From the University of Washington and the department of Trenberth’s missing heat comes a claim that we’ll have to wait another 15 years for global warming to resume. Sounds like a goalpost mover to me.
The Oceans that Slowed 21st Century Global Warming
Why did the rapid global warming that characterized the latter part of the 20th century slow down over the last 15 years or so? Many different theories have been proposed, but a new study suggests that a massive movement of heat from shallow surface waters to deep regions of the Atlantic and Southern Oceans — but not the Pacific Ocean, as many researchers had predicted — might be responsible. Xianyao Chen and Ka-Kit Tung analyzed data from profiling floats, or oceanographic sensors that can move vertically throughout the water column, and traced the pathways that heat has taken through the world’s oceans since the turn of the 21st century. The oceans are capable of storing about 90% of the world’s surface heat content, and the researchers suggest that most of the excess heat that would have otherwise continued to fuel global warming is currently stored in the basins of the Atlantic and Southern Oceans.
The researchers also suggest that a sudden shift in salinity that corresponded with the slowdown of global warming at the beginning of the 21st century may have triggered this migration of heat to deeper waters. Historically, similar events have lasted 20 to 35 years, according to Chen and Tung. Consequently, the researchers suggest that global warming will pick back up in 15 more years or so, when heat returns to the surface waters.
Article #11: “Varying planetary heat sink led to global-warming slowdown and acceleration,” by X. Chen at Ocean University of China in Qingdao, China; X. Chen; K.-K. Tung at University of Washington in Seattle, WA.
Following rapid warming in the late 20th century, this century has so far seen surprisingly little increase in the average temperature at the Earth’s surface. At first this was a blip, then a trend, then a puzzle for the climate science community.
More than a dozen theories have now been proposed for the so-called global warming hiatus, ranging from air pollution to volcanoes to sunspots. New research from the University of Washington shows that the heat absent from the surface is plunging deep in the north and south Atlantic Ocean, and is part of a naturally occurring cycle. The study is published Aug. 22 in Science.
Subsurface warming in the ocean explains why global average air temperatures have flatlined since 1999, despite greenhouse gases trapping more solar heat at the Earth’s surface.
“Every week there’s a new explanation of the hiatus,” said corresponding author Ka-Kit Tung, a UW professor of applied mathematics and adjunct faculty member in atmospheric sciences. “Many of the earlier papers had necessarily focused on symptoms at the surface of the Earth, where we see many different and related phenomena. We looked at observations in the ocean to try to find the underlying cause.”
The results show that a slow-moving current in the Atlantic, which carries heat between the two poles, sped up earlier this century to draw heat down almost a mile (1,500 meters). Most of the previous studies focused on shorter-term variability or particles that could block incoming sunlight, but they could not explain the massive amount of heat missing for more than a decade.
“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.”
Tung and co-author Xianyao Chen of the Ocean University of China, who was a UW visiting professor last year, used recent observations of deep-sea temperatures from Argo floats that sample the water down to 6,500 feet (2,000 meters) depth. The data show an increase in heat sinking around 1999, when the rapid warming of the 20th century stopped.
“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.”
Rapid warming in the last three decades of the 20th century, they found, was roughly half due to global warming and half to the natural Atlantic Ocean cycle that kept more heat near the surface. When observations show the ocean cycle flipped, around the year 2000, the current began to draw heat deeper into the ocean, working to counteract human-driven warming.
The cycle starts when saltier, denser water at the surface northern part of the Atlantic, near Iceland, causes the water to sink. This changes the speed of the huge current in the Atlantic Ocean 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. Recent observations at the surface in the North Atlantic show record-high saltiness, Tung said, while at the same time, deeper water in the North Atlantic shows increasing amounts of heat.
The authors dug up historical data to show that the cooling in the three decades between 1945 to 1975 – which caused people to worry about the start of an Ice Age – was during a cooling phase. (It was thought to be caused by air pollution.) Earlier records in Central England show the 40- to 70-year cycle goes back centuries, and other records show it has existed for millennia.
Changes in Atlantic Ocean circulation historically meant roughly 30 warmer years followed by 30 cooler years. Now that it is happening on top of global warming, however, the trend looks more like a staircase.
The temperature oscillations have a natural switch. During the warm period, faster currents cause more tropical water to travel to the North Atlantic, warming both the surface and the deep water. At the surface this warming melts ice. This eventually makes the surface water there less dense and after a few decades puts the brakes on the circulation, setting off a 30-year cooling phase.
This explanation implies that the current slowdown in global warming could last for another decade, or longer, and then rapid warming will return. But Tung emphasizes it’s hard to predict what will happen next.
A pool of freshwater from melting ice, now sitting in the Arctic Ocean, could overflow into the North Atlantic to upset the cycle.
“We are not talking about a normal situation because there are so many other things happening due to climate change,” Tung said.
The research was funded by the U.S. National Science Foundation and the National Natural Science Foundation of China.
(I expect we’ll see a rebuttal from Bob Tisdale soon) UPDATE: We have and it is here.
UPDATE2: The list of excuses is up to 38 now, and an updated permanent count is here on this WUWT page under “Climate FAIL files”: http://wattsupwiththat.com/climate-fail-files/list-of-excuses-for-the-pause-in-global-warming/