Man-made heat has raised ocean temperatures, twice of what was experienced 150 years ago, researchers have found.
"Since the 1990s, the total amount of heat content change in the oceans is twice that of what we'd seen up until that point in the past 150 years," says Penn State meteorology professor Chris Forest.
In the 18 years since 1997, the oceans have absorbed the same amount of energy — about 150 zetajoules — as they did between 1865 and 1997, researchers report in the journal Nature Climate Change.
Scientists have long been aware that 90 percent of all man-made heat energy ends up in the globe's oceans. For the latest study, they used historical data going back to British research ships in the 1870s. They used high-tech monitors in seas around the world and sophisticated computer models to track the changes.
While the majority of the additional heat going into the oceans is trapped in the top 2,300 feet, that's changing, researchers say.
"Over the past few decades the ocean has continued to warm substantially, and with time the warming signals are reaching deeper into the ocean," says study lead author Peter Gleckler, a scientist at the Lawrence Livermore National Laboratory in California.
What is most worrying is the rate at which the warming is increasing, which is accelerating the rate at which energy is trapped in planet's climate system as a whole, he says.
The vast extent of the world's oceans means the absorbed heat is raising temperatures by just a few tenths of a degree, but what is important is the effect on the globe's energy balance, researchers say.
Every bit of temperature rise decreases the amount of heat the oceans can absorb, meaning more and more heat stays in the atmosphere and on land, the researchers explain.
The result is changes both at sea and on land, they say.
"These findings have potentially serious consequences for life in the oceans as well as for patterns of ocean circulation, storm tracks and storm intensity," says Jane Lubchenco, who was former chief of the National Oceanic and Atmospheric Administration.
The bottom line, experts say, is that the study's findings are hard evidence that human activity is dramatically heating the planet.