Multiple oceans may help stall global warming

The Atlantic and Southern oceans may be covering up global warming by hoarding heat. The finding could explain a puzzling plateau in Earth’s surface temperatures that many scientists have blamed on the Pacific Ocean.

Since the turn of the century, global average surface temperatures have remained flat despite an unabated rise in greenhouse gas emissions. Scientists have developed several theories to account for the lost heat, including that it is getting trapped in the oceans. Using climate simulations, many studies have pointed to the Pacific Ocean, where unusually strong trade winds may have shoved warm water deep beneath the surface (SN: 3/22/14, p. 12; SN: 10/5/13, p. 14).

The new research, however, suggests that the Pacific plays only a minor role. Instead, the study finds that the Atlantic Ocean and the Southern Ocean, which surrounds Antarctica, are stashing most of the warmth. The authors — atmospheric scientist Ka-Kit Tung and oceanographer Xianyao Chen, both at the University of Washington in Seattle — speculate that cyclical changes in salinity and water circulation can account for the heat grab. Their results appear in the Aug. 22 Science.

“I still think the Pacific Ocean is playing the lead role in this ocean heat uptake,” says climate scientist Matthew England of the University of New South Wales in Sydney. But he says the study is important for pointing out that the Atlantic and Southern oceans are also involved.

Tung and Chen used measurements from global ocean monitoring systems and climate simulations to estimate the inventories of heat in the world’s oceans between 1970 and 2012. Since 1999, they found, the oceans as a whole have amassed more heat in deep waters than in previous decades, supporting the idea that salty seas are masking global warming. Of the extra heat, the researchers found that nearly all of it sunk into the Atlantic and Southern oceans, with each claiming nearly half the surplus.

Though it’s unclear why the Atlantic and Southern oceans may be taking up extra heat, the researchers think a natural phenomenon may explain how the Atlantic does it. The Atlantic naturally acts as a conveyor belt for heat, Tung says, moving warm waters from the tropics northward. In hot, shallow pools of the tropics, evaporation leaves behind saltier — and denser — water. When that warm, dense water travels north and meets colder, less salty water, it sinks, he says. “And when it goes down it brings the heat along with it.” That cycle of heat stashing may currently be stuck on high speed, he says. The conveyor belt could stay that way for a few decades, Tung says, but it will eventually slow down and release heat back into the atmosphere, resuming global warming.  

But the theory may be an oversimplification, says oceanographer Igor Polyakov of the University of Alaska Fairbanks. The waters of the North Atlantic are complex and experience other short-term temperature variations and localized cooling cycles, he says, which aren’t explained by the conveyor belt idea. “We need to go a long way,” he says, “before we will be able to provide a detailed description of the mechanisms driving changes in the oceans and global climate system.”