sentence: As coauthor Shayne McGregor of the University of New South Wales explains , `` the main cause of the Pacific wind , temperature , and sea level trends over the past 20 years lies in the Atlantic Ocean [ ... ] We saw that the rapid Atlantic surface warming observed since the early 1990s , induced partly by greenhouse gases , has generated unusually low sea level pressure over the tropical Atlantic .
question: what was _ warming _ _ _ ?
answers: Atlantic surface