Tree-ring records from the region, and the analysis of pollen sequences preserved in lake sediments near Cahokia, show evidence of significant climate variability during the period of Cahokia’s peak and decline.

The Medieval Warm Period, which affected much of the Northern Hemisphere between roughly 950 and 1250 CE, created conditions that were, in the Mississippi Valley, generally favorable for agriculture — warmer temperatures and reliable precipitation supported the corn-based agriculture that fed the city.

As the Medieval Warm Period gave way to the more variable climate of the early centuries of the second millennium, conditions in the Mississippi Valley became less reliably favorable.

Evidence of drought episodes has been identified in the regional climate record for the twelfth and thirteenth centuries.