Though the end of the 20th century looked like we were going to see runaway temperatures around the globe, that hasn't really happened despite countries like China and Russia and Mexico and India continuing to belch CO2 into the atmosphere.
More than a dozen hypotheses have been proposed for the so-called global warming hiatus, ranging from air pollution to volcanoes to sunspots and now the University of Washington has entered the fray, saying 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.
Subsurface ocean warming would explain why global average air temperatures have flatlined since 1999, despite greenhouse gases trapping more solar heat at the Earth’s surface, the authors write.
(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: University of Washington
“Every week there’s a new explanation of the hiatus,” said corresponding author Ka-Kit Tung, a University of Washington 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 two and a half decades of the 20th century, they proposed in an earlier study, 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, in about 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 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 slowly 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.
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 have been 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.
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, for example, 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.
Citation: Xianyao Chen, Ka-Kit Tung, 'Varying planetary heat sink led to global-warming slowdown and acceleration', Science, 22 August 2014: 897-903. DOI:10.1126/science.1254937. Source: University of Washington
- PHYSICAL SCIENCES
- EARTH SCIENCES
- LIFE SCIENCES
- SOCIAL SCIENCES
Subscribe to the newsletter
Stay in touch with the scientific world!
Know Science And Want To Write?
- Top Secret: On Confidentiality On Scientific Issues, Across The Ring And Across The Bedroom
- The Mystery Of The Red Sea
- Would New Planet X Clear Its Orbit? - And Any Better Name Than "Planet Nine"?
- Stop Using BMI To Determine Health
- Make Sexual Harassment in Federally Funded Institutions a Crime.
- First-semester GPA A Better Predictor Of College Success Than ACT Score
- The Greenhouse Effect Fallacy
- "Hello Vance,yes I know about that model, and about at least two experimental attempts, one with..."
- "No, I am not from the states. It sounds like an astonishing arrangement to have multiple jurisdiction..."
- "Very true. The observation I've made is that in 2001 it was John F. Nash and Alicia DeLarde ..."
- "The male female difference you think is so basic isn't so simple. Consider this bird. http..."
- "It is a matter of jurisdiction. In the USA the federal government can only claim jurisdiction..."
- BMI is Bologna
- Energy Drinks: The Dose Makes the Poison
- California’s Prop 65: Bad For Public Acceptance Of Science, About To Get Worse
- Wear Red Today! It’s Women’s Heart Health Awareness Day
- Can Marijuana Ease NFL Players’ Pain? Claims Are All Over The Field
- Mid-Life Crisis Clusters Found In 4 US Cities
- Cambridge researcher develops smartphone app to map Swiss-German dialects
- Studies link healthy workforces to positive stock market performance
- Pioneering discovery leads to potential preventive treatment for sudden cardiac death
- Online shopping might not be as green as we thought
- Gene family turns cancer cells into aggressive stem cells that keep growing