I'm remembering a very old Sci Fi story by an almost forgotten writer. And earthquake removed a portion of the narrow part of South America apart and the Pacific and Atlantic were now naturally connected (yay, no more Panama Canal tolls!) But this eliminated a vital Atlantic current which brought warm water and weather to Northern Europe, which began to go into a modern ice age. An incipient war of immigration was developing between the U.S. and its once European friends such as Gt. Britian, France and Germany.

It was solved by using a big bomb to restore the land portion and get that damn current moving again!