Scientists on Tuesday reported having unlocked the mystery of a slowdown—a false pause— in global warming since the late 20th century, saying the answer lies in the Indian Ocean.

Until now, climate scientists believed the slowdown, which has been observed since 1998, was related to declines in surface temperature of Pacific Ocean, and tied to a prevalence of La Nina climate conditions.

In a paper published in the Nature Geoscience Journal on Tuesday, Sang-Ki Lee, an oceanographer at the University of Miami, and his colleagues said when they tried to track the heat missing from the atmosphere in the oceans, they did not find it in the Pacific Ocean.

Hydrographic records indicated that the Pacific Ocean’s heat content had been decreasing, said the scientists, who then analysed their observations along with simulations from a global ocean–sea ice model to track the pathway of heat.

“We find that the enhanced heat uptake by the Pacific Ocean has been compensated by an increased heat transport from the Pacific Ocean to the Indian Ocean, carried by the Indonesian throughflow," the paper said.

The scientists reported that the Indian Ocean heat content has risen sharply, accounting for more than 70% of the global ocean heat gain in the upper 700 metres of the Indian Ocean over the past decade.

The scientists conclude the Indian Ocean has become increasingly important in altering global climate variability.

“This finding is consistent with our own work (and related previous work). We found a tendency for colder sea surface temperatures in the eastern Pacific, warmer sea surface temperatures in the western Pacific and Indian, and increased heat burial below the surface in the western Pacific and Indian," Michael Mann, director of the Earth System Science Center at Pennsylvania State University, said in an email.

Mann explained that while the authors emphasize the Indian Ocean heat burial, this finding is broadly consistent with their own work and other recent studies that have identified La Nina-like conditions over the past decade for the slowdown in surface warming.

The slowdown in global warming is often mistaken for or referred to as a hiatus, but the warming has continued—albeit at a slower rate over the past decade. In fact, the UN’s World Meteorological Organization last year warned in a report that the warming of oceans has accelerated, and is happening at lower depths.

“More than 90% of the excess energy trapped by greenhouse gases is stored in the oceans," the UN report had observed.

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