A U.S.-led international scientific team says it has determined that Earth's river deltas are sinking due to human activity, many having recent severe flooding.

The researchers, led by University of Colorado-Boulder Professor James Syvitski, found 24 of the world's 33 major deltas are sinking, making them increasingly vulnerable to flooding from rivers and ocean storms and putting tens of millions of people at risk.

The study concludes the sinking of deltas from Asia and India to the Americas is exacerbated by the upstream trapping of sediments by reservoirs, dams, man-made channels and levees that carry sediment into the oceans, as well as the accelerated compacting of floodplain sediment caused by the extraction of groundwater and natural gas.

About 500 million people in the world live on river deltas.

The researchers, using data supplied by NASA satellites, predict global delta flooding could increase by 50 percent under current projections of about 18 inches in sea level rise by the end of the century, as forecast by the 2007 Intergovernmental Panel on Climate Change report.

The research that included Dartmouth College, Louisiana State University, the City College of New York, the Geological Survey of Japan, the Woods Hole Oceanographic Institute and the University of Southampton in England appears in the Sept. 20 issue of the journal Nature Geoscience.

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