From NPR News, this is ALL THINGS CONSIDERED. I'm Melissa Block.

Over the past 50 years, the global sea level has risen by four inches. Scientists say that's largely due to global warming, but they can't fully explain where all that water came from. And even more puzzling, it turns out dams have played a surprising role in influencing the sea level.

NPR's Richard Harris has the story.

RICHARD HARRIS: As the planet warms, sea level rises for two main reasons: First, ocean water expands as it gets warm, second, glaciers melt and that water runs into the sea. The problem is it's not all adding up right now. Scientists think they know how quickly the sea water is expanding, and they think they know how fast glaciers, such as Greenland, are melting. But that's not enough to explain sea level rise over the past half century.

Professor BENJAMIN CHAO (National Central University of Taiwan): And scientists have been scratching their heads trying to explain this sea level rise.

HARRIS: Ben Chao is with the National Central University of Taiwan. He's just published a paper online in Science magazine on the topic, and his data make the puzzle even more puzzling. Chao estimated how much water has been caught over the past 50 years, behind the world's big dams. It turns out to be a huge amount of water. And if it had float into the ocean, it would have raise sea levels by more than an inch.

Prof. CHAO: It's equivalent to three centimeters in total of all the whole global sea. It is surprisingly large.

HARRIS: Sea level has risen four inches, but it would have risen five if we hadn't been so busy buildings dams to catch all that runoff. So, scientists trying to balance the ocean's books really need to find five inches of water.

Prof. CHAO: That's the million-dollar question now. Where does the water come from?

HARRIS: It's possible that glaciers have melted more than scientists realized. Only in the past decade if they had new and pretty reliable satellite instruments to measure that directly. Anny Cazenave from Toulouse University says it's also possible that other human activities are adding water to the oceans. For example, we have been pumping water up from deep underground, and once it's done irrigating crops, that water could end up in the ocean.

Dr. ANNY CAZENAVE (Senior Scientist, Toulouse University): You can expect contribution from deforestation, from urbanization, from irrigation and so on.

HARRIS: In some, she says those processes could add as much water to the oceans as dams hold back. But that's still not enough to explain the long-term rate of sea level rise. Dork Sahagian at Lehigh University says that is a conundrum, but he's more concerned about what could lie ahead.

Professor DORK SAHAGIAN (Director of Environmental Initiative, Lehigh University): If we stop building dams, the rate of sea level rise will increase immediately and significantly and our projections for the whole next century may be significantly underestimated.

HARRIS: United Nations scientists say sea levels could rise by one or two feet by the end of the century, and that could turn out to be an underestimate. Of course, if we keep building new dams, that has an environmental impact as well.

Richard Harris, NPR News.

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