The world's oceans are heating up at an accelerating pace and faster than previously estimated, setting a new temperature record in 2018 as global warming threatens a diverse range of marine life, scientists have warned.

New measurements, aided by an international network of 3,900 floats deployed in the oceans since 2000, showed more warming since 1971 than calculated by the latest UN assessment of climate change in 2013, the researchers said.

The findings published on Thursday in the US journal Science, led by the Chinese Academy of Sciences, debunk previous reports that suggested a so-called pause in global warming in recent years.

"Global warming is here, and has major consequences already. There is no doubt, none!" the authors wrote in a statement.

Man-made greenhouse gas emissions are warming the atmosphere, according to the overwhelming majority of climate scientists, and a large part of the heat gets absorbed by the oceans. That in turn is forcing fish to flee to cooler waters.

"Ocean heating is a very important indicator of climate change, and we have robust evidence that it is warming more rapidly than we thought," said co-author Zeke Hausfather, a graduate student in the Energy and Resources Group at the University of California, Berkeley.

About 93 percent of excess heat - trapped around the Earth by greenhouse gases that come from the burning of fossil fuels - accumulates in the world's oceans.

The latest report relied on four studies, published between 2014 and 2017, that gave more precise estimates of past trends in ocean heat, allowing scientists to update past research and hone predictions for the future.

A key factor in the more accurate numbers is an ocean monitoring fleet called Argo, which includes nearly 4,000 floating robots that "drift throughout the world's oceans, every few days diving to a depth of 2,000 metres and measuring the ocean's temperature, pH, salinity and other bits of information as they rise back up", said the report.

Argo "has provided consistent and widespread data on ocean heat content since the mid-2000s", it said.

Record temperature

The new analysis shows warming in the oceans is on pace with measurements of rising air temperature.

And if nothing is done to reduce greenhouse gases, "models predict that the temperature of the top 2,000 metres of the world's oceans will rise 0.78 degrees Celsius by the end of the century", it said.

The thermal expansion - water swelling as it warms - would raise sea level 30cm, above any sea level rise from melting glaciers and ice sheets.

"While 2018 will be the fourth warmest year on record on the surface, it will most certainly be the warmest year on record in the oceans, as was 2017 and 2016 before that," Hausfather said.

"The global warming signal is a lot easier to detect if it is changing in the oceans than on the surface."

Lead author Lijing Cheng, of the Institute of Atmospheric Physics at the Chinese Academy of Sciences, said records for ocean warming had been broken almost yearly since 2000.

Overall, temperatures in the ocean down to 2,000 metres rose about 0.1C from 1971-2010, he told Reuters news agency. The 2013 United Nations assessment estimated slower rates of heat uptake but did not give a single comparable number.

Almost 200 nations plan to phase out fossil fuels this century under the 2015 Paris climate agreement to limit warming. President Donald Trump, who wants to promote US fossil fuels, plans to pull out of the pact in 2020.

A separate study on Monday, by the European Union's Copernicus Climate Change Service, said 2018 was the fourth warmest year for global surface temperatures in records dating back to the 19th century.

Ocean temperatures are less influenced by year-to-year variations in the weather. It can take more than 1,000 years for deep ocean temperatures to adjust to changes at the surface.

Among effects, extra warmth can reduce oxygen in the oceans and damages coral reefs that are nurseries for fish, the scientists said.

Warmer seas release more moisture that can stoke more powerful storms.

Warmer ocean water also raises sea levels by melting ice, including around the edges of Antarctica and Greenland.