Off the east coast of Greenland, where these icebergs are, salty water sinking into the deep may carry atmospheric heat with it—helping to slow global warming.
Has the Atlantic Ocean Stalled Global Warming?
New research suggests that heat trapped by atmospheric greenhouse gases is getting buried in the Atlantic.
PUBLISHED AUGUST 21, 2014
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.
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.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.
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.
Transporting Heat
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.
Watch: How Carbon Dioxide Kills Ocean Life
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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"What Is The Ocean Conveyor?
Woods Hole Oceanographic Institution
Woods Hole Oceanographic Institution
The deeper researchers dig into the dramatic slowdown in global warming during the past 15 years, the more their findings point to natural climate variability as the cause – rather than any purported end to human-triggered climate change.
The latest example of this conclusion comes from a pair of scientists from the United States and China trying to identify where the expected heat has been hiding.
They attribute the slowdown in warming, as others do, to heat taken up by the world's oceans, especially at depths below about 2,300 feet.
But the duo argues that, in a case of "right church, wrong pew," other studies have mistakenly placed the uptake largely in the western tropical Pacific.
Instead, the Atlantic and southern oceans have done the heavy lifting, say Xianyao Chen of the Ocean University of China and Ka-Kit Tung of the University of Washington.
They base their conclusion on tens of millions of measurements of ocean heat content gathered between 1970 and 2013, from the surface to depths of 5,000 feet. Their findings will be published Friday in the journal Science.
"Oceanographers have long known that the Atlantic and southern oceans are more efficient at storing heat than the Pacific," Dr. Tung says. Based on the measured distribution of heat, trade winds alone – which have blown at unusually high levels during the hiatus – aren't powerful enough to drive warm water very deep, he says.
By contrast, in the Atlantic, the heat is carried north as part of a powerful current system known as the Atlantic thermohaline circulation. The north-flowing Gulf Stream is the most visible manifestation of this circulation.
By the time it reaches the far North Atlantic, the dense, salty water has cooled and sinks. It plunges toward the seafloor and heads south at depth, retaining some of the heat it accumulated on the surface.
Tung and others have identified what they see as multi-decadal swings in the intensity of the circulation, driven by changes in the water's salinity. Fresh water is more buoyant than its salty counterpart, so when the water freshens because of changes in rainfall or snowfall patterns or because of melting Arctic ice, the circulation slows.
Tung and Dr. Chen note that a slight cooling period in global average temperatures in the mid-20th century – another pause in the longer-term warming trend – coincided with a fast phase in the Atlantic's thermohaline circulation. Indeed, plots of global average temperatures since about 1910 give climate change a stair-step look, when the current hiatus in warming is factored in.
Their conclusions, de-emphasizing the Pacific’s role, differ from others, Tung says, because he and his partner focused on measurements instead of on models or model-measurement blends. For instance, models used in some studies reproduce the variability that the duo sees in the Atlantic, but it's weaker and the swings occur more frequently than they do in the real world.
Some other studies have acknowledged an important role for the Atlantic and southern oceans as heat sinks. In a paper on the warming hiatus published on Sunday, a team led by Kevin Trenberth, a researcher with the National Center for Atmospheric Research in Boulder, Colo., noted that heat had been driven to depths of 2,300 feet or lower in those oceans.
Still, Dr. Trenberth's team concludes, the Pacific has been the center of the heat-sink action, driven largely by a natural climate swing known as the Pacific Decadal Oscillation.
Coinciding with the hiatus, the PDO has been in a phase that intensified trade winds blowing west across the Pacific. They draw warm water with them to pile up deep beneath the western Pacific, even as the winds pull deeper, colder water to the surface in the eastern Pacific.
Trenberth is not convinced that Chen and Tung have it right.
The new paper adds nice details on the heat content of different oceans, writes Trenberth in an e-mail. But, he adds, the study fails to acknowledge the long-range effect that conditions in the tropical Pacific can have on the Atlantic, nor does it reflect an understanding of the effects that those long-range "teleconnections" have on heat and water.
"What they have is not really new," he says.
Others say they see an influence from the Atlantic different from the one that Chen and Tung emphasize. In a paper published in early August, a team led by Shayne McGregor, a researcher at the University of New South Wales in Australia, reports the results of a modeling study showing how nearly 20 years of unusually warm water in the Atlantic have contributed to the unusually strong trade winds in the Pacific. In essence, air was rising over the warm waters, setting up a region of low pressure over the Atlantic and a region of falling air – high pressure – over the eastern Pacific.
"This pressure difference intensifies the equatorial Pacific trade winds, which in turn suck up colder water in the eastern Pacific and pile up warm water in the western Pacific," explains University of Hawaii oceanographer Axel Timmermann, a member of the team conducting the study.
Chen and Tung, for their part, say the hiatus in measured global warming could persist for a while longer – the effect of natural variability superimposed over the long-term warming trend.
Each phase of the variations they see in the Atlantic's thermohaline circulation lasts on average about 30 years. So Chen and Tung say the pause should last another 10 to 15 years before warming accelerates again, even if a long-predicted El Niño weather pattern in the Pacific occurs over the next 12 months.
El Niño years tend to be warmer than normal globally. Based on studies indicating that the missing heat has been pooled beneath the western tropical Pacific, many scientists have posited that the next El Niño could be a monster as trade winds relax and the large pool of warm water sloshes back eastward.
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