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Why is global warming slowing down? Atlantic and Southern oceans hold the answer

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Scientists around the globe are in agreement that global warming or temperature on Earth has not increased at the rate that was expected by many. It remains a debatable question what caused such a halt and a new theory suggests that Atlantic and Pacific oceans transported surface heat of the seas to deeper parts of the oceans.

The unexpected slowdown in the rise of global temperature is sometimes referred as the global warming hiatus or global warming pause and scientists have derived a number of theories that explain such a pause. Now, scientists at the University of Washington (UW) have come up with a new theory, which hints at a global warming hiatus.

Ka-Kit Tung, a professor of applied mathematics and assistant faculty member in atmospheric sciences at UW, who is also the corresponding author of the study, explains that by using Argo floats, the researchers examined deep sea water temperatures at a depth of 6,500 feet or 2,000 meters. The researchers found that the heat was sinking in deep waters from around 1999, from when scientists believe that the global warming hiatus started.

Tung explains that there are natural current cycles in the Atlantic and Southern oceans that are salinity-driven, which stores heat deep in the oceans.

"After 30 years of rapid warming in the warm phase, now it's time for the cool phase," says Tung.

The researchers cite previous studies that suggest that temperature increases in the later part of the 20th century was partly due to global warming and partially because of the Atlantic Ocean cycle, which kept heat close to the surface. However, from around 2000, the cycle changed and started to gobble up surface heat and transferred it to the deeper regions of the ocean and worked to offset the warming caused by human activities.

The scientists suggest that the cycle begins when denser and saltier water at the surface of northern Atlantic Ocean results in the sinking of water. The sinking of water also alters the speed of the Atlantic Ocean current, which circulates heat throughout Earth.

The researchers reveal that by examining previous data, they found that the Atlantic and Southern oceans witnessed a similar phenomenon between 1945 and 1975. Looking at historical data, the researchers suggest that there's a period of 30 warmer years followed by 30 cooler years. Some scientists believe that the current global warming hiatus may continue for a decade or more before rapid warming starts again. Tung says even though the latest theory suggests the arrival of rapid warming in the near term, it cannot be predicted what will happen next.    

The study has been published in the online journal Science.

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