The oscillations have a natural switch. During the warm period, faster currents cause more tropical water to travel to the North Atlantic, warming both the surface and the deep water. At the surface this warming melts ice. This slowly makes the surface water there less dense and after a few decades puts the brakes on the circulation, setting off a 30-year cooling phase.
The authors dug up historical data to show that the cooling in the three decades between 1945 to 1975 – which caused people to worry about the start of an ice age – was during a cooling phase. (It was thought to have been caused by air pollution.) Earlier records in Central England show the 40- to 70-year cycle goes back centuries, and other records show it has existed for millennia.
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, however, the trend looks more like a staircase.
This explanation implies that the current slowdown in global warming could last for another decade, or longer, and then rapid warming will return. But Tung emphasizes it’s hard to predict what will happen next.
A pool of freshwater from melting ice now sitting in the Arctic Ocean, for example, could overflow into the North Atlantic to upset the cycle.
“We are not talking about a normal situation because there are so many other things happening due to climate change,” Tung said.
The research was funded by the U.S. National Science Foundation and the National Natural Science Foundation of China.
Check the following link to read/download the Full Study – “Varying Planetary Heat Sink Led to Global Warming Slowdown and Acceleration”:
http://www.sciencemag.org/content/345/6199/897
Source: University of Washington.
