attributed, in part, to the plant’s return to its equilibrium, i.e., to temperatures held prior to the Little Ice Age.
Beginning and ending points for the Little Ice Age cannot be precisely determined. It was a generalized trend. Some scholars mark the endpoint nearer to 1800 than 1850.
But, although an exact endpoint cannot be given, it nonetheless makes sense that the end of the Little Ice Age would mark the beginning of a non-anthropogenic warming trend around the globe. The IPCC reports that “most permafrost has been degrading since the Little Ice Age.”
It would be remarkable if, at the end of a four-century-long cold era, the plant did not warm. Tautologically, that is the definition of the end of a cold age: planetary warming.
As the two extremes of measured historic planetary temperature, the Medieval Climate Anomaly and the Little Ice Age constitute reference points for other eras, including the current one.
By contrast, previous Ice Ages were not historic, but prehistoric, there being no direct observations and written records of them.
What caused these two outliers? Any answer must be merely speculative, but some scholars note that ice core samples from polar regions show that wide swings in atmospheric carbon dioxide levels, non-anthropogenic in origin, may have accompanied both the Little Ice Age and the Medieval Warm Period.
