By Alisha A. Renfro, Ph.D., National Wildlife Federation
Rising sea levels expose low-lying coastal areas to increased flooding, saltwater intrusion, and erosion, which can further increase their vulnerability to the effects of storms. The Mississippi River Delta along coastal Louisiana is particularly sensitive to sea level rise, as the river-driven geology causes the land to sink, amplifying local sea level rise.
In order for coastal managers to design restoration projects that will anticipate patterns of land loss and protect coastal communities, sea level rise estimates must be based on the most up-to-date research available. The Louisiana Applied Coastal Engineering and Science Division (LACES) recently released a technical report, “Recommendations for Anticipating Sea-Level Rise Impacts on Louisiana Coastal Resources during Project Planning and Design.” The report’s findings recommend that coastal managers and planners anticipate an average increase in sea level of 3.3-feet by 2100.
Predicting future sea level requires a thorough understanding of drivers behind sea level as well as the historical and current sea level trends. Today, tide gauges, satellite readings, measurements from ships and instruments attached to floats are used to measure the worldwide increase in the average sea surface elevation. Additionally, they allow researchers to better understand the relative contributions of freshwater influx from melting glaciers and other land ice as well as the expansion of ocean water volume from increased temperatures. The global sea level rise trend from 1992-2011 was approximately 0.11 inches per year.
The sea level rise trend in the Gulf of Mexico is less certain than the global trend due to the absence of consistent ship-based measurements and few instrument mounted floats. However, sea surface elevation derived from satellites shows that the sea level rise trend varies along the coast of Louisiana.
Understanding current sea level rise and the factors that contribute to it are the first step in predicting sea level rise over the next century. At present, the largest uncertainty is if ice sheets located on land will melt with warming temperatures, eventually collapsing and causing rapid sea level rise. As a result, LACES recommends assuming a 3.3-foot rise (range of 1.6 to 4.9-feet) in sea level by 2100. However, for specific projects LACES recommends using the historic sea level rise trend for that part of the Louisiana coast as the base for predicting future sea level.
Relative sea level rise is the elevation of the sea measured from a particular location. Relative sea level is the combination of global sea level and a change in the local elevation, such as subsidence. This is an especially important consideration in coastal Louisiana where, due to the Mississippi River Delta-driven geology, land in some areas is sinking 3 to 5 times faster than global sea levels are rising. Unfortunately, subsidence rates vary across Louisiana and are currently not well understood on a local level, but it is hoped that monitoring stations now spread throughout the coast will refine the understanding of subsidence rates.
It is imperative that coastal managers consider the future conditions of sea level to better design and implement projects that can be sustained with these changing conditions. Louisiana’s 2012 Coastal Master Plan uses recommendations from the LACES report to predict the Louisiana coastal landscape in 2060 and outlines a coastal restoration and protection strategy that will help protect the economically and ecologically vital resources of Louisiana.