How did ice-age climates cause savannas to spread into temperate regions?

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Flat savanna plain with scattered acacia trees under a partly cloudy sky in Serengeti National Park, Tanzania. — Savanna A savanna marked with acacia trees in Serengeti National Park, Tanzania. Africa's Serengeti Plain is commonly used as an example of the savanna biome—a hot, seasonally dry ecological region characterized by an open tree canopy (i.e., scattered trees) above an understory of continuous grasses.

During the Pleistocene Epoch, savannas expanded significantly into temperate regions at the expense of forests during long, cool, and dry intervals known as glacial periods or ice ages.1

This expansion was driven by a global cooling trend that reduced ocean surface temperatures, which in turn decreased evaporation and slowed the hydrologic cycle. This resulted in less cloud formation and reduced precipitation globally. As these conditions persisted, the drier, cooler climate favored the development of seasonally dry habitats, allowing savanna vegetation to replace forests in midlatitude regions. Evidence for this shift is supported by studies of fossilized pollen found in sediments across South America, Africa, and Australia.1