That Sinking Feeling

While government geologists argue over subsidence and sediment loading, the rest of us are wondering: Is the Gulf Coast toast?

IF I INTERPRET IT CORRECTLY, a controversial July 2004 report from the National Oceanic and Atmospheric Administration ( NOAA) predicts that Galveston Island will be a memory by the time my great-great-great grandchildren are born. Or maybe it’s their great-great-great grandchildren. We’re speaking here in geological time, and in an arcane, apocalyptic language that is mostly gibberish to laypeople like me. According to one of the report’s co-authors, Louisiana State University geologist Roy Dokka, southern Louisiana is sinking into the Gulf of Mexico at a rate of five feet a century, and the upper coast of Texas can’t be far behind. The good news is that some scientists in other government agencies think Dokka is full of beans. The bad news is that nobody really knows for certain, and even if they did, there’s very little we can do about it.

Nearly everyone agrees that our coasts are vanishing, that sea levels are rising, and that the earth is warming. The question is why. Clearly, the coastal problem is not as bad in Texas as it is in Louisiana, which lost more than four thousand square kilometers of coast during the twentieth century. New Orleans is almost under water already. But so are parts of Houston after a good rain; there are places where the surface level is eight to ten feet lower than it was in 1930. In the community of Surfside, in Brazoria County, you can drive along the beaches and see waves lapping around the pilings of cottages that once stood hundreds of feet from the shoreline. In the village of Treasure Island, just across the San Luis Pass bridge from Galveston Island, homes built fifty yards from the shore in the seventies now stand with their decks overlooking a breakwater instead of a beach (the Gulf shoreline was more than one hundred miles south of Galveston during the last ice age). Thirty miles south of the mouth of the Sabine River, geologists have found evidence of encampments—arrowheads, pottery, mastodon teeth—in 75 feet of water.

Beach loss is caused mainly by erosion, especially surges that accompany hurricanes and tropical storms. The larger problems, however, are rising sea levels and subsidence, the sinking or settling of land, which can be either man-made or natural. Scientists have known since 1926 that land sinks when groundwater, oil, or gas is extracted. Harris County has spent billions of dollars converting from groundwater to surface water, and the Harris-Galveston Coastal Subsidence District has spent about $10 million monitoring subsidence with a network of “benchmarks,” devices anchored to stable places on the earth’s surface to measure elevation and subsidence rates. Ron Neighbors, the district’s general manager, told me that subsidence today is negligible, maybe three quarters of a foot per century. But here’s what makes the NOAA report so controversial and fraught with intrigue: Dokka and his colleagues claim that the real monster is geological subsidence, a force of nature that is inexorable and irreversible.

The Gulf Basin, a giant, seething cauldron of tectonic mischief that extends inland nearly as far as Dallas, started sinking millions of years before man made his appearance, sagging under the weight of billions of tons of sediment pouring down the Mississippi River and Texas rivers like the Trinity, Colorado, Brazos, and Rio Grande. Geologists call this sediment loading: Houston Chronicle reporter Eric Berger likens the phenomenon to a bowling ball weighing down the center of a trampoline. As sediment accumulates and is compacted, the earth’s crust begins to crack under the weight, splintering the subsurface of the basin into thousands of faults and squeezing ancient deposits of salt and soft shale into amazing formations, some of them miles inland. (The numerous salt domes around Houston are the result of this big squeeze; in the deepest part of the Gulf is a mountain range of salt and sediment called the Sigsbee Escarpment, six hundred miles across, rising three thousand feet.) As pressure mounts, parts of the coast slip off into the Gulf through a process known as gravity gliding. Take Galveston: Although sea levels worldwide are rising about a foot a century, the Island is losing more than twice that when subsidence is factored in. “You have to ask, Does Galveston have two and a half feet to lose?” said Houston geologist Art Berman, whose recent article in the Houston Geological Society Bulletin, “The Anatomy of a Silent Disaster,” comes down on the side of Dokka’s research.

On the other hand, research geologist Robert Morton, of the U.S. Geological Survey in St. Petersburg, Florida, told me that Dokka’s research was “scientifically questionable”—a serious allegation against a Ph.D. in geology who is trained in geodesy, the science of measuring the earth’s elevations. Even more damning, Morton suggests that Dokka may be more interested in grabbing headlines than in scholarly pursuit. “Terms like ‘sediment loading’ and ‘gravity gliding’ made perfect sense millions of years ago, but they don’t necessarily apply today,” Morton said. “We know that Galveston Island began forming several thousand years ago, and we know its elevation has not changed much since then. If the rates of subsidence had been as fast [as the NOAA report claims], Galveston would have been permanently submerged a long time ago.”

The quarrel between Dokka and Morton has been roiling for some time, and the NOAA’s National Geodetic Survey, which sets the standards for benchmarks and is the final authority on geodetic measurements in the country, appears caught in the crossfire. “A lot of geologists don’t understand what we do,” said Dave Zilkoski, the NGS’s deputy director. “The report is the best available information on subsidence rates we have. How they are interpreted is something else. Let the geologists work it out.” In a paper released after our conversation, Morton wrote that the rapid subsidence in Louisiana was largely the result of oil and gas production and that there was no evidence that the subsidence that had occurred in the past few thousand years was a result of natural processes like sediment loading

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