How Antarctica got its ice sheets —In the continual movement of Earth’s tectonic plates, Antarctica was severed from the southern tip of South America about 34 million years ago, creating the Drake Passage. Antarctica became completely surrounded by ocean. The powerful Antarctic Circumpolar Current began to sweep around the continent, isolating Antarctica from the warmth of the global oceans and provoking large-scale cooling.
Today’s climate system is influenced by the ocean’s conveyor-like global circulation. Cold, salty waters sink to drive the conveyor, and warm surface currents complete the loop. (Jayne Doucette.)
Surface waters flowed from the Pacific into the Atlantic 10 million years ago via an ocean gateway called the Central American Seaway, and both oceans had the same salinity.
About 5 million years ago, the North American, South American, and Caribbean Plates converged. The rise of the Isthmus of Panama restricted water exchange between the Atlantic and Pacific, and their salinities diverged. The isthmus diverted waters that once flowed through the Seaway. The Gulf Stream began to intensify.
Today, evaporation in the tropical Atlantic and Caribbean leaves behind saltier ocean waters and puts fresh water vapor into the atmosphere. Trade Winds carry the water vapor westward across the low-lying isthmus, depositing fresh water into the Pacific through rainfall. As a result, the Atlantic is saltier than the Pacific. (Illustration by Jack Cook, WHOI)