Temperatures at Earth's surface aren't rising as fast as they did in the 1990s, even though the amount of heat-trapping greenhouse gases in the atmosphere continues to increase steadily. This apparent hiatus in global warming has been fodder for skeptics—but among climate scientists, it has sparked a search for the "sink" that is storing all the missing atmospheric heat.
Locating that sink matters, because it could tell researchers how long our current hiatus might last, says Ka-Kit Tung, an atmospheric scientist and applied mathematician at the University of Washington in Seattle.
In this week's issue of Science, Tung and Xianyao Chen at Ocean University of China in Qingdao suggest that much of the missing heat has gone into the deep waters of the Atlantic Ocean. This is in contrast to a previous study that argued the heat went into the Pacific Ocean.
The two different sinks correspond to two different mechanisms for transporting heat in and out of the ocean. The earlier study suggested that intensifying trade winds, related to the El Niño-La Niña cycle, were sinking heat into relatively shallow layers of the Pacific. If that's true, then the current hiatus could be over as soon as the next El Niño, says Tung.
But if most of it is ending up deeper in the Atlantic Ocean, then we may have another 10 to 15 years before global warming resumes with its previous intensity. That could buy us a little more time to deal with the problem—but also give skeptics more ammunition.
Unlike some of the previous studies, which relied on computer models, Tung and Chen also used observations from an oceanwide network of sensors to locate the missing heat. The sensors included floats that dive nearly a mile (1,500 meters) beneath the surface to measure the temperature and salt content of the water.
The researchers found that warm temperatures penetrated much deeper in the Atlantic Ocean and in the waters surrounding Antarctica than in the Pacific or Indian Oceans. This heat storage deep in the Atlantic, below about a thousand feet (300 meters), is what has allowed the current pause in rising average surface temperatures globally, Tung says.
The researchers suggest that a global system of ocean currents known as the conveyor belt has been responsible for burying heat in the Atlantic. At the ocean surface, the conveyor transports warm, salty water from the Caribbean to the North Atlantic. North of Iceland, the water becomes colder and saltier, and so dense that it sinks into the deep, where it flows south again.
This sinking branch of the conveyor is what transports heat into the deeper Atlantic Ocean, the researchers write—and during the mid-to-late 1990s, it started to accelerate. For reasons that still aren't clear, the saltiness of the Atlantic water varies cyclically. When the water is saltier, it sinks faster and carries more heat into the deep.
The changes operate on a roughly 30-year cycle, says Tung. The current hiatus has lasted about 15 years. Since 2006, Tung adds, the conveyor belt has been slowing. But for now, the amount of heat being drawn out of the atmosphere is still enough to allow the global warming hiatus to persist.
A Murky Picture
There is some heat going into the Atlantic, writes Kevin Trenberth, a researcher at the National Center for Atmospheric Research (NCAR) in Boulder, Colorado, in an email. But Trenberth—who was not involved in the current study—disagrees with how it's getting there.
Trenberth argues that processes in the Pacific Ocean drive changes in the North Atlantic current. The same basic mechanism that may drive heat into the Pacific—intense trade winds that pile up warm water in the western Pacific—has large ripple effects on the atmosphere.
Those ripples influence jet streams, or currents of air flowing through the atmosphere, across the U.S., and over the North Atlantic Ocean. And those atmospheric currents can drive changes in ocean currents.
Josh Willis, a climate scientist at the Jet Propulsion Laboratory in Pasadena, California, isn't entirely convinced by the new study either. "I think this is an interesting hypothesis," he says. But he would like to see a more detailed look at the ocean's heat content using other observational data—not just the information collected by one network of instruments.
Satellite measurements of sea-surface height could be one avenue, Willis says. "When the ocean warms up, seawater expands," he explains. That expansion changes the height of the ocean, which certain satellites in orbit are sensitive enough to pick up.
The Hiatus Will End
It's important to note that a pause in rising temperatures doesn't mean global warming isn't happening, writes Gerald Meehl, a senior scientist at NCAR, in an email. "Global warming hasn't stopped, it has temporarily shifted to the subsurface ocean," says Meehl, who first proposed that the Atlantic Ocean was storing some of the missing heat.
Indeed, it's just a matter of time before this heat is reflected in atmospheric temperatures, says Tung. If this 30-year cycle holds, we're starting to climb out of the current pause, he explains.
"The frightening part," Tung says, is "it's going to warm just as fast as the last three decades of the 20th century, which was the fastest warming we've seen." Only now, we'll be starting from a higher average surface temperature than before.
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