One of the great history generating mysteries of the middle ages was
the Medieval Warm Period. Why this climate pattern happened is unknown. Equally, why it stopped, and stopped with such a brutal weather event as nearly a year of hard rains in Northern Europe, is unknown. However, Centrum knows weather. We're shown this in a vignette of Grant being rained out at Vicksburg.
Either ending the Medieval Warm Period early or keeping it in place longer would bring massive changes to world history. In Europe alone the Medieval Warm Period coincides with the rise of feudalism. The end of the Medieval Warm Period seems to start the decay and collapse of feudalism. If the climatic effects of the Medieval Warm Period were vital to feudalism, and I've read authors who made solid arguments that way, any plot needing to either weaken or strengthen feudalism could focus on weather control.