Upsala glacier. Image via LongHornDave on Flickr

As most of us are already aware, a rapid (and somewhat alarming) period of warming characterized the closing decades of the 20th century. This trend was caused, in part, by human-produced greenhouse gases. However, despite the fact there has been no dramatic changes in our greenhouse gas production (in fact, greenhouse gases are increasingly entering Earth's atmosphere), there has been very little increase in average temperatures at the Earth's surface over the last 30 years.

This fact left climatologists and Earth scientists stumped. Likewise, this (so-called) global warming hiatus has been fuel for climate change deniers, who argue that, since global temperatures are not significantly increasing, human-induced global warming (anthropogenic global warming) cannot be a viable model. Rather, they assert that warming is just a part of Earth's natural cycle.

And unfortunately, most of the research that has been previously put forth only offered an explanation for short periods of reprieve in the warming trend. Ultimately, these studies could not explain the massive amount of heat missing for more than a decade. However, new research from the University of Washington indicates that the slowdown in global warming will likely last for about another 10 years, and then rapid warming will return.

It is all linked to Earth's oceans.

This new research diverges from many other studies that explain this hiatus, as the research from Washington focuses on Earth's oceans as an explanation of the decline in warming. Specifically, the team looked at the Atlantic ocean. “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.”

Tung and co-author Xianyao Chen used data of deep-sea temperatures obtained from Argo floats (which sample the water down to 6,500 feet [2,000 meters]), as well as older oceanographic measurements and computer reconstructions in order to form a vision of what kind of heat can be found within the planet's oceans. Ultimately, their research reveals that the slow-moving current in the Atlantic, which is responsible for carrying heat between Earth's poles, sped up earlier this century to draw heat down almost a mile (1,500 meters). As the heat was drawn into the ocean, it caused the average global temperatures to decline. Notably, the results show an increase in heat sinking starting around 1999, when the rapid warming of the 20th century stopped.

The study was published Aug. 22 in Science.

How The Cycle Works:

(Top) Average surface temperatures. Black dots are yearly averages. (Middle) Heat content compared to the average, in the north Atlantic Ocean. (Bottom) Salinity of the seawater in the same part of the Atlantic.  via K. Tung

The authors state that the Earth is on a 30-year cycle where, every three decades, heat will be drawn into the ocean and the planet will slightly cool. At the end of this 30 year period, the sinking will stop, effectively ending the cool period, and warming will increase. Tung asserts, "There are recurrent cycles that are salinity-driven that can store heat deep in the Atlantic and Southern oceans. After 30 years of rapid warming in the warm phase, now it’s time for the cool phase."

More specifically, at the ocean's surface, there is a conveyor system. This conveyor consists of oceanic currents that transport warm, salty water from the Caribbean to the North Atlantic. North of Iceland, the water becomes colder and saltier as the newly introduced warm water causes glacial melting. When enough cool water in introduced, it gets so dense that it sinks deep into the ocean, where it flows south again

“When it’s heavy water on top of light water, it just plunges very fast and takes heat with it,” Tung said.

These statements are supported by the fact that recent observations at the surface in the North Atlantic show record-high saltiness, while at the same time, deeper water in the North Atlantic shows increasing amounts of heat.

The authors also found historical data to show that the cooling follows roughly a 30 year pattern. Between 1945 and 1975 there was a cooling phase. According to the authors, earlier records in Central England show the cycle goes back centuries, and other records show it has existed for millenia.

So what does this mean as far as global warming goes? Previously, changes in circulation patterns in the ocean meant roughly 30 warmer years followed by 30 cooler years. However, now that we have to take global warming into consideration, the trend looks more like a staircase (the warming may slow, but there is still a notable warming trend).

Yet, Tung emphasizes it’s hard to predict what will happen next. "We are not talking about a normal situation because there are so many other things happening due to climate change," Tung said. For example, a pool of freshwater from melting ice, which is now sitting in the Arctic Ocean, could overflow into the North Atlantic to upset the cycle.

Kevin Trenberth, a researcher at the National Center for Atmospheric Research (NCAR), agrees that the oceans are responsible for the recent decline in warming; however, he argues that the Pacific is the main driving force behind this new trend. Trenberth claims that processes in the Pacific Ocean cause changes in the North Atlantic current. He explains by asserting that intense trade winds pile up warm water in the western Pacific, and that these have large "ripple effects" on the atmosphere. These ripples influence currents of air that are flowing through the atmosphere (called "jet streams"). Ultimately, he believes that these air currents cause changes in ocean currents (which causes the melting of ice and subsequent sinking).

But whatever is causing the warm currents to move North and cause heat sinks, scientists agree that this hiatus will end, and soon, the rapid warming will return.


 

The research was funded by the U.S. National Science Foundation and the National Natural Science Foundation of China.


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