Wednesday, November 11, 2009

Global warming won't affect all deltas

Richard Lovett in Nature News: Whether river deltas become swamped by rising sea levels will depend on a multitude of factors, including the type of soil and the tectonic action of any nearby plates, say researchers. "In coastal systems we have to think about combined impacts," said oceanographer Richard Feely of the Pacific Marine Environmental Laboratory in Seattle, Washington, at this year's meeting of the Coastal and Estuarine Research Federation in Portland, Oregon on 3 November. Every system is different, he says.

In the Mississippi Delta, for example, not only is the sea level rising, but the soils are subsiding, causing the land to submerge more rapidly than the river can deliver new sediment. "It's quite clear that if we try to focus on conserving the outer areas, it's going to be almost impossible" to save the delta, says Carles Ibáñez Martí, director of the Institute for Food and Agricultural Research and Technology's Aquatic Ecosystems unit in Sant Carles de la Ràpita in Spain.

…A very different combination of factors threatens California's Sacramento–San Joaquin Delta, east of San Francisco. There, the region's natural wetlands were long ago drained and dyked to create intensively farmed islands. Unfortunately, the soils consist mostly of peat, which has slowly oxidized — releasing carbon dioxide — or blown away so the islands are now as much as 8 metres below sea level.

…One delta that seems set to survive is the Danube Delta in the Black Sea. Because it is in a region of tectonic uplift creating central Europe's Carpathian Mountains, says Liviu Giosan, a sedimentologist at the Woods Hole Oceanographic Institution in Massachusetts, the sea level has remained stable for several thousand years. Furthermore, he says, even though upstream dams have cut down its supply, the delta seems to be receiving enough sediment to remain stable.

Somewhere in the middle of the Danube delta, shot by T. Lessiak, Wikimedia Commons

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