Average global temperatures haven't increased in the past 15 years, and scientists have been working to determine why.
A number of recent studies have fingered the ocean as the cause. Researchers believe heat that might otherwise warm the planet is being stored -- hidden, in a way -- in the sea.
A paper released today in the journal Nature adds to that body of evidence by linking the recent global warming hiatus to cooling in the equatorial Pacific Ocean.
"The Pacific cycle happens to be in a cold state, kind of dipping down. It drags the global mean temperature downward," said Shang-Ping Xie, a climate scientist at the Scripps Institution of Oceanography and co-author of the study.
In order to figure this out, Xie and his colleague Yu Kosaka, also at Scripps, devised a way to shift ocean temperatures in global climate models just a small amount in one part of the Pacific Ocean.
"Climate models consider anthropogenic forcings like greenhouse gases and tiny atmospheric particles known as aerosols, but they cannot study a specific climate event like the current hiatus," Kosaka said.
"We devised a new method for climate models to take equatorial Pacific ocean temperatures as an additional input. Then, amazingly, our model can simulate the hiatus well."
The researchers were able to use the model to document that, from 1970 to 2012, changes in tropical Pacific surface temperatures tracked very closely with global temperature.
Gerald Meehl, a climate scientist at the National Center for Atmospheric Research who has researched the ocean's effect on global temperatures, said Xie's study confirmed, using new methods, some of the work he and other colleagues have done showing that in periods of a global warming hiatus, sea surface temperatures in the tropical Pacific are lower.
This cycling of temperatures in the Pacific is often referred to as the Pacific Decadal Oscillation.
Meehl's work has also shown that when this happens, more of the heat gets mixed into the deeper ocean, although this paper did not address that, he said.
John Wallace, a climate scientist at the University of Washington who is familiar with the paper, said there have been other periods where warming has paused or slowed down, sometimes followed by a rapid rise in temperatures.
One such period of rapid warming was in the early part of the last century, and another, a warming slowdown, occurred in the 1950s to the early 1970s.
To explain these past events, researchers have looked at variation in the Atlantic Ocean as another cause.
"The people who favor the Atlantic have usually look at the whole century and suggest that these two periods of rapid warmth and the two hiatuses are manifestations of a long, cyclic phenomenon in the Atlantic that is sometimes referred to as the Atlantic Multi-Decadal Oscillation," Wallace said.
But whether it's cycles in the Atlantic Ocean or the Pacific, Wallace said, the research firmly points out that natural variability can play a big role in how the globe warms up.
This isn't to say that greenhouse gases don't have a big effect on climate. It's just that other processes can work to either strengthen or work against that effect.
It's almost like two forces pulling in the opposite direction, said Xie. Emissions of greenhouse gases are working to push global temperatures up. Right now, the tropical Pacific is working to push them down.
One example of this is in summer average temperatures, which have continued to warm even as the year-long average stays flat. That's because the equatorial Pacific Ocean does not have as big an influence in the summer, Xie noted.
When the cycle shifts, though, the warming will return -- and fast.
"People want to ignore climate change saying, 'Look, the temperature hasn't changed [in over a decade].' But if you wait for another five, 10 years, you will be seeing even faster global warming," Xie said. "It's just a natural cycle."
Clarification: In an earlier version of the story, it was unclear whether a reference to two periods of temperature variation referred to more rapid or slower warming. The period in the early 20th century was one of rapid warming, while the period in the 1950s to early 1970s had slower warming.