A slowdown in global warming that has scientists puzzled and led to considerable debate among climate researchers could go on for at least another decade, experts say.
Beginning in the late 1990s, the rate of world temperature rises slackened even as levels of greenhouse gases increased, in a phenomenon that has been dubbed a "global warming pause."
A number of theories have been proposed in an attempt to explain it; now researchers are suggesting a naturally occurring cycle of Atlantic Ocean currents that takes about 30 years to complete may have hit the "pause" button on warming.
The cycle is known as the Atlantic Meridional Overturning Circulation current.
The heat may have gone into the ocean, the researchers suggest, citing evidence that the 30-year Atlantic current cycle alternatively warms and then cools the globe by pulling large amounts of heat from the atmosphere into the ocean depths.
That suggests that global warming has not really eased, but rather is ongoing in the world's oceans while the temperature of the atmosphere has temporarily stabilized, they say.
"It's important to distinguish between whether ocean heat storage is responsible for the hiatus versus not enough heat reaching the surface of the Earth," says study co-author Ka-Kit Tung of the University of Washington. "We did find enough heat stored in the North and South Atlantic that, if it had remained on the surface, it would have resulted in rapid warming."
The researchers said they based their conclusions on data gathered by a grid of ocean-sampling devices known as Argo floats capable of monitoring ocean temperatures down to as deep as 6,500 feet.
Historical records show heat being pulled down into the ocean caused a similar warming "hiatus" from about 1945 to 1975, the researchers said, after which the cycle of currents reversed and caused a warming trend that lasted until around the year 2000.
Since then, another "flip" has once again drawn heat down into the ocean, holding atmospheric temperatures at the level recorded in 1998.
Scientist had suggested theories and models of an ocean-induced pause in global warming before, but had considered the Pacific Ocean the prime suspect, something not borne out by temperature measurements there.
"If these models are true, we should be able to find the missing heat, and under the Pacific we couldn't find enough heat to explain the hiatus," Tung says.
Tung and his colleagues began to look elsewhere, finally determining that the AWOL heat was in fact disappearing into the Atlantic.
Temperature data from there showed Atlantic regions were storing more heat energy than the total going into all the rest of the world's oceans combined, they said.
