Combine the populations of Brazil and the United States and you get a half a billion people. You also get the number of people who live within the world’s major river deltas, people who are susceptible to the double whammy of sinking land and rising seas, according to a study published last Sunday.

The study, published in the journal Nature Geoscience, used satellite imagery to show that 85 of the world’s major deltas–again that affects half billion people–experienced severe flooding over the past decade. The study also projects that such flooding could expand by a whopping 50% if sea levels continue to rise, which is expected under what the study calls moderate climate change conditions.

Asia is expected to be the worst continent hit, but intensely farmed and highly populated areas of every continent except Australia and Antarctica will be affected. China alone has three of the deltas most in danger–the Yangtze, the Pearl and the Yellow River deltas. Egypt’s Nile is also high-risk along with the Rhone River delta in France. Other river deltas in Asia rank second from the top, including the Ganges in Bangladesh.

Deltas are natural floodplains. But, over the last several decades, especially the last 50 years, human activity has caused the land to subside, or sink, in many regions. The cause of this sinking is manifold. First and foremost, dams and other manmade river diversions hold back sediment layers that would normally build up naturally via flooding.

Another cause of sinking deltas is subsurface mining, which involves the digging of tunnels or shafts into the earth to reach mineral and other deposits.

Water, as you know, is vital to human survival. But extensive groundwater withdrawal for long periods of time can lead to subsidence. For instance, the Chao Phraya in Thailand has sunk two to six inches per year as a result of groundwater withdrawal.

Another type of mining, the extraction of oil and gas, is causing what is called “accelerated compaction” in many of those delta regions already in danger. (In the last century, Italy’s Po Delta, for example, subsided 12 feet due to methane mining.)

The other side of the double whammy is climate change-driven sea level rise. The UN’s Intergovernmental Panel on Climate Change has predicted that levels will rise by 7.2-23.6 inches by the year 2,100. Combine that with climate change-related increasing storm intensity, such as hurricanes and typhoons, and you get loss of natural shoreline barriers like mangrove forests.

The study, which was conducted by the Institute of Arctic and Alpine Research at the University of Colorado, says there are indications that more and more river deltas will sink below mean sea level.

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Photo of Yangtze River courtesy of livepine’s photostream/Creative Commons