Q: Is Louisiana losing a football field of land to the ocean every hour?
A: Yes. Both natural processes and human activities contribute to the land loss, though humans are primarily to blame.
FULL QUESTION
Is Louisiana losing a football field of land to the ocean every hour?
FULL ANSWER
Yes. Torbjörn E. Törnqvist, a geology professor at Tulane University in New Orleans, told us by email, “Estimates vary a bit, but by and large the one football field per hour metaphor is very reasonable.”
According to the U.S. Geological Survey’s most recent analysis in 2011, Louisiana lost an average of 16.6 square miles of land a year from 1985 to 2010, which equates to roughly a football field per hour. In total, the state lost 1,883 square miles of land between 1932 and 2010 — an area over 1.2 times larger than Rhode Island.
Scientists say Louisiana’s land loss involves at least three main factors — (1) reduced sediment flow from the Mississippi River and its tributaries, (2) subsidence, or the sinking of land, and (3) sea-level rise. These factors come about via natural processes, human interference or both.
However, before human interference, the interaction of natural processes led to a net increase of land in the region, which has led scientists to conclude that land loss in Louisiana is a human-caused phenomenon.
The effects of the state’s land loss include decreased flood protection during hurricanes and tropical storms and threats to the state’s commercial fishing and oil and gas industries.
Why Losing Wetlands Matters
When scientists talk about land loss in Louisiana, they’re referring to the conversion of wetlands to open water.
Wetlands, which make up roughly 11 percent of the state, are areas where the water level hovers around the surface of the soil. They are among the most diverse and productive ecosystems in the world, alongside rainforests and coral reefs. They provide food for a host of different animals, from insects to mammals — including humans.
In Louisiana in particular, 75 percent of commercial fish and shellfish species depend on the state’s wetlands for breeding and survival, says the Louisiana Coastal Protection and Restoration Authority. As a result, when wetlands are lost, so are the habitats that sustain the fishing industry.
The oil and gas industry is also threatened by wetland loss in the state. “As the coastline recedes, tangles of pipeline are exposed to corrosive seawater,” according to a piece published in Bloomberg in August 2016. With refineries and ports also at risk, wetland loss threatens $100 billion in energy infrastructure, Bloomberg says.
Lastly, wetlands protect against floods by soaking up and slowing flood waters.
One acre of wetland, an area smaller than a football field, can store roughly 1 million gallons of water, according to a 2006 Environmental Protection Agency summary of the subject. Wetland vegetation also slows the speed of floodwaters. Consequently, wetlands “can actually lower flood heights and reduce the water’s destructive potential,” the EPA explains.
Coastal wetlands, such as those in Louisiana, especially protect against coastal floods and storm surge caused by hurricanes and tropical storms.
Coastal floods come about during periods of heavy rainfall, high tides and strong winds blowing inland from the ocean. Similarly, storm surge is “caused by forces generated from a severe storm’s wind, waves, and low atmospheric pressure,” explains the National Oceanic and Atmospheric Administration. In other words, both of these phenomena are defined by water moving inland from the coast.
For example, the extent of Hurricane Katrina’s impact on New Orleans and surrounding areas in 2005 could have been partly mitigated had the wetlands remained better intact. Katrina “is a prime example of the damage and devastation that can be caused by [storm] surge,” says NOAA. “At least 1500 persons lost their lives during Katrina and many of those deaths occurred directly, or indirectly, as a result of storm surge,” the agency says.
Cause 1: Reduced Sediment Flow
Reduced sediment flow from the Mississippi River and its tributaries is largely responsible for wetland loss in Louisiana.
Before humans settled in the region, sediment flow created enough land in southeastern Louisiana to more than compensate for natural sea-level rise (from natural climatic variability), sinking land and the occasional storm.
In fact, researchers estimate that over the thousands of years prior to human interference in the Mississippi River Delta, sediment flow led to a net increase of more than 9,650 square miles of wetlands, an area larger than the size of Vermont.
When humans settled in the area, they began managing the Mississippi River by building levees (see image left) along the river’s borders to prevent river flooding.
Different from coastal flooding, river flooding occurs when rivers overflow after excessive rain or snowmelt, for example. River flooding can also be caused by upstream precipitation, whereas coastal flooding, by default, comes from the coast (downstream).
By building levees, humans prevented significant amounts of sediment from reaching wetland floodplains. The sediment built up the wetlands as they submerged due to sinking land and natural sea-level rise.
This means there’s a tradeoff between mitigating river flooding versus mitigating coastal flooding and storm surge in Louisiana, Robert R. Twilley, a professor of oceanography and coastal sciences at Louisiana State University, and other researchers, explain in a July 2016 paper.
Published in the journal Sustainability Science, that paper compared changes in the ratio of land-to-water between 1932 and 2010 in two different regions of wetlands along Louisiana’s coast — the Atchafalaya and Terrebonne Basins.
The Atchafalaya Basin has maintained sediment flow, and, as a result, “is the only region of coastal Louisiana that is building deltaic wetlands and confirms the ability of the river to sustain delta landscape if sediment delivery is allowed to occur across the coastal floodplain,” the researchers write.
In the Terrebonne Basin, sediment supply from the Mississippi River was eliminated in 1903 due to the building of levees. Consequently, the region has experienced relatively drastic land loss.
By comparing these two regions, the researchers concluded that reduced sediment flow has been a significant driver of wetland loss in Louisiana. “Subsidence certainly contributes to land loss, but in these cases, we propose that sediment supply exerts stronger influence,” the authors said.
Cause 2: Subsidence
Reduced sediment flow is a human-caused contributor to wetland loss in Louisiana. But subsidence, or the sinking of land, involves both natural and human-caused factors, such as sediment compaction and groundwater and oil extraction, respectively.
Sediment compaction is the process by which sediment underground becomes progressively more dense and less porous over time. Regardless of human activity, this process is always occurring in the region.
Törnqvist, at Tulane, told us by email that for Louisiana’s wetlands overall, “Present-day subsidence rates are likely predominantly natural.” But human activity may have amplified the rate of sinking land in the region in the past, he added.
In a November 2011 paper published in Geophysical Research Letters, Alexander S. Kolker, an associate professor at the Louisiana Universities Marine Consortium, and others pointed to oil production as a potential exacerbator of sinking land in the 20th century.
When fluids like oil are withdrawn from the ground, it decreases the volume of the reservoir, thereby causing the land to sink.
South Louisiana produced 114 million barrels of oil in 1945, up to 437 million barrels in 1968 and back down to 55 million barrels in 2005. Kolker’s group found a “close correspondence” between these changes in oil production in the region and changes in subsidence rates, which suggests these human activities can have a “rapid influence” on the rate of sinking land in the region.
This means that the oil and gas industries are not only threatened by land loss in the region, as previously mentioned, but they are also part of the cause of the phenomenon.
Törnqvist also told us that in localized areas like New Orleans, “human influences” have been and continue to be a dominant cause of sinking land.
In May 2016, NASA radar scientist and engineer Cathleen E. Jones and others found that surface and groundwater extraction drive subsidence in New Orleans and surrounding areas. Published in the Journal of Geophysical Research: Solid Earth, the study found especially high rates of sinking land around “major industrial facilities” near New Orleans.
While sinking land in New Orleans hasn’t directly led to wetland loss, it may have made the city even more susceptible to flooding during storms like Hurricane Katrina.
Cause 3: Sea-Level Rise
Törnqvist added that while reduced sediment flow due to river management has been the main factor leading to land loss in Louisiana to date, “in the longer run, [absolute] sea-level rise driven by increased greenhouse gas emissions will be the greatest challenge.”
Scientists look at sea-level rise in two different ways. Absolute sea-level rise takes into consideration only increased ocean height due to melting ice caps and warming seas. Relative sea-level rise, however, includes both absolute rise and land changes, including sinking land, in specific areas.
In fact, Törnqvist and others at Tulane found that 35 percent of the wetlands in the Mississippi Delta and 58 percent in the Chenier Plain — which together span southern Louisiana — may not be able to keep pace with relative sea-level rise. The rate of relative sea-level rise in the region is currently between 4 and 20 millimeters per year, one of the world’s highest.
Published in Nature Communications this month, their research means that — even if the rates of absolute sea-level rise and sinking land remain the same — the majority of Louisiana’s current coast is at risk of being lost.
But the rate of absolute sea-level rise will likely not remain the same, making it an increasing threat to Louisiana’s wetlands.
Prior to human interference, absolute sea level had risen globally due to natural climatic variability — about 400 feet in the past 20,000 years. As we already mentioned, sediment flow from the Mississippi River was able to keep pace with this rise, along with the rate of sinking land.
Human activity, including the burning of fossil fuels, has caused the rate of absolute sea-level rise to roughly triple since the beginning of the 20th century. And the rate is expected to increase further by the end of the 21st century.
Over the phone, Törnqvist made a point to tell us that his group’s estimates were “conservative.” Yet, “when you low-ball everything, it still looks bad,” he said.
Still More Causes
Other factors also contribute to land loss in Louisiana. For example, river management goes beyond the building of levees to ward off river flooding.
Wetlands were dredged to build over 10,000 miles of transport canals used by oil and gas industries. These canals made the remaining wetlands susceptible to saltwater intrusion from the Gulf of Mexico. Some of Louisiana’s wetlands are freshwater ecosystems, which means that saltwater causes their vegetation to deteriorate.
While many complex processes contribute to wetland loss in Louisiana, scientists agree reduced sediment flow due to the building of levees to ward off river flooding has been a significant factor. Without sediment flow, the region can’t keep up with sinking land and sea-level rise — both of which occur via natural and human-caused means.
But in the future, rising seas may pose the largest threat to Louisiana’s coast, given that the rate of absolute sea-level rise is increasing due to global warming — another predominately human-caused phenomenon.
Editor’s Note: SciCheck is made possible by a grant from the Stanton Foundation.
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