Past Sea-Level Rise Points to Troubling Future

New research by Rice geologists indicates that a rise in sea levels of as little as a half-meter per century has been sufficient to dramatically change the shoreline of the U.S. Gulf Coast within the past 10,000 years. That’s not good news, because half-meter increases are within the moderate range of predictions for the Gulf Coast during the century to come.

The findings are drawn from experimental data collected by a group led by John Anderson, the W. Maurice Ewing Chair in Oceanography and professor of earth science. Other group members were Rice graduate students Jessica Maddox, Kristy Milliken, Alex Simms, and Patrick Taha and one of Anderson’s former students, University of Alabama professor Tony Rodriguez. The researchers drilled core samples and took underwater soundings from the southern shores of Texas to Alabama’s Mobile Bay, and analyses have begun to yield a catalogue of reactions that coastal features undergo in response to specific rates of sea-level rise.

“About 5,000 years ago, when the sea level was rising approximately 50 centimeters per century, the upper part of Corpus Christi Bay increased by about one-third over the span of about 200 years,” Anderson explains. “Even without factoring in any effects from global warming, that’s only about 20 percent slower than the projected sea-level increases along the Texas and Louisiana coasts this century.”

Scientists know from oceanographic records that sea level has been rising worldwide for at least 10,000 years. The exact cause is unknown, but the rates of rise for specific eras have been well-documented from marine sedimentary records worldwide. Until this century, however, the rates of sea-level rise have gradually fallen to about 20 centimeters per century today from more than 100 centimeters per century 10,000 years ago—a rate that’s been confirmed using global tide gauge records from the past 100 years.

But the trend toward slowing rates of sea-level rise is expected to reverse this century as global warming pushes rates back up. According to estimates by the International Panel for Climate Change, rising ocean temperatures will add about 30 centimeters to sea level this century, and glacial runoff from Antarctica—the least understood of all the phenomena involved—could add another 40 centimeters. Thus, the worst-case scenario could translate into a 90-centimeter boost in sea level by the year 2100—a rate of increase that global coastlines haven’t experienced in about 8,000 years.

Estimating the effects of rising seas on a particular coastline requires more than an accurate assessment of sea-level rise, Anderson says. Local geography and geology also play a role. For instance, the coastal plains of southeast Texas and Louisiana are a vast sedimentary plain that is sinking at the rate of about 20 centimeters per century. Moreover, there are other geological forces at work that can be impacted or even overwhelmed by rising seas.

“On geologic time scales, barrier islands like Galveston and Padre Islands retreat toward land,” Anderson says. “The Galveston shoreline, for example, is moving about 1.5 meters inland every year. But the same forces that are slowly eroding the beaches on the windward side of the island deposit that sand on the leeward side, so the island itself remains a stable barrier, even though it marches slowly toward shore.”

Anderson, who began studying the geography of the U.S. Gulf Coast about 15 years ago, notes that rising seas can overwhelm such fragile coastal structures as barrier islands. “We know, for example, that Sabine Bank, a seafloor formation about 20 miles offshore along the Texas–Louisiana border, was a barrier island until just a few thousand years ago,” Anderson says. “We know that it was drowned in place, because we drilled core samples there. We still don’t know what rate of sea-level rise it took to drown the island, but we’re studying that.”

Rising seas also can inundate river deltas and tidal wetlands, such as those at the head of the Trinity River in upper Galveston Bay or the head of the Nueces River in upper Corpus Christi Bay. Anderson’s group also is finding evidence that a catastrophic collapse of one coastal structure—like a barrier island—can result in stress or even in collapse of interconnected structures, such as tidal wetlands.

“Even with past examples to guide us, devising an accurate model for local shoreline change will be difficult because there is a complex interplay between global events—like what’s happening in Antarctica—and local geologic, oceanic, and atmospheric phenomena,” Anderson says. “There are many more variables that go into this than I ever would have believed when I started studying the Gulf Coast in the 1980s.”

—Jade Boyd