By Sid Perkins, Science News
Residents of Louisiana, take note: If engineers don't divert sediment-rich waters from the Mississippi River to help replenish a sinking river delta, about 10 percent of your state will slip beneath the waves by the end of this century. However, even if the engineers do try to abate the subsidence, the Mississippi doesn't carry enough sediment to offset more than a small fraction of that loss, a new analysis suggests.
Over the past few centuries, about a quarter of the wetlands in the Mississippi River delta have been lost to the ocean, says Harry Roberts, a marine geologist at Louisiana State University in Baton Rouge. Several factors have contributed to that loss, he notes, including sea-level rise and the settling of land as ancient sediments gradually become compacted under their own weight. Now, Roberts and colleague Michael Blum--now at the ExxonMobil Upstream Research Company in Houston--use computer models to estimate the effect that these processes will have on the Mississippi delta in the next few decades. The news, reported online June 28 in Nature Geoscience, isn't good.
Tidal gauges at Grand Isle, La.--near the tip of the Mississippi delta, where river-dumped sediments lie about 60 meters thick--indicate that land there is sinking as much as 8 millimeters each year. At Baton Rouge, about 250 kilometers upstream, sediments are thinner and the land subsides up to 3 millimeters per year. In Roberts and Blum's new analysis, regions between those points sink at intermediate rates.
Not only is the land sinking, but the sea is also rising. Scientists on the Intergovernmental Panel on Climate Change report that since 1993, sea level has risen about 3 mm/yr. That rate is expected to accelerate as the planet's climate warms, heating Earth's surface waters and causing them to expand, says Roberts. The melting of land-based ice such as glaciers will also contribute. So, the researchers presume--conservatively, Roberts says--that in the year 2100 sea level will be rising 4 mm/yr.
Roberts and Blum estimate that between 2000 and 2100, the combined effects of subsidence and sea-level rise will swamp as much as 13,500 square kilometers--about 10 percent of the area of Louisiana.
"Geologists have known about [these effects] for a long time, but only in a qualitative way," says Roberts. "For a lot of people, this will be shocking news."
"This is an outstanding study," says Torbjorn Tornqvist, a coastal geoscientist at Tulane University in New Orleans. He suggests that some researchers may quibble about the team's relatively simple approach to a complex problem, but he adds that even if a more sophisticated approach had been used, "it's unlikely that the results would have been fundamentally different."
Finally, Tornqvist notes, the estimates for future subsidence and sea-level rise used by Roberts and Blum are conservative--meaning that Louisiana could easily end up losing more than a tenth of its coastal lands this century.
Today, the Mississippi River carries only about 205 million tons of sediment to its delta each year. That's less than half the amount it transported in the era before people built dams upstream that interrupt that material's journey to the sea. Also, only 40 percent or so of a river's sediment accumulates in a delta; the rest washes out to sea, the researchers note. So, even if engineers began to divert sediment-laden water from the Mississippi into the marshes at the head of the delta, they'd only save about 900 square kilometers of land from sinking beneath the waves in the next century, the scientists estimate.
Roberts and Blum "have come up with a result that we should clearly be concerned about," Tornqvist says.