Tung and Chen noticed that the North Atlantic's heat content (a measure of stored energy) shifted in 1999, about when the hiatus began. The ocean started absorbing heat at depths below 984 feet (300 m). (The South Atlantic Ocean also took up some heat.) These regions stored more heat energy than the rest of the world's oceans combined, even the enormous Pacific Ocean, the researchers' temperature data show.
So how does the Atlantic cool an entire planet? The likely culprit is a natural climate cycle linked to the Atlantic Meridional Overturning Circulation (AMOC) current, Tung said. The AMOC is part of a worldwide ocean conveyor belt. Here's how the AMOC works: In the North Atlantic, salty tropical water flowing north cools off and sinks. This water, dense because it is cool and salty, heads south toward the equator, then eventually rises again in the South Atlantic. When the water sinks, it traps heat in the ocean depths. Ocean surface temperatures drive the current: fast when cold, slow when warm. [Images: The World's Biggest Oceans and Seas]
Between 1945 and 1975, the cycle was in a cool phase, sucking up atmospheric heat at a rapid pace. Toward the end of this cycle, in the 1970s, scientists noticed a suspected "global cooling" that was touted as the beginning of a possible Ice Age. But then the AMOC flipped to warming, corresponding to the rapid uptick in global temperatures. Finally, in 1999, the current switched back to a cold, speedy plunge into the ocean depths, taking extra heat along with it.
Such natural cycles make global warming look more like a staircase than a steady rise in temperatures, Tung said. "Right now, we're on the flat part of the staircase. We still have a few more years of the hiatus."
However, others scientists remain convinced the Pacific plays an important role in the global warming hiatus. Several recent studies affirm the link between the pause and the changes in the Pacific. An Aug. 3 study in the journal Nature Climate Change found that faster trade winds over the Pacific bring up cold water and cool the atmosphere. An Aug. 17 study, also in Nature Climate Change, suggested the Pacific Decadal Oscillation climate cycle might be responsible for the hiatus. That cycle flips every 20 to 30 years.
"I still think the Pacific Ocean is playing the lead role in this ocean heat uptake, but this study is important as it points to an additional role from the Atlantic and Southern Oceans," said England, who co-authored the Aug. 3 Nature Climate Change study.
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