Read here. Map source here. Scientists studied the extended warm period (interglacial) prior to the last major ice age. That prior warm period happened some 125 thousand years ago. Using a sediment core from a German lake, the scientists analyzed the quantity of pollen grains found in the core. Approximately every 1,500 years during the ancient interglacial, the climate changed from a warming phase to a cooling phase. The current interglacial, since the end of the last ice age about 15 thousand years ago, shares this same, every 1,500 year oscillation of warming-to-cooling climate changes.
The scientists involved in the study suggest this natural 1,500 year oscillation, found in both the ancient interglacial and the one we live in, is likely due to solar forces.
"The results of the authors' analysis revealed the presence of 11 major cold events having an average recurrence time of approximately 1450 years over the course of the last interglacial, which periodicity is essentially identical to the millennial-scale oscillation of climate throughout the current interglacial (Bond et al., 1997, 2001; deMenocal et al., 2000; McDermott et al., 2001; Gupta et al., 2003; Hu et al., 2003).....This study adds to the growing body of evidence that earth's climate oscillates in a well-defined manner on a timescale of approximately 1500 years. This knowledge is very important, for it suggests that something other than the historic buildup of carbon dioxide in the atmosphere may be responsible for 20th-century warming..... Indeed, there is absolutely no evidence for any concomitant oscillation in the air's CO2 content accompanying the 1500-year oscillation of climate that was responsible for the warmings that produced the prior Medieval Warm Period and the still earlier Roman Warm Period,"
