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The Earth has warmed substantially over the past 130 years. But since the late 1990s, the global average temperature has not increased. Scientists have been trying to explain this pause in global warming. And the latest study, in the journal Nature, points to a natural cooling trend in the Pacific Ocean.

NPR's Richard Harris reports.

RICHARD HARRIS, BYLINE: It's no secret that the Pacific Ocean can profoundly affect the weather. You may recall the withering heat wave in 1998, triggered by a big ocean warming event, an El Nino. Well, Shang Ping Xie at the Scripps Institution of Oceanography has been looking at longer-term cycles of heating and cooling in the Central Pacific.

SHANG PING XIE: We started the study trying to resolve several contradictions.

HARRIS: Including the pause in global average temperature over the past 15 years. Oddly, during that time period, we still had record melting of the ice in the Arctic Ocean, and we broke all sorts of 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.

XIE: It's gaining extra heat during the past 15 years, and that heat is being stored.

HARRIS: Stored deep in the ocean. Xie 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.

XIE: And then we believe that the natural cycle will eventually swing up. And when that happens, we will be seeing unprecedented rates of climate warming when this happens.

HARRIS: 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. But how to explain the continuing summer heat waves and melting arctic ice? Xie says that isn't so mysterious. 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.

XIE: But if you go to the summer season, actually, the global mean temperature has kept rising for the past 15 years. That allows heat waves to set records, and it allows the Arctic Ocean to melt at a record pace.

HARRIS: 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 at MIT thinks there's more to this story.

SUSAN SOLOMON: Well, I think the ocean's playing a role. I'm not sure it's playing the largest role. And don't think that the Pacific Ocean alone is the only place to look.

HARRIS: Solomon says it could well be that the oceans aren't simply in a natural cycle. They could be affected by changes in the atmosphere, including air pollution.

SOLOMON: It's been really quite incredible how much air pollution is going on in China, and that reflects energy out to space, and that certainly can be a cooling effect. In other places, though, the amount of air pollution has actually decreased, in parts of Europe and the United States, for example.

HARRIS: So it's not clear what the overall effect is. Another factor is volcanic activity. There's been a string of small volcanoes since 2005, and those have produced a steady stream of sulfur particles, which also reflect energy back out into space.

SOLOMON: I don't think it's the whole story, but the measurements are very clear that it is part of the story - maybe 25 percent, maybe 30 percent.

HARRIS: Even with all this, Solomon says, remember, the global average temperature hasn't been dropping.

SOLOMON: It just hasn't continued to increase, as it was doing before.

HARRIS: So global warming is very much with us today.

Richard Harris, NPR News.

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