The Medieval Quiet Period
By Raymond S Bradley, Heinz Wanner and Henry F. Diaz
The Holocene, Vol 26, Issue 6 (2016)
Abstract: For several centuries in early Medieval times the climate system was relatively unperturbed by natural forcing factors, resulting in a unique period of climate stability. We argue that this represents a reference state for the Common Era, well before anthropogenic forcing became the dominant driver of the climate system.
Introduction: The notion of a “Medieval Warm Period” (MWP) has become a basic paradigm in the literature on climate variations of recent millennia, though the timing and duration of that interval is ill-defined. Numerous articles have compared the average temperature in recent decades, with paleotemperature estimates for different intervals of time during the Medieval period sensu lato (from the Fall of Rome in 476 CE to the beginning of the Renaissance, ~1500 CE). Such comparisons have assumed particular significance in terms of the role of greenhouse gases in recent decades.
Critics have argued that, if temperatures were as warm or warmer than current conditions before the onset of anthropogenic forcing, this would provide evidence that “natural” fluctuations alone could explain current conditions, since greenhouse gases were only ~280 ppmv during Medieval time (versus 400 ppmv today). Hence the debate about the timing, extent and exact level of global temperature in the past has raged on, a hostage to the conflicting views about the nature and magnitude of anthropogenic climate change.