Here's the scary part:
There is a link between general periods of decline (dark ages) when the temps are cooler, and periods of civilization and growth when temps are warmer.
Also, the Black Death happened because of a cold spell. The pestis bacteria, which causes Bubonic Plague, causes a change in the digestive tract of fleas, such that they end up constantly ravenous and bite everything in sight, trying to get full. They say this cooling happened due to a volcanic eruption (another natural cause). If that volcano hadn't erupted -- and brought on a global cooldown -- the Black Death wouldn't have happened. Think about the ramifications.