New measurements, aided by an worldwide network of 3,900 floats deployed in the oceans since 2000, showed more warming since 1971 than calculated by the latest United Nations assessment of climate change in 2013, they said.
The thermal expansion - water swelling as it warms - would raise sea level 30 centimeters, above any sea level rise from melting glaciers and ice sheets.
"If you want to see where global warming is happening, look in our oceans", said Zeke Hausfather, a graduate student in the Energy and Resources Group at the University of California, Berkeley, and co-author of the paper.
Ocean heating is critical marker of climate change because an estimated 93 percent of the excess solar energy trapped by greenhouse gases accumulates in the world's oceans.
'Ocean heating is a very important indicator of climate change, and we have robust evidence that it is warming more rapidly than we thought'. And, unlike surface temperatures, ocean temperatures are not affected by year-to-year variations caused by climate events like El Nino or volcanic eruptions.
Overall, temperatures in the ocean down to 2,000 metres rose about 0.1 degree Celsius from 1971-2010, he told Reuters news agency. The report also found that the warming rate has accelerated since 1991.
"Global warming is close to ocean warming, and 2018 will be the warmest year on record, followed by 2017, then 2015".
The oceans are warming faster than previously estimated, setting a new temperature record in 2018 in a trend that is damaging marine life, scientists said on Thursday.
He told Reuters that records for ocean warming had been broken nearly yearly since 2000.
For the new study, scientists used data collected by a high-tech ocean observing system called Argo, an global network of more than 3,000 robotic floats that continuously measure the temperature and salinity of the water.
Prior to Argo, ocean temperature data was sparse at best, relying on devices called expendable bathythermographs that sank to the depths only once, transmitting data on ocean temperature until settling into watery graves.
Argo "has provided consistent and widespread data on ocean heat content since the mid-2000s", it said.
A warmer ocean causes sea level to rise, bringing problems like unsafe coastal flooding. "These four new records that have been published in recent years seem to fix a lot of problems that were plaguing the old records, and now they seem to agree quite well with what the climate models have produced".
'It´s mainly driven by the accumulation of greenhouse gases such as carbon dioxide in the atmosphere due to human activities, ' said Lijing Cheng, a lead author of the study from the Chinese Academy of Sciences.
This study was supported by the National Key R&D Program of China (2017YFA0603202).