WASHINGTON, Aug. 21 (Xinhua) -- Researchers from the United States and China on Thursday explained why the rapid global warming in the late 20th century has been slowing down over the last 15 years.
The researchers reported in the U.S. journal Science that the so-called global warming hiatus may be mainly due to a massive movement of heat from shallow surface waters to deep regions of the Atlantic and Southern Oceans.
Over a dozen theories have been proposed for the slowdown, ranging from air pollution to volcanoes to sunspots, but they could not explain the massive amount of heat missing for more than a decade, they said.
"Every week there's a new explanation of the hiatus," corresponding author Ka-Kit Tung, a professor of applied mathematics at the University of Washington, said in a statement.
"Many of the earlier papers had necessarily focused on symptoms at the surface of the Earth, where we see many different and related phenomena. We looked at observations in the ocean to try to find the underlying cause."
Tung and co-author Xianyao Chen of the Ocean University of China used recent observations of deep-sea temperatures from a network of devices called Argo floats that sample the water down to 2,000 meters.
The data showed an increase in heat sinking in Atlantic and Southern Oceans around 1999, when the rapid warming of the 20th century stopped.
"The finding is a surprise, since the current theories had pointed to the Pacific Ocean as the culprit for hiding heat," Tung said. "But the data are quite convincing and they show otherwise."
The researchers also suggested that a sudden shift in salinity may have triggered this migration of heat to deeper waters.
Recent observations at the surface in the North Atlantic find record-high saltiness, Tung said, while at the same time, deeper water in the North Atlantic shows increasing amounts of heat.
"When it's heavy water on top of light water, it just plunges very fast and takes heat with it," Tung said.
As a result, a slow-moving current in the Atlantic, which carries heat between the two poles, sped up earlier this century to draw heat down almost 1,500 meters, the researchers said.
The researchers also dug up historical data to show that there was another hiatus between 1945 and 1975, which caused people to worry about the start of an Ice Age.
Earlier records in Central England show the 40- to 70-year cycle goes back centuries, and other records show it has existed for millennia.
The researchers concluded that changes in Atlantic Ocean circulation meant roughly 30 warmer years followed by 30 cooler years.
"There are recurrent cycles that are salinity-driven that can store heat deep in the Atlantic and Southern oceans," Tung said. " After 30 years of rapid warming in the warm phase, now it's time for the cool phase."
Since similar events have lasted 30 years historically, the researchers suggested that the current slowdown in global warming could last for another 15 years, then rapid warming will return.
The research was funded by the U.S. National Science Foundation and the National Natural Science Foundation of China.