Many theories have been suggested to explain why the Earth is currently in a global warming hiatus, from volcanoes to air pollution to sunspots. But new research indicates that this missing heat is trapped deep in the Atlantic Ocean.
Scientists from the University of Washington (UW) say that heat from the surface is plunging nearly a mile down in the north and south Atlantic, affecting the balance between incoming heat and heat radiated back into the atmosphere.
Despite the fact that greenhouse gases are trapping more solar heat at Earth's surface, we are currently in a global cooling period, which researchers say can last for more than a decade.
The study findings were published in the journal Science.
"Every week there's a new explanation of the hiatus," corresponding author Ka-Kit Tung, a UW professor of applied mathematics, said in a statement. "Many of the earlier papers had necessarily focused on symptoms at the surface of 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 observed deep-sea temperatures from Argo floats that sample the water down to 6,500 feet (2,000 meters). They found that fluctuations in surface temperature are part of a cycle involving ocean circulation and its response to changes in density of the Atlantic Ocean's upper layers.
The cycle starts when saltier, denser water at the surface northern part of the Atlantic, near Iceland, causes the water to sink. This changes the speed of the huge current in the Atlantic Ocean that circulates heat throughout the planet.
"When it's heavy water on top of light water, it just plunges very fast and takes heat with it," Tung explained.
Changes in Atlantic Ocean circulation historically meant roughly 30 warmer years followed by 30 cooler years. Now that it is happening on top of global warming, the authors say the trend looks more like a staircase.
"After 30 years of rapid warming in the warm phase, now it's time for the cool phase," Tung said.
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