The heat trapped by greenhouse gases is raising ocean temperatures faster than previously thought, according to new research showing that previous claims of a global warming deceleration or "gap" in the last 15 years were unfounded.
Ocean warming is a critical marker of climate change as it is estimated that 93 percent of excess solar energy trapped by greenhouse gases accumulates in the world's oceans.
Unlike surface temperatures, ocean temperatures are not affected by year-on-year variations caused by weather events such as El Niño or volcanic eruptions.
"If you want to see where global warming is happening, look at our oceans," said Zeke Hausfather, a graduate student at the University of California (UC), Berkeley, USA.
"Ocean warming is a very important indicator of climate change, and we have strong evidence that it is heating up faster than we thought," Hausfather said.
The new analysis, published in the journal Science, shows that trends in the heat content of the oceans correspond to those predicted by major climate change models and that global warming is accelerating.
Assuming a business as usual scenario in which no effort was made to reduce greenhouse gas emissions, the Coupled Model Intercomparison Project 5 (CMIP5) predicts that the temperature of the world's 2,000 meters of oceans will increase by 0 , 78 degrees Celsius by the end of the century.
The thermal expansion caused by this increase in temperature would raise sea levels by 30 centimeters beyond the already significant rise in sea level caused by the melting of glaciers and ice sheets. Warmer oceans also contribute to stronger storms, hurricanes and extreme precipitation.
"While 2018 will be the fourth hottest year on the surface, it will certainly be the hottest year ever recorded in the oceans, as it was 2017 and 2016 before that," said Hausfather.
"The global warming signal is much easier to detect if it is changing in the oceans than on the surface," he said.
Four studies, published between 2014 and 2017, provide better estimates of past trends in ocean heat content by correcting discrepancies between different types of ocean temperature measurements and better explaining gaps in measurements over time or location.
"The Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC), published in 2013, showed that major climate models seem to predict a much faster increase in ocean heat content in the last 30 years than observed observations," said Hausfather.
A fleet of nearly 4,000 floating robots roams the world's oceans, every few days plunging to a depth of 2,000 meters and measuring ocean temperature, pH, salinity and other information as they rise.
This ocean monitoring battalion, called Argo, provided consistent and widespread data on ocean heat content since the mid-2000s.
Prior to Argo, ocean temperature data were scant at best, using devices called disposable ultrasound that sank to depths only once, transmitting data on ocean temperature until they settled in aquatic burials.