Palynology, or the study of pollen, is used to reconstruct ancient environments and document environmental changes that had significant impacts on past human societies.
As the climate changes—growing warmer or colder, drier or wetter—so do the natural ranges of various plant species with specific temperature and precipitation requirements. These changes are reflected in the relative quantities of different types of pollen found in layers of sediment dating from different time periods—sediment retrieved through a process called coring, shown here:
Peat bogs and lake beds are particularly good pollen "traps," but other types of sediment can be tested as well.
For a simple introduction, see Palynology on the University of Texas website.