Published online 10 November 2009 | Nature | doi:10.1038/news.2009.1077

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Global warming won't affect all deltas

Rising sea levels could submerge Mississippi Delta but leave other systems intact.

Birdsfoot delta of the Mississippi RiverThe Mississippi delta could disappear as a result of climate change.US Geological Survey

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.

The battle to preserve the delta need not be fought at all, Ibáñez Martí says. The last time that seas were rising rapidly, at the end of the ice age, the Mississippi Delta simply retreated upriver, he explains. One option would be to let the same happen again. That, however, might prove politically unfeasible as people are likely to be unwilling to relocate. If efforts focus on protecting existing buildings from flooding, though, the delta might be squeezed between the rising sea and inland development.

Race against time

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.

"The system is in a crisis," says Laura Doyle, a graduate student at the Center for Watershed Sciences at the University of California, Davis. "Fortress levees" have to hold back rising seas and protect lands that continue to subside, she says. "The levees are up against time. There are a number of factors making [them] really fragile."

Even if rising sea levels don't eventually overcome the levees, an earthquake might breach many of them at once. One solution might be to deliberately flood some of the islands to restore habitat, particularly for phytoplankton that might rebuild the natural food web.

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"The delta is headed toward an inevitable change," Doyle says, "but flooded islands have the potential to be a source of primary [ecological] production."

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. 

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