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article imageWhat has happened to global warming?

By Karen Graham     Aug 22, 2014 in Environment
There has been an apparent hiatus in global warming, much to the delight of the climate-change naysayers. Climate scientists are perplexed over the slowdown in rising surface temperatures, even though atmospheric greenhouse gases continue to increase.
There may be an explanation for the stall in rising surface temperatures. New research from the University of Washington purports to show where all that heat has been going for the last decade or so. There have been dozens of theories proposed as an explanation for the halt in rising temperatures, from sun-spots to volcanic eruptions or trade winds.
In a study published in the August 22 issue of Science, Ka-Kit Tung, an atmospheric scientist and applied mathematician at the University of Washington, Seattle, explains how research has shown that the heat absent from the earth's surface has been plunging deep into the North and South Atlantic Ocean.
The study basically offers the explanation that subsurface warming is the reason for global surface temperatures flat-lining since around 1999. This is despite greenhouse gases trapping more solar heat at the surface. This also explains why, even though greenhouse gases continue to increase, the temperature remains static.
(Top) Global average surface temperatures  where black dots are yearly averages. Two flat periods (h...
(Top) Global average surface temperatures, where black dots are yearly averages. Two flat periods (hiatus) are separated by rapid warming from 1976-1999. (Middle) Observations of heat content, compared to the average, in the north Atlantic Ocean. (Bottom) Salinity of the seawater in the same part of the Atlantic. Higher salinity is seen to coincide with more ocean heat storage.
Credit: K. Tung / Univ. of Washington
"Every week there's a new explanation of the hiatus," said author Ka-Kit Tung. "Many of the earlier papers had necessarily focused on symptoms at the surface of Earth, where we see many different and related phenomena. We looked at observations in the ocean to try to find the underlying cause."
Previous studies looking for the hidden heat
As readers will remember, studies have been focusing on the South Pacific and Pacific ocean currents as the reason for the "hidden heat" and cause of the current changes in temperature variations, but the research has proven that this is not the case. A study published in Nature Climate Change in February this year suggested equatorial trade-winds were driving the surface heat down into the ocean.
What the study shows is that the slow-moving current in the Atlantic Ocean that carries heat between the two poles had sped up earlier this century, and while doing so, drew surface heat down into the depths of the ocean almost a mile (1,500 meters). While earlier studies were focusing on short-term variables or particulates that could block the sun light, no one could explain what had happened to the missing heat.
"The finding is a surprise, since the current theories had pointed to the Pacific Ocean as the culprit for hiding heat," Tung said. "But the data are quite convincing and they show otherwise."
The distribution of active floats in the Argo array  colour coded by country that owns the float  at...
The distribution of active floats in the Argo array, colour coded by country that owns the float, at the end of May 2014.
Tung and co-author Xianyao Chen of the Ocean University of China who was a visiting professor at the University of Washington last year, used recent data from deep-sea temperatures recorded by Argo floats that sample water at a depth of 2,000 meters. The data clearly showed heat sinking occurring from around 1999, when the rapid warming of the 20th century appeared to stop.
Discovery of a naturally occurring cycle
"There are recurrent cycles that are salinity-driven that can store heat deep in the Atlantic and Southern oceans," Tung said. "After 30 years of rapid warming in the warm phase, now it's time for the cool phase."
The Atlantic Ocean was actually doing something akin to naturally protecting the environment. The researchers found that the rapid warming seen in the last 30 years of the 20th century was only half due to global warming and half due to a naturally occurring ocean cycle that kept more heat near the surface.
Around 2000, the ocean's cycle seemed to flip, and deep water temperatures began to rise, indicating the Atlantic Ocean was removing the surface heat and driving it deep into the depths to counteract human-driven warming. A simplified explanation shows that the cycle starts when saltier, denser water near the northern extent of the Atlantic, near Iceland begins to sink.
This changes the speed of the huge Atlantic current that circulates heat throughout the planet. "When it's heavy water on top of light water, it just plunges very fast and takes heat with it," Tung said. The explanation coincides with recent readings at the surface in the North Atlantic showing increased saltiness. At the same time, deeper water in that same area showed increased temperatures.
Historical evidence of a thirty-year cycle
Historical data shows a cooling cycle between the years 1945 to 1975. During this time, it was thought this period was the start of an ice age caused by air pollution. Other records showed that even earlier in central England, a 40 to-70 year cycle went back centuries. The historical records paint a picture of 30 year warming periods cycling with 30 year cooling periods. But Tung says that with global warming added to the mix, the trend looks like a staircase.
Canyons and Glaciers Along the Northwest Coast of Greenland.
Photo taken: July 13  2012.
Canyons and Glaciers Along the Northwest Coast of Greenland. Photo taken: July 13, 2012.
NASA/Michael Studinger
The temperature cycles have an "off-switch." For example, during the warming cycle, faster currents cause warm tropical water to travel to the North Atlantic, warming both surface and deep water. On the surface, the warm water melts the ice. This essentially makes the surface water less dense, and finally after a period of time, say 30 years, we begin a 30 year cooling phase.
What this implies is we have another decade of the cooling cycle to go through before temperatures start heating up again, although Tung admits it is difficult to predict what will happen next. He says, "We are not talking about a normal situation because there are so many other things happening due to climate change."
More about Atlantic ocean, dozens of theories, naturally occurring cycle, missing heat, cool phase
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