For example, the proposition that “the climate is changing” and that “climate change in anthropogenic” are separate. One might be false, and the other true, or vice-versa.
Phrases like ‘climate change’ and ‘climatic instability’ also need careful definition. Because the earth’s climate is essentially erratic and unpredictable, it is not clear what would constitute ‘change’ or ‘instability.’
Long before the Industrial Revolution and the widespread use of fossil fuels, droughts and floods appeared without discernable pattern, cause, or predictability. Mild winters and harsh winters arrived capriciously.
If one had unlimited access to data, and sufficient power to construct mathematical models, perhaps some of the historic climate events would have been predictable, or perhaps their causes discernable.
Given however, the data which researchers in fact have, there are at best vague hypotheses about what cause the Medieval Warm Period, an era from around 850 A.D. to around 1250 A.D.
During that era, exceptionally warm temperatures - outliers - are evidenced around the globe. Some data comes from direct observations: records of snowfall or lack thereof, glacial retreat, and harvests. Some of the data is indirect: tree-ring measurements and ice core samples.
Globally, droughts during the Medieval Warm Period seem to be greater in scope than anything observed in the twentieth or twenty-first centuries, as the Intergovernmental Panel on Climate Change (IPCC) notes:
Compelling arguments both for and against significant increases in the land area affected by drought and/or dryness since the mid-20th century have resulted in a low confidence assessment of observed and attributable larges-cale trends. This is due primarily to a lack and quality of direct observations, dependencies of inferred trends on the index choice, geographical inconsistencies in the trends and difficulties in distinguishing decadal scale variability from long term trends. On millennial time scales, there is high confidence that proxy information provides evidence of droughts of greater magnitude and longer duration than observed during 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 to 1850) compared to the Medieval Climate Anomaly (950 to 1250).
Likewise, the Little Ice Age represents a statistical outlier of lower temperatures than anything recorded in the last century or two.
In sum, change and instability seem to be the defining characteristic of the earth’s climate. It would be truly unusual if the climate were predictable or if it repeated its patterns from one year to the next.
This instability seems to predate the advent of the widespread use of coal, oil, and gas. The instability seems to be no greater after the introduction of fossil fuels, given the major outliers which antedate it.
