In Ice Age, what natural event forces the animals to migrate south?

In Ice Age, an approaching ice age forces the animals to migrate south.

The 2002 film begins during a prehistoric cold period when many animals are moving toward warmer regions. Manny, Sid, and Diego become an unlikely group after their paths converge around a human baby. Their journey takes them through frozen landscapes, dangerous cliffs, and encounters with other prehistoric creatures.

The wording can cause a small mix-up: the film is titled Ice Age, but the immediate story is not about the beginning of a scientifically dated glacial period. It uses the broad idea of advancing cold and ice as the reason animals are on the move.

The movie combines this migration premise with comedy, family adventure, and references to prehistoric life. Its setting is fictionalized rather than a precise reconstruction of one specific ice-age location or year.