Deltas Sink Worldwide, Increasing Flood Risk

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BangladeshMost of the world's major river deltas are sinking, further increasing the regions' vulnerability to severe floods, new satellite studies find.

Of the world's 33 major river deltas, 24 are sinking due to flood-control efforts and other human-caused changes to the river systems.

The combination of sinking deltas and rising seas will increase the damages caused by hurricanes and other flooding events, according to the study, published in the current issue of Nature Geoscience.

As climate change causes glaciers and ice sheets to melt, sea levels are expected to rise. The study "conservatively" estimates that the area vulnerable to flooding could increase by 50 percent worldwide.

"It remains alarming how often deltas flood, whether from land or from sea, and the trend seems to be worsening," the authors wrote.

An estimated 500 million people live on river deltas.

Deltas are landforms that are created as sediments flow downstream and are deposited at the mouths of rivers. Seasonal floods typically build up and expand deltas, but flood-control efforts such as levees and dams have blocked the movement of sediment and prevented deltas from regenerating, even during severe flooding events, the study noted.

Deltas are also being tapped for underground water sources and oil and natural gas resources. These extractive activities compact the soil, causing the ground to subside faster than new sediments arrive.

"There are a host of human-induced factors that already cause deltas to sink much more rapidly than could be explained by sea level alone," said Albert Kettner, a research associate at the University of Colorado at Boulder who co-authored the study, in a statement.

The international team of scientists used global satellite data to analyze current delta conditions. The data was compared against historical maps of major low-lying rivers published between 1760 and 1922.

The study based its estimate of a 50-percent increase in global delta flooding on the 2007 Intergovernmental Panel on Climate Change (IPCC) projection of 0.45 meters of sea level rise by 2100.

However, the IPCC's sea-level rise forecast is now considered to be too modest. Glacial ice across Greenland has since melted at much faster rates. At this week's United Nations Climate Summit, IPCC Chair Rajendra Pachauri warned of the "possible elimination of the Greenland ice sheet and a resulting contribution to sea level rise of about 7 meters. Without mitigation, future temperatures in Greenland would compare with levels estimated for 125,000 years ago when [historical modeling] suggests 4 to 6 meters of sea level rise," he said [PDF].

The combination of sinking deltas and rising seas will increase the damages caused by hurricanes and other flooding events. In the past decade, 85 percent of the world's major deltas experienced severe floods, the study found.

The Mississippi Delta has been sinking due to the artificial levee system built along the river and the removal of the delta's oil and gas reserves. Before Hurricane Katrina inundated New Orleans in 2005, the Louisiana coast was losing 62 square kilometers of land each year. New Orleans' best hurricane defense, the Louisiana wetlands, continues to wash away into the Gulf of Mexico.

Similar delta conditions have imperiled thousands of flood victims in the Irrawaddy Delta of Myanmar and the Ganges-Brahmaputra Delta of India and Bangladesh in recent years.

The study warned that the Chao Phraya River, which runs through Bangkok, Thailand, may be the worst affected by delta loss. Parts of the delta have sunk 1.5 meters below sea level.

Ben Block is a staff writer with the Worldwatch Institute. He can be reached at bblock@worldwatch.org.

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