Big dam in China may warm Japan

The link between widespread industrial activity and global warming has long been on the world’s list of worries, but now there’s concern that a single construction project–the Three Gorges Dam in China–could cause regional warming hundreds of miles away in Japan.

Map of eastern Asia shows the flow of ocean currents (red arrows) into and out of the Japan Sea. Shades of blue represent ocean depths. Nof/Bulletin of the AMS

The 610-foot-high dam, scheduled for completion later this decade, would stretch more than 1.3 miles across the Yangtze River and create a reservoir the length of North America’s Lake Superior. Although primarily built to generate hydroelectric power, to extend navigation upstream, and to provide flood control downstream, the project would impound more than 10 trillion gallons of water that could be used for irrigation.

If such agricultural use significantly diminishes the Yangtze River’s flow, it could instigate convection in the Japan Sea, argues Doron Nof, an oceanographer at Florida State University in Tallahassee. That process could bring relatively warm water to the sea’s surface and raise temperatures over Japan. Nof presents his analysis in the April Bulletin of the American Meteorological Society.

China’s Yangtze River, the third longest river in the world, currently dumps up to 8 million gallons of fresh water into the Yellow Sea each second. This fresh water, along with flow from the Yellow River, mixes with ocean currents and sweeps northward into the Japan Sea. There, the low-salinity water flows along the surface and forms an insulating blanket that impedes heat transfer to the atmosphere from the warmer, saltier waters trapped below, says Nof.

Even in the coldest part of the winter, when Siberian winds chill the surface water, the Japan Sea’s top layer doesn’t become dense enough to sink, says Nof. The last bout of large-scale convection in the Japan Sea occurred more than a half-century ago. Since then, warming farther north in the Sea of Okhotsk has melted ice there and sent additional fresh water southward.

Nof contends that if China diverts as little as 10 percent of the Yangtze’s flow, the Japan Sea’s surface layer at times would become salty enough to sink. Warmer water coming to the surface would cause the temperature of the overlying air to rise. Such convection elsewhere in the world raises regional air temperature several degrees Celsius, he adds.

Alberto D. Scotti, an oceanographer at the University of North Carolina at Chapel Hill, says it’s “definitely plausible” that increased salinity in the upper layers of the Japan Sea could restart the convection there, especially during cold winter conditions. Despite the argument’s simplicity and elegance, at this point it’s only speculation, Scotti says. “Without more detailed analysis, the only way to know [if the argument is valid] is to see what happens when the dam is completed,” he notes.

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