Defining the Threat: How Will Climate Change Affect Already Weakened Wetlands?

COASTAL LOUISIANA is more vulnerable to the effects of global climate change than any other region in the United States. It's low elevation, high rate of subsidence and rapid loss of wetlands expose this area to the worst consequences of climatic change — a rising Gulf, possibly stronger storms, unpredictable rainfall and warmer weather.

Louisiana has already lost more than 1,500 square miles of coastal wetlands in the past century due to human and natural causes. These causes range from saltwater intrusion along industrial and navigational canals and flood-control levees to the natural settling of muddy soils and erosion of the coastline.

Sea-Level Rise

Rising seas, produced by melting glaciers and warming and expanding oceans, could contribute to further loss of wetlands. In the last 100 years, water levels along the state's coast have increased by as much as 40 inches from a combination of rising oceans and sinking land known as "relative sea-level rise." In fact, Louisiana has the highest rate of relative sea-level rise in the nation. What's more, scientists predict sea level could rise another 20 to 44 inches by the year 2100. Because most of the state's coastal areas are flat, even a conservatively estimated 20-inch rise in sea level would put more than 3,500 square miles of Louisiana underwater. In marsh and mangrove habitats, rising seas would submerge land, waterlog soils, threaten plants and possibly reduce fish and shellfish productivity.

Marsh Changes as Sea-level Rises

Courtesy of NOAA

Hurricanes and Storms

Although climate models disagree on how often hurricanes and tropical storms will strike, evidence suggests that future storms may be more powerful. Rising sea levels will magnify the tidal surges, further eroding beaches and flooding interior marshes with salt water. Such forces could undermine ongoing efforts to protect and restore the coastline and its wetlands.

A powerful hurricane can devastate a coastline, destroying barrier islands, eroding beaches and flooding freshwater marshes. Climate models predict that such storms are likely to increase in intensity during this century. Courtesy of NASA

Consider the devastation inflicted by the dual blows of Tropical Storm Isidore and Hurricane Lili, which struck Louisiana within a two-week period last fall. The combined impact of the storms caused more damage to Louisiana's barrier islands than Hurricane Andrew did in 1992, said Dr. Shea Penland, a geologist with the University of New Orleans and an expert on Louisiana's coastal wetlands. Isidore and Lili caused severe erosion, sediment overwash and extensive damage to Breaux Act restoration projects, such as those in the Isle Dernieres barrier islands, said Penland. Trinity Island, the largest in the chain, lost 231 feet of beach — roughly 30 percent of the island — unraveling much of the restoration work done after Andrew stripped 160 feet from the beach 10 years ago. "Without these Breaux Act barrier-island restoration projects in place, the destruction to the Isle Dernieres would have been much greater," said Penland.

Prior to Hurricane Lili, East Island, part of the Isle Dernieres barrier island group, was a strong barrier with wide beaches, accumulating sand on its eastern end and protecting the marshes and waters behind it. Courtesy of Dr. Shea Penland

After Hurricane Lili's devastation, much of the eastern end of this barrier island is gone, gouged out by the hurricane's wind and waves. The hurricane almost split the island in two parts, nearly breaching it at the point indicated by the arrow. Courtesy of Dr. Shea Penland

Rainfall and Runoff

In addition to the harmful effects of rising seas and perhaps stronger storms, Louisiana's wetlands face the threat of serious changes in precipitation. The two climate models project opposite scenarios for both rainfall and river runoff in the state's coastal zone.

"The more recent droughts may be signatures of global climate change," said Penland. Dry conditions, such as those experienced during the drought that extended from 1998 - 2000, could increase the dieback of marshes as a result of the "brown marsh" phenomenon. Reductions in rainfall and river flow would degrade water quality, altering food webs in estuaries and increasing the risk for harmful algal blooms and the accumulation of contaminants in animals and humans.

Increases in freshwater runoff to the Mississippi River, on the other hand, could exacerbate conditions like the "dead zone" off Louisiana's coast, where fertilizer-rich runoff has formed an 8,500-squaremile area of oxygenstarved marine life. Such conditions could decrease yields of shrimp and other catches.

Drainage from the Continent to the Gulf

The Mississippi River Basin, the source of nutrients causing the 6,000-to 7,000-square-mile "dead zone," or hypoxia in the Gulf of Mexico.

Courtesy of NOAA

Higher Temperatures

Warmer weather would pose another threat to the state's wetlands. A rise in temperature of even a few degrees could be deadly for the larval forms of many ocean species that spend the early stages of their lives in estuaries. An acceleration of wetlands loss due to changes in global climate would have significant consequences, exposing the region to more damage from storms and threatening navigation, industry, agriculture, fishing and the people of Louisiana.