Last year was the warmest year on record for the world's oceans, part of a long-term warming trend, according to a study released on Monday. “If you look at the ocean heat content, 2019 is by far the hottest, 2018 is second, 2017 is third, 2015 is fourth, and then 2016 is fifth,” said Kevin E. Trenberth, a senior scientist at the National Center for Atmospheric Research, NCAR, and an author on the study.