Alex J. Chepstow-Lusty, from the Institut Franc¸ais d’Etudes Andines, and his colleagues examined sediment core samples from lakes in the area as well as archaeological evidence of mite populations and pollen density, and concluded that -- among other factors -- the Incas were helped by the warm climate, which allowed them to plant crops at higher altitudes than during earlier parts of their history.
Soil core samples from the area showed maize pollen at altitudes up to 3300 meters above sea level during the warmer period, whereas pollen from earlier periods is found up to only about 3100 meters. This indicates that a great deal more of the steeply-sloping land was arable by the Incas during this period than previously.
The article does not conclude that the climatic trend was solely responsible for the Inca's expansion, and also lists factors such as abundant labor pool and a well-organized standing army.
The notion of a Medieval Warm Period (MWP) is not universally accepted among scientists. The article notes:
The prevailing view of this interval, known commonly as the “Medieval Warm Period” (MWP), is that elevated temperatures were often only intermittently experienced and, in some regions, was apparently characterized instead by climatic anomalies such as prolonged drought, increased rainfall or a stronger monsoon system (Hughes and Diaz, 1994; Bradley et al., 2003; Stine, 1994; Zhang et al., 2008). However, evidence for the MWP being a global phenomenon is contentious, especially in the Southern Hemisphere, where there are few continuous, detailed palaeoclimatic records spanning this interval (Bradley, 2000; Broecker, 2001). Nevertheless, from the Marcacocha dataset we can infer that temperatures increased from ca. AD 1100 (after a period of relative aridity in comparison to much of the first millennium AD) and that conditions remained warm and stable for several centuries thereafter.
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