The Warming was a period of time in Ocampan history in which Ocampa's surface became a total desert. The Caretaker came and opened a chasm which allowed the Ocampa to flee underground.
The story of The Warming was told to Harry Kim and B'Elanna Torres by an Ocampan doctor. It was also told to Kathryn Janeway and Tuvok by the Caretaker himself. (VOY: "Caretaker")