Pollen may seem like ephemeral dust when it's aloft on the breeze, but those grains can have surprising staying power in the environment. After plants spread theirs far and wide it can be preserved in lakebeds or fossilized in soil. When scientists take a sediment core, they sometimes find that pollen from different species dominates in different eras. And since plants thrive best under particular temperatures and conditions, researchers can use the pollen mix to determine the climate of an era.

In one 2009 study, researchers used pollen traces to study the climate of the Andes mountains in South America, and found evidence that a warm spell between 1100 and 1533 AD allowed for more agriculture on the mountain slopes. That may explain the Inca civilization's expansion during those centuries, researchers suggest.