Global Climate Change
Subsidence & Erosion
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Lessons on the Lake

The Forces are with Us:
Natural Forces Affecting the Lake Pontchartrain Basin

illustration of the shore in a hurricane


  • Understand the direct and implied relationships among global climate change,

    storm events, subsidence, and erosion.

  • Understand the potential impact of global climate change and related issues on

    the Lake Pontchartrain Basin.

Multiple Intelligences Learning Activities:

Develop a student newspaper on factors affecting the Lake Pontchartrain Basin.

Plot hurricane data sets on a tracking map.

Draw illustrations and take photographs for student newspaper.

Develop a multimedia project to foster environmental

Musical Rhythmic
appreciation or depict environmental degradation.

Write C-Mail messages and responses to classmates.

Write a reaction paper on hurricane impact on coastal parishes of the Basin. Associate feelings with facts regarding factors that affect the Lake Pontchartrain Basin.

Natural Forces Affecting Lake Pontchartrain

Humanity and the natural world are interconnected in the Lake Pontchartrain Basin, as everywhere on the planet. Precipitation, temperature, and sea level have always influenced where people live, grow food, and do business. We influence the natural world, though, by the things we do or do not do in the local environment. In this section, we will focus on certain factors affecting the Lake Pontchartrain Basin, including: global climate change or global warming, sea level rise, hurricanes, subsidence, erosion.

Global Climate Change
Subsidence & Erosion

Global Climate Change

The earth's atmosphere is a thin blanket of gases which serves many important functions. It protects the planet from harsh ultraviolet (UV) rays from the sun, and it warms the Earth's surface and modifies our climate. This naturally occurring phenomenon is called the greenhouse effect.

illustration of the greenhouse effect Click the illustration for a larger view.

Earth's atmosphere is made up of about 78% nitrogen, 21% oxygen, and about 1% of trace gases that absorb infrared radiation. These trace gases, which trap heat to warm the earth, are known as the greenhouse gases. The two most important ones in producing the insulating effect of Earth's atmosphere are carbon dioxide (CO2) and water vapor (H20), but others include methane (CH4), chlorofluorocarbons (CFC's), nitrous oxide (N20). and ozone (03). The following chart lists the major greenhouse gases and their sources.

Greenhouse Gasses and their Surces

Greenhouse Gasses Natural Sources Manmade Sources
Carbon Dioxoide
  • respiration of animals & plants
  • volcanic eruptions
  • decay of organisms
  • burning of fossil fuels
  • clearing land for agriculture
  • cement manufacturing
  • livestock
  • wetlands
  • termites
  • landfills
  • coal mines
  • rice paddies
  • natural gas leaks
  • burning of biomass
  • none
  • refrigerators
  • auto air conditioners
  • solvents
  • insulation
  • aerosols
Nitrous Oxide
  • soil microbes' digestion
  • burning coal & wood
  • chemical fertilizers
  • lightning
  • pine trees
  • photochemical reacton between nitrogen oxides & organic compounds
Water Vapor
  • evaporation
  • transpiration
  • photochemical reaction between nitrogen oxides & organic compounds
  • will increase as evaporative effect rises proportionally to increasing amounts of other greenhouse gases

To help you understand more about this complex topic, review this chart of greenhouse gases from a historical perspective:

Atmospheric Greenhouse GasRange from Dawn of Humans to 1750 A.D. Concentration in 1990 A.D.Annual Rate of Increase & Pre-industrial Levels (PL)
Carbon Dioxide
180-295 parts per million353 parts per millionannual growth: 25% PL: 0.5%
0.3-0.8 parts per million1.72 parts per millionannual growth: 115% PL: 0.5-1.0%
Nitrous Oxide
275-295 parts per billion310 parts per billionannual growth: 9% PL: 0.25%
CFC-11Not invented yet.280 parts per trillionPL: 4.0%
CFC-12Not invented yet.484 parts per trillionPL: 4.0%

During those same time periods, the human population of the planet increased from 760 million people in 1750 to 5.3 billion people by 1990.

How do you think this affected the amounts of greenhouse gases?

What other sources of greenhouse gases can be associated with increasing human population growth?
Refer to the Greenhouse Gasses chart.

The amount of carbon dioxide in the atmosphere has been increasing rapidly, as has methane, which traps heat 20-30 times more efficiently than carbon dioxide. One CFC molecule traps 20,000 times the heat of one carbon dioxide molecule.

There is much concern about the negative effects on our climate which can be produced by these atmospheric changes. The increase in the amount of heat trapped by the greenhouse effect due to higher greenhouse gas concentrations will make the earth's climate warmer than it would otherwise be. This is called global warming.

There is some uncertainty and disagreement in the scientific community about the likelihood of global warming, even though scientific predictions of global temperature increase have remained quite consistent for almost 100 years now. It is estimated that the temperature increase resulting from a doubling of CO2 concentrations over pre-industrial levels would occur in the range of 3-8 degrees Fahrenheit (1.5-4.5 degrees Celsius). Scientists believe that this could occur by the middle of the 21st century--or sooner, if CO2 levels more than double. As long as amounts of greenhouse gases continue to escalate, the earth's average global temperatures are expected to increase. As average temperature climbs, so will the likelihood of unusual or disturbed weather patterns around the globe---including the Lake Pontchartrain Basin! Severe storms may increase in frequency and intensity, including tropical storms and hurricanes. Droughts may be commonplace in some areas, resulting in habitat loss, water shortages, and more forest fires. But weather patterns are not the only things that will be affected.

Is Sea Level Rising?
A Major Coastal Concern

A number of studies indicate that the sea has been rising 1-2 millimeters each year for the last century. This is not all due to global warming, though, because the land itself rises and subsides over time due to natural processes. But if atmospheric CO2 concentrations double, the Intergovernmental Panel on Climate Change (IPCC) estimates that planetary warming could cause global sea level to rise from 3-11 inches (8-29 centimeters) to 39 inches (1 meter) in the next 50 to 100 years. This action, which would result from water's tendency to expand when heated as well as from melting of glacial ice and polar ice caps, would send hundreds of meters of seawater encroaching over gently sloping coastal land. This would permanently flood hundreds of square miles of low-lying coastal areas along the Gulf. For those of us who live in the Lake Pontchartrain Basin, it means that barrier islands, tourist beaches, fishing centers, cultural and historical sites, business and industry, groundwater supplies, wildlife habitat, and residential areas would be severely impacted. Coastal wetland habitats might be lost or greatly reduced. We could probably prevent these negative effects from happening, but it would cost over $50 billion for bulkheads and levees to hold back the sea.

Global Climate Change
Subsidence & Erosion

What is that Sinking Feeling? or Are We Slip-Slidin' Away?

Subsidence and Erosion in the Lake Pontchartrain Basin

Mississippi River sediment has formed most of Louisiana's coastal wetlands over the past 5000 years or more. This happened when river water regularly deposited sediments over land areas during the river's annual floods, accumulating thick layers of sand, mud, and silt. Over time, a series of overlapping deltas formed, eventually building a wide deltaic plain that forms Louisiana's coast. Lake Pontchartrain was formed when Mississippi River deltaic deposits partially closed it off from the Gulf of Mexico. All the land in the Pontchartrain Basin south of Interstate 12 consists of Mississippi River sediments deposited over time.

Fresh sediment is needed to nourish the wetlands vegetation and to insure the buildup of land. This was never a problem, because periodic flooding of the Mississippi River spread millions of tons of sediment across coastal marshes every year. This natural process of sedimentation might have continued indefinitely if it were not for human intervention.

The annual flooding caused problems for people who had settled along the river and its distributaries. This resulted in a system of flood control by building levees to help keep the Mississippi River, as well as rivers and bayous throughout the Lake Pontchartrain Basin, confined to narrow channels. While the levee system prevents damage to people and property, including loss of lives, it also prevents river water from flowing through the wetlands. The sediment that once nourished coastal marshes now flows directly into the Gulf of Mexico, where it sinks out of sight off the continental shelf.

Without the annual deposition of sediment from the Mississippi, most of Louisiana's coastal marshes are sinking, too. This is called subsidence, and it is a serious problem in the Lake Pontchartrain Basin. However, levees are not the only factor affecting sedimentation. The Mississippi River itself carries less sediment than it did many years ago, and much of its remaining sediment never reaches south Louisiana. Dams have been constructed on every major tributary of the Mississippi River, trapping sediment so it can't bring new life to coastal marshes. In addition, better farming practices in the Midwestern states prevent erosion of sediment from farmland, so less finds its way into the river.

Subsidence is a serious factor affecting the Lake Pontchartrain Basin, but coupled with sea level rise, it contributes to another problem, erosion. Even before levees lined the banks of the Mississippi River and its tributaries, Louisiana's coastal wetlands were being affected by the digging of ditches and canals through the marsh. First dug by trappers, hunters, and farmers for navigational purposes, larger canals were made by cypress loggers to remove big trees from our once-extensive forested wetlands. Scars from these operations are still visible in the LaBranche Wetlands and in the Manchac Wildlife Management Area near Turtle Cove. The majority of canals in coastal marshes, though, were dug by the oil industry for pipelines and access to drilling sites.

How do canals contribute to subsidence and erosion in the marsh? Spoil banks formed from the dredged material add weight to the spongy marsh soil, causing it to compact and subside. As it sinks, "ponding" occurs, or the formation of large areas of open water in the marsh. Over time, the canals widen, and more wetlands are converted to open water. These factors change the hydrology, or water flow, in the wetlands, preventing much-needed sedimentation and allowing saltwater to intrude farther and farther inland. Freshwater marsh vegetation cannot tolerate these environmental changes and, as it dies, the soil weakens and is subject to erosion by tidal action and severe storms. Shoreline erosion in some areas is greater than 100 feet per year.

In the Lake Pontchartrain Basin, the most dramatic effect of saltwater intrusion can be witnessed at the navigational ship channel known as MRGO, or the Mississippi River Gulf Outlet. Cutting through both fresh and brackish coastal marshes of lower St. Bernard Parish, as well as areas of forested wetlands, the MRGO has contributed in a major way to existing subsidence and erosion problems. Ponding has left a swath of damage threatening St. Bernard and Orleans parishes that will take many more years to repair the damage than it did to create it.

As sea level rise exacerbates problems associated with subsidence and erosion, areas of the Basin especially affected by those factors are the barrier islands, such as the Chandeleur Islands. Formed by the same coastal processes that build wetlands, these fragile fringes are slowly disappearing. While serving to protect the coast from severe storms and hurricanes, barrier islands bear the brunt of such storms. Barrier islands also provide important habitat for marine life. Yet, they are deteriorating faster than natural processes can rebuild them. Sea level rise is slowly flooding the islands, sedimentation remains low, and recent storms have contributed severe erosional damage. Without our barrier islands, the factors associated with global climate change will wreak even more havoc on the fragile coastal marshes.

Human intervention has unintentionally created these problems, and it will require human intervention to do something about them. Several projects already at work are in the experimental stages, and it will be interesting to follow their progress as we hope for positive results in rebuilding wetlands in the Lake Pontchartrain Basin. Mitigation projects have filled in existing pipeline canals and areas of open water in the LaBranche wetlands. Rock dikes near Martello Castle in Lake Borgne and between Lakes Maurepas and Pontchartrain are helping to prevent further erosion. Every year, thousands of Christmas trees are placed in the marshes of Orleans, St. Bernard, Jefferson, St. Tammany and St. Charles parishes, where they serve to slow down erosion from wave action and to trap sediment.

Proposed river diversion projects are being suggested to rebuild our wetlands. These are ambitious projects, costing many millions of taxpayer dollars. Will they be worth the cost? Can we afford to do it? Will they work? Scientists are in heated debate about these projects, as more solutions to our wetland loss problem are being sought.

You can find more information on global climate change and how it affects coastal wetlands in these excellent resources:

(1) Project TELLUS Teaching Modules for Global Change Issues (Lyle Soniat, Ph.D., Sea Grant Education Office, LSU, Baton Rouge, LA 70803)

(2) Global Environmental Education Resource Guide (Sharon Walker, Ph.D., Gulf Coast Research Laboratory P.O. Box 7000, Ocean Springs, MS 39566-7000)

Global Climate Change
Subsidence & Erosion


Those of us who live in the Lake Pontchartrain Basin are always wary of tropical storms or hurricanes that enter the Gulf of Mexico. These severe weather patterns are usually the result of meteorologic activity in the south Atlantic Ocean or in the Caribbean Sea from May through November. Hurricanes are born and grow in the steamy tropics. The connection is energy transfer and the role of water in that process. The sun and the ocean are the furnace for the formation of a hurricane. Toward the end of the summer the accumulated solar warmth in ocean waters may raise the sea surface temperature to as much as 26.5 degrees C.

Convert 26.5 degrees Celsius to ______________degrees Fahrenheit.
(HINT: multiply by 9; divide by 5; add 32)

In the tropical Atlantic, this heating of the ocean's surface can result in the formation of a weak low pressure system with scattered, thin clouds. If the low pressure system is pushed toward the west by the trade winds, it may gain strength with increasing winds and thickening clouds. In this manner, the low pressure system may advance to a tropical depression, then to a tropical storm, and finally to hurricane strength. illustration of low pressure pattern

About one of every eight tropical depressions becomes a tropical storm, and the chance of a selected tropical storm becoming a hurricane is only about 60%.

Water provides the transportation for a hurricane's energy, similarly to the way it may move heat from the furnace to a radiator in your school's heating system. The solar energy in the ocean causes water to evaporate from the surface, bringing heat with it. The water vapor condenses, forming hurricane clouds, and the latent heat is released into the air. This is the power that drives the hurricane. As the warm, moist air from the sea surface cools and the water vapor condenses, huge cumulonimbus clouds form. Precipitation forms in these clouds, and rain falls back to the sea surface in a continuous process. The amount of rain that falls gives us an idea of the strength of the storm. A well-developed hurricane can deliver 24 cm (about 10 inches) of rain per day!

Hurricane Definitions

Tropical Disturbance:
Poorly organized counterclockwise circulation.

Tropical Depression:
Organized counterclockwise circulation, winds up to 39 mph.

Tropical Storm:
Well organized counterclockwise circulation, winds 39-73 mph.

Tropical Storm Watch:
The possibility of winds 39-73 mph within 48 hours.

Tropical Storm Warning:
The likelihood of winds 39-73 mph within 24 hours.

Winds 74 mph or more. Heavy rains and storm surge.

Hurricane Watch:
The possibility of hurricane force winds in excess of 74 mph within 48 hours.

Hurricane Warning:
The likelihood of hurricane force winds within 24 hours.

Saffir/Simpson Hurrican Scale Ranges

Scale# (Category)12345
Winds (mph)74-9596-110111-130131-155156 or greater
Storm Surge (Feet)4-56-89-1213-1918 or greater
Damage (Impact)minimalmoderateextensiveextremecatastrophic

Global Climate Change
Subsidence & Erosion


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©1998 Lake Pontchartrain Basin Foundation

Lessons on the Lake is published by the
Lake Pontchartrain Basin Foundation
Metairie, LA

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Global Climate Change
Subsidence & Erosion