Ocean temperatures are rising far faster than previously thought, according to a climate change report on climate change.
Last year is expected to be recorded as the hottest year yet, beating the 2017 record, according to an analysis by the Chinese Academy of Sciences.
The ocean's heat is recorded by thousands of floating robots.
Lijing Cheng, a lead author of the study from the Chinese Academy of Sciences, said for the last 13 years an ocean observing system called Argo has been used to monitor changes in ocean temperatures, leading to more reliable data.
The robots dive to a depth of 2000 metres every few days, recording the temperature as they float back up to the surface.
Scientists warn these temperatures are driving sea levels and helping to fuel more intense hurricanes and other extreme weather.
And the warming is happening faster than predicted by scientists in 2013, according to the study, which was published this week in the journal Science.
"It's mainly driven by the accumulation of greenhouse gases such as carbon dioxide in the atmosphere due to human activities," Lijing said.
Leading climate scientists said in October that the world has about 12 years left to shift from still rising emission toward cleaner renewable energy systems, or risk facing some of the worst impacts of climate change.