SAN FRANCISCO -- The Earth's oceans are absorbing more and more heat from a warming world, and the pace of the temperature rise is increasing with each passing decade, researchers have discovered.
In the past two decades alone, the oceans have taken up more than half the increase in heat created by the world's outpouring of greenhouse gases during the entire industrial era, scientists at the Lawrence Livermore National Laboratory reported Monday following a study of ocean temperature records dating back more than a century.
Most of that heat has been absorbed in the upper 2,300 feet of the oceans, but an ever greater amount is reaching into deeper ocean levels -- a mile and more down, said Peter J. Gleckler, a Livermore physicist and climate scientist who led the research.
"We're now seeing that more and more of the heat from global warming is going into the deeper layers of the oceans," Gleckler said in a telephone interview.
Among the warming influences carrying heat into the oceans are receding sea ice in the Arctic, melting glaciers over much of Greenland and collapsed ice shelves in Antarctica, the researchers said.
"The heat capacity of the ocean system is huge," said Paul J. Durack, a Livermore oceanographer on the research team. "And that melting land-based ice adds to the total mass of the warming ocean. As the oceans absorb more heat they expand and become a major cause of the sea-level rise we're already seeing."
The researchers conceded in their report that the environmental effects of the increasing heat at deeper ocean levels is poorly understood, if at all. And as a physicist and climate modeler, Gleckler said in the interview that he cannot predict what those effects might be.
Some data in the new research comes from computer models of the ocean's response to the changing heat output generated by the world's greenhouse gases. But much of it has come from an international project called Argo that is financed by more than 30 countries and has been gathering data over the past decade from an array of more than 4,000 robotic floats deployed in every ocean in the world.
Each robot hovers at about 3,300 feet deep and periodically dives more than a mile down before surfacing to transmit temperatures, salinity and changing ocean currents to shore stations around the world.
Another climate research team led by Dean Roemmich at the Scripps Institute of Oceanography in La Jolla has also used the Argo floats to study how heat produced on the Earth's surface by greenhouse gases is absorbed into the oceans.
Roemmich's group, separately from Gleckler's team, has estimated from the data that the oceans' heat may be increasing only by thousandths of a degree Fahrenheit per year, but that the "warming signal" extends down beyond 6,500 feet. It is most noticeable in the oceans of the Southern hemisphere, the researchers said.
The oceans below that depth remain largely unknown, and the Argo floats cannot dive there, but scientists at the 30 nations -- including the United States -- that support the $25 million Argo project are already testing deeper floats that could sample temperature, salinity and ocean currents down to depths of more than 31/2 miles.
"It's already one of the great international successes in measuring the world's climate system," Durack said.
"The Argo floats have been providing us with global heat coverage since 2005, and they've greatly improved our confidence in the models we've been testing and using," Gleckler said.
Another valuable source of information on the heat of the oceans came to the Livermore researchers from the voyage of H.M.S. Challenger around the world from 1872 to 1876, before the industrial era.
That epochal scientific expedition sailed nearly 80,000 miles, and its hundreds of thermometer records taken by sailors from the surface down to nearly 27,000 feet are part of the Gleckler group's report.
The Gleckler team's report was published Monday in the journal Nature Climate Change, and the Roemmich group's report was published earlier in the same journal.
The Livermore team's colleagues include Ronald J. Stouffer of Princeton University, Gregory C. Johnson of the NOAA/Pacific Marine Environmental Laboratory in Seattle, and Chris E. Forest of Pennsylvania State University.