Where, oh where, is the planet's missing heat?
In 1999, the feverish rise in Earth's surface temperatures suddenly slowed, even as greenhouse gas emissions escalated. This unexpected slowdown has been called a global warming hiatus or global warming pause. Most climate scientists don't think this hiatus means global warming went kaput, but the reason (or reasons) for the slowdown has scientists flummoxed. Researchers have offered more than two dozen ideas to explain the missing heat.
Now, a study published today (Aug. 21) in the journal Science suggests a natural climate cycle in the North Atlantic Ocean gobbled Earth's extra heat. While the study is unlikely to settle the scientific debate, it does support the idea that Earth's global warming continues in the ocean, even when air temperatures stay flat.
"It's important to distinguish between whether ocean heat storage is responsible for the hiatus versus not enough heat reaching the surface of the Earth," said study co-author Ka-Kit Tung, of the University of Washington in Seattle. "We did find enough heat stored in the North and South Atlantic that, if it had remained on the surface, it would have resulted in rapid warming." [Infographic: Earth's Tallest Mountain to Deepest Ocean Trench]
Scientists have blamed the oceans for the global warming pause before, but they pointed their fingers at the Pacific, not the Atlantic. However, in seeking to test this idea with temperature data, oceanographer Xianyao Chen, of the Ocean University of China in Qingdao, and Tung, an atmospheric scientist, said they couldn't find the missing heat in Pacific Ocean temperature measurements.
"If these models are true, we should be able to find the missing heat, and under the Pacific we couldn't find enough heat to explain the hiatus," Tung told Live Science.
Tung and Chen then searched ocean by ocean until they hit on the North Atlantic, where the heat was playing hooky. The pair primarily relied on Argo floats, which record ocean temperature and salt content down to 6,560 feet (2,000 meters). These worldwide floats reached their most comprehensive levels beginning about 2005. Other records from floats, ships and buoys filled in the timeline since 1970.
But the millions of data points don't conclusively prove that the North Atlantic Ocean is devouring heat. "Unfortunately, the massive array of ocean temperature measurements by Argo floats has only been made after the early 2000s, just when the present hiatus in surface warming was starting," said Matthew England, a climate scientist at the University of New South Wales in Australia, who was not involved in the study. "So being conclusive about each ocean basin is limited by data availability."