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For years now, sea levels have been rising steadily as a result of global warming. But something curious happened in 2010 and 2011. Sea level actually dropped slightly. Scientists puzzling over that hiccup say they now have an explanation. It has something to do with rain in Australia. Here's NPR's Richard Harris.

RICHARD HARRIS, BYLINE: The world's oceans are warming up, and as they do, the water expands and the sea level rises. Glaciers are also melting, adding more to higher seas. But John Fasullo, at the National Center for Atmospheric Research, says three years ago, scientists monitoring sea level by satellite noticed that sea level - which normally rises by an eighth of an inch a year - actually dropped by about a quarter of an inch.

JOHN FASULLO: So it was a real surprise to see the sea level drop in 2010, 2011, and it was a big question at first as to what was going on.

HARRIS: Something else big was going on at that time, as well. Rain that normally falls into the ocean instead fell onto the Australian landmass. That summer deluge created the worst floods on record, as captured by TV crews like this one from Britain's Channel 4.

(SOUNDBITE OF CHANNEL 4 BROADCAST)

UNIDENTIFIED WOMAN: An inland tsunami swept through the town of Toowoomba. As the waters rose, people scrambled to their roofs, clinging on, desperately waiting for rescue.

HARRIS: Now, some of those floodwaters simply ran right back into the ocean, so they didn't affect sea level. But that was not true for a lot of this excess rain. Fasullo says move farther inland, and rivers don't simply carry rainwater right back to the sea.

FASULLO: It's kind of like if you took a plate and turned it upside down, the whole center area doesn't run off back to the ocean. In fact, there are major river basins that run back towards the center of the continent.

HARRIS: Some years, rainfall pools up and creates a temporary freshwater sea called Lake Eyre.

FASULLO: You get millions of birds migrating to the area, and the whole ecosystem transitions from a desert into an inland sea in a few months.

HARRIS: Fasullo and some colleagues are publishing a paper in Geophysical Research Letters that concludes that the inland sea and similar features elsewhere are enough to explain the quarter-inch drop in global sea level. The inland sea has gradually evaporated, and a lot of that water has rained back into the oceans, raising sea levels once again. But that's not the end of the story.

FASULLO: Since then, we've shot past the long term trend, actually, and we're way up above it.

HARRIS: In fact, over the past two years, global sea level has risen by nearly an inch. That's more than three times faster than normal.

FASULLO: And with that, we have a new mystery.

HARRIS: Once again, fingers are pointing toward odd rainfall patterns. There's drought in Australia and the Americas right now, which means more rain is falling into the ocean and less is falling on land. John Nielsen Gammon at Texas A&M University says we wouldn't even know about these small variations, if not for very precise satellite observations.

JOHN NIELSEN GAMMON: It's not something you can go to the beach and notice, but we can detect it now.

HARRIS: And no question, the long term trend is toward higher sea level. Richard Harris, NPR News.

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