Rock islands dot the ocean in Palau, Micronesia, May 26, 2012. The waters of the tropical Pacific Ocean have been relatively cool for the last 15 years.
Now scientists at the Scripps Institution of Oceanography have more evidence that this global "pause" has to do with conditions in the Pacific Ocean.
"We started the study trying to resolve several contradictions," says Shang-Ping Xie.
He and a colleague asked why the global average temperature has bucked its long-term upward trend. And they also set out to explain why — even during this hiatus — there has been record melting of ice in the Arctic Ocean, and why there have been many new summertime heat records.
Xie says he can explain a lot of that simply by looking at what's been happening in the tropical waters of the Pacific Ocean. Waters there have been relatively cool, and that means the ocean can take up more heat than usual.
"It's gaining extra heat during the past 15 years, and that heat is being stored" in the deep ocean, he says.
There's no telling how long this cool phase will persist. But the previous Pacific cool phase, which started in the 1940s, lasted about 30 years. It can't last forever — the ocean will eventually return to a warm phase, "and when that happens, we will be seeing unprecedented rates of climate warming," he says.
Not only will we get the natural heat wave, but on top of that we'll get all the warming from greenhouse gases that have been building up during this cold cycle.
Xie says he can also explain the continuing summer heat records and melting Arctic ice. It turns out that the plateau in global average temperatures is mostly the result of lower temperatures during the wintertime.
That drags down the average, "but if you go to the summer season, actually the global mean temperature has kept rising for the past 15 years," he says. "That allows heat waves to set records, and it allows the Arctic Ocean to melt at a record pace."
In fact, when he runs a computer simulation that includes the cooling of the Pacific Ocean, he also sees a seasonal pattern that matches the real world ups and downs.
But Susan Solomon, an atmospheric scientist at the Massachusetts Institute of Technology, says there's more to this story.
"I think the ocean's playing a role. I'm not sure it's playing the largest role, and don't think the Pacific Ocean alone is the only place to look," she says.
It could well be that the oceans aren't simply in a natural cycle, Solomon says. The water temperature could be affected by changes in the atmosphere, including air pollution.
"It's been really quite incredible how much air pollution has been going on in China, and that reflects energy out into space. And that certainly can be a cooling effect," she says. "In other places though, the amount of air pollution has actually decreased, in Europe and the United States for example," so it's not clear what the overall effect is.
Another factor is volcanic activity. There have been a string of small volcanoes since 2005, and those have produced a steady stream of sulfur particles, which also reflect solar energy back out into space.
"I don't think it's the whole story," Solomon says, "but the measurements are very clear that it's part of the story — maybe 25 percent, maybe 30 percent."
Even with all this push towards cooler temperatures, Solomon points out, the global average temperature has still been rising decade by decade. "It just hasn't continued to increase as it was doing before," she says.
The 2000s were warmer than the 1990s, which were warmer than the 1980s. Put another way, the last decade was the warmest since scientists started taking methodical measurements around the world more than a century ago. So global warming is very much with us today.