Northern Gulf of Mexico (NGOM) Ecosystem Change and Hazard Suseptibility Project - Overview

Statement of Problem

Storm Climate Change:

Figure 2.  Population density increase from 1900 to Present with counties along the Gulf of Mexico coastline (U.S. Census, 2006) [enlargement]

Following the devastation wrought by Hurricanes Katrina and Rita in August - September 2005, it is apparent that a better understanding of the northern Gulf of Mexico (NGOM) coastal system, and its response to human activities, is necessary for sustainable restoration, redevelopment, and sound natural resource management strategies.

Global climate projections that suggest more intense Atlantic hurricanes over the next several decades provide further justification for investigations of the geomorphological structure, ecological function, and hazard vulnerability of the NGOM coast.

Most scientists agree that the global climate is changing, and model results and theoretical considerations support the idea that ocean warming will affect tropical storm characteristics. Ocean sea surface temperature changes modify atmospheric circulation, which in turn produces conditions that are favorable to hurricane formation. Hurricanes act as immense heat engines that run on energy transferred from the ocean surface, and accordingly, some researchers have proposed that increases in North Atlantic sea surface temperature may boost mean storm intensity. Such an increase in the baseline level of hurricane activity can also be expected to impact coastal zones by influencing local sediment supplies, altering marsh accretionary processes, and changing rates of wind and wave erosion.

Analysis of global meteorological data collected since 1970 has revealed that the annual number of tropical cyclones occurring in the World Ocean has been nearly uniform during the last several decades. Typically, 80 to 90 storms form each year in the tropical oceans, with an average of eleven percent forming in the Atlantic Basin. However, since 1995 in all ocean basins the proportion of intense cyclones within the total number of storms has increased, and the lifetime of these major storms has increased on average by about twelve hours. In the North Atlantic, this global change has meant a statistically significant increase in the annual count of hurricanes in the decade since 1995. Also, the Atlantic hurricane season has lengthened during the last decade, with more storms occurring early, in July, and late, in October.

During summer 2005, there were 27 named Atlantic hurricanes, and nineteen of these formed in the eastern North Atlantic Basin from waves in the westward flowing tropical trade winds. The observed trends in sea surface temperature and Atlantic storminess cannot be fully explained by natural variability, because the North Atlantic Oscillation accounts for only 10 percent of the sea surface temperature changes over the last ten years. Further, recent change in hurricane activity is not likely to be driven by sea surface temperature increases alone, because global circulation model simulations require doubled atmospheric carbon dioxide to achieve similar upshifts, and those impacts are global.

Rather, it appears that persistent changes in the wind patterns over the tropical North Atlantic since 1995 have been the main cause of the altered number and characteristics of developing hurricanes. Note that in 2005, 30 percent of the atmospheric tropical waves entering the North Atlantic Basin from the east developed into tropical storms, but prior to 1995, only an average of ten percent reflected this evolution.

This general change in the fate of tropical waves is linked to a marked decrease in vertical shear in the atmosphere over the tropical Atlantic that is anticipated to persist over the coming decades. Accordingly, both theory and observations strongly suggest that longer and more intense hurricane seasons can be expected in the future. The elevated mean level of North Atlantic hurricane activity seen since 1995 should be taken as an indication of the expected conditions over the coming decades.

In other words, the hurricane climatology of the 20th century should not be used in preparing for the hazards posed by hurricanes in the 21st century. Clearly, post - Hurricane Katrina and Rita, reconstruction policies, ecosystem susceptibility to storm modification, and human vulnerability to storm hazards should be evaluated with respect to the probable new storm climate.

SeaLevel Change:

The NGOM coastal region is dynamic; changes in the physical system have been driven by natural variability in the earth’s climate system and, during the last thousand years, human activities. An understanding of rates of relative sea-level rise in the region will impact restoration and storm protection plans but the extent to which sea-level projections can be factored in depend on a better understanding of climate change. Today, climate change and variability in the region are driven by a complicated combination of natural and anthropogenic forcing. To unravel the impacts of either on the system, we must estimate the type and range of natural variability likely to occur in the next 50 - 100 years. Several of the tasks in this project will address this issue.

Wetland Loss:

Along with the increased threat of hurricane activity, dramatic landscape change in the northern Gulf of Mexico region during the last century has reduced the level of hurricane protection afforded to northern Gulf of Mexico (NGOM) human populations by destruction of coastal wetlands and barrier islands. During the 60 year period between 1930 and 1990, coastal Louisiana lost an estimated 3950 square kilometers, or 1526 square miles, of wetlands, defined as periodically flooded land containing emergent vegetation. In the north-central Gulf Coast, dramatic wetland loss during the last century is tied to the history of human development, and was primarily due to inundation or erosion. Over 95 percent of the land lost in southern Louisiana was due to the conversion of wetland to open water.

Moreover, large areas of coastal Louisiana wetlands have become more saline as salt water has encroached on the deteriorating coastal zone. Overall, some 40 percent of all U.S. coastal wetlands are found in the north - central Gulf Coast, and the loss of wetlands in Louisiana since 1930 is roughly equal to 80 percent of the total U.S. national wetland loss over the last 60 years.

Although the long term mean rate of loss from 1956 to 2000 was 88 square kilometers per year, regional rates of wetland land loss have varied since 1930. A maximum loss rate of about 100 square kilometers per year occurred during the 1970s, but this rate declined in the 1990s to about 60 square kilometers per year. Although lower than previous rates, these high rates of southern Louisiana land loss are projected to continue during at least the next 50 years, and one recent prediction estimates an additional loss of wetlands exceeding 1329 square kilometers by the year 2050.

The observed accelerated rates of land loss in coastal Louisiana most likely a result of human disturbance and intervention in the processes that maintain coastal wetlands. Human activities that have exacerbated land loss in the northern Gulf of Mexico include the construction of flood protection structures, alteration of the hydrologic regime, canal and channel dredging and fluid extraction associated with maritime commerce and energy production, and wildlife management practices.

Barrier Island Change:

Barrier island chains in the northern Gulf of Mexico extending from Mobile Bay, Alabama to Atchafalya Bay, Louisiana are disintegrating rapidly as a result of combined physical processes involving limited sediment availability, alteration of alongshore sediment transport, and rising absolute sea-level. The cumulative loss of land area and rates of land loss from these ephemeral islands are both astonishing and to some extent expected because present physical conditions are different from those that existed when the islands first formed.

Demographic Change:

Extreme change to northern Gulf of Mexico coast landscapes that diminished protection from severe storms coincided with a general shift of the U.S. population to the nation’s coasts over the last several decades. Today, 559 counties with centers that lie within 80 kilometers of a coastline comprise just 13 percent of the continental U.S. land area, but account for 51 percent of the 2000 population and 57 percent of 2000 civilian income. The U.S. economy is overwhelmingly concentrated at its ocean and Great Lakes coasts.

For example, the mean income per square kilometer in coastal counties is more than eight times that of the remaining inland counties. This coastal concentration of human and economic density within the conterminous U.S. increased throughout much of the twentieth century, and continues to the present. Since 1900, population density has sharply increased in all 16 coast - adjacent counties within the Louisiana – Mississippi – Alabama Gulf of Mexico coastline. Across the eastern portion of this coastal reach, population density has typically increased by more than 50 people per square mile, and in the area south of New Orleans, such increases have exceeded hundreds of people per square mile. (Figure 2) (U.S. Census Bureau, 2006*).