The U.S. coastline is a dynamic and important region that hosts a large percentage of the population, has a critical role in the economy, and is made up of physically, geologically, ecologically, demographically, economically, and socially diverse environments (NOAA, 2013). With the number of people living in coastal areas increasing every year (NOAA, 2013), long-term planning to sustain healthy, productive U.S. coastlines and coastal communities depends on an understanding of the natural processes and human activities—and their interactions and feedbacks—that shape and change the coastal zone. Long-term changes resulting from these processes may affect the coasts’ future habitability, health, and productivity.
The U.S. Gulf of Mexico Coast (hereafter referred to as “the Gulf Coast”) provides a particularly relevant setting to study deeply connected natural and human interactions. This is in large part because of the historical, and continuing, concentration of a wide range of infrastructure and coastal development, much of it energy related, within a region that is highly vulnerable to hurricane landfalls and is experiencing the most rapid rise in relative sea level in the United States (Marcy et al., 2012, 2018). This confluence of intense, varied coastal development and rapid change in environmental stressors makes the Gulf Coast a harbinger of what is to come globally as the rates of sea level rise approach those at the end of the last glacial period—a time when many coastal systems were unable to keep pace and rapidly migrated landward.
Over various timescales, changes to the physical landscape, including natural and human-induced coastal subsidence and erosion; river, delta, and inlet migrations; and modifications from human activity, have significantly and in some places irreversibly changed the coastline. The construction of engineered infrastructure, such as levees and navigation channels, has affected delta morphology and the health and salinity of coastal marshes and related ecosystems. River systems that empty into the Gulf carry sediment and nutrient loads that have also changed over time due to agriculture, engineering, changes in the built
environment, and other human activities. Such modifications can alter the natural (physical and ecological) system, often prompting people to make further modifications, thus creating a cycle that, over time, increases the likelihood that infrastructure, communities, and ecosystems will sustain damage due to sea level rise, coastal storms, and further development.
Over the next 10 to 200 years, the Gulf Coast shoreline will respond to a number of different environmental stresses, such as accelerating relative sea level rise, increasing frequency of intense hurricanes, and warming temperatures (e.g., Bender et al., 2010; NOS, 2011; Biasutti et al., 2012; Wright et al., 2015; Yan et al., 2017). Some of these stresses will lead to dramatic changes along the coast. For example, rapid sea level rise can lead to the loss of barrier islands, thus exposing the mainland to more energetic waves; the loss of wetlands due to their inability to keep up with rising seas; or abandonment of coastal communities and infrastructure when the cost of coastal protection becomes too great. These “tipping points” (discussed in more detail in Chapter 2) may be reached earlier along the Gulf Coast than in other U.S. coastal settings, due to its low-lying sedimentary coastline and vulnerable ecosystems.
Another stressor for the Gulf region is coastal storms. Widespread flooding and associated coastline erosion and ecosystem disturbance1 during the 2017 Atlantic hurricane season led to major disruptions in energy, water supply, and wastewater treatment throughout the Gulf of Mexico, the Southeast Atlantic, and Caribbean coastal zones2 (e.g., Egan, 2017; Jaimes et al., 2017; Jermoe, 2017; Halverson, 2018; Smith et al., 2018). Cascading impacts of the 2017 hurricane season—on commerce, the political landscape, and socioeconomic vulnerability, for example—are as yet unknown, but based on impacts from the past decade (e.g., the 2005 hurricane season), will likely be significant (Halverson, 2018). One consequence of the recent storm season was the outmigration of hundreds of thousands of people from Puerto Rico to the U.S. mainland (Alvarez, 2017); whether it is permanent or temporary remains unknown. Recent work by Hauer (2017) predicts that by 2100, sea level rise could lead to the outmigration of tens to hundreds of thousands in some Gulf Coast counties. Such an egress could tax inland urban centers such as Atlanta, Denver, and Phoenix (Hauer, 2017).
Currently, there are coastal ecosystem and shoreline restoration initiatives in place (e.g., the State of Louisiana’s 2017 Coastal Master Plan). In the case of Louisiana, however, these plans are only being implemented after significant and irreversible land loss has already occurred. Even if Louisiana’s Coastal Master Plan is fully implemented, substantial net land loss will continue to occur over the next half-century (LACPRA, 2017). Thus, the Gulf
1 For example, see pre- and post-Hurricane Harvey comparisons from Texas: https://www.usgs.gov/center-news/pre-and-post-storm-photo-comparisons-texas?qt-news_science_products=2#qt-news_science_products.
2Response and Recovery to Environmental Concerns from the 2017 Hurricane Season: Hearing Before the Members, Subcommittee on the Environment, 115th Congress. 2017. Statement of the Majority Staff of the Committee on Energy and Commerce.
Coast offers a unique opportunity to gain fundamental understanding of coastal system evolution—both human and natural—that will be globally relevant. A major scientific challenge, with large societal implications, is whether projections of the conditions under which such coastal tipping points are reached can be improved.
Solutions to the Gulf Coast region’s contemporary problems can be informed by improved understanding of the potential evolution of the system. Though still challenged by uncertainty, such understanding could enable objective evaluation of long-term outcomes of different mitigation approaches and thus allow assessment of different courses of action. The recent past and future evolution can only be understood, projected, and affected if the physical, ecological, and human components of the system, as well as their interactions and feedbacks, are well understood.
The National Academies of Sciences, Engineering, and Medicine’s (the National Academies’) Gulf Research Program (GRP) asked the National Academies’ Board on Earth Sciences and Resources, Ocean Studies Board, and Board on Environmental Change and Society to undertake a study on long-term coastal zone dynamics and the interactions and feedbacks between human and natural processes that occur in the coastal zone. The complete Statement of Task (SOT) is provided in Box 1.1. The project originated as a way to better understand the multiple factors that influence long-term processes along the Gulf Coast, and was envisioned to help inform decision making and planning related to the four strategic initiatives of the GRP:
- Healthy Ecosystems – Advance understanding of ecosystem processes and dynamics to facilitate sustainable use of natural resources.
- Thriving Communities – Enable people and coastal communities to successfully prepare for, respond, and adapt to stressors and adverse events.
- Safer Offshore Energy Systems – Foster minimization and management of risk to make offshore operations safer for both people and the environment.
- Capacity Building – Enhance the ability of researchers, decision makers, and communities to use science to solve challenges at the intersections of human, environmental, and offshore energy systems.
The committee believes that, in addition to informing the GRP, this report can assist with possible research directions and decision making among relevant scientific disciplines, private-sector organizations, and local, state, and regional governments.
The Committee on Long-Term Coastal Zone Dynamics: Interactions and Feedbacks Between Natural and Human Processes Along the U.S. Gulf Coast was convened to answer this request. Twelve committee members brought to this task a broad spectrum of knowledge and expertise related to coastal geology, ecology, and engineering; geography; energy systems development; economics; and social and behavioral science. Committee member and staff biographies are provided in Appendix A.
Toward the beginning of the study process, the committee determined that several terms in or related to the SOT needed a common definition to better define the report scope. Definitions for these key terms are found in Box 1.2. Because of the emphasis on energy and energy-related infrastructure in the SOT, the committee’s approach was to focus the report content slightly more toward the Louisiana and Texas coasts, which have a higher
prevalence of such infrastructure compared with Alabama, the Gulf Coast of Florida, and Mississippi. However, there are discussions of and examples from all of the Gulf states throughout the report and most of the processes addressed are relevant to all Gulf states.
During the study, the committee convened three information-gathering meetings (May 17–18, 2017, Washington, DC; September 18–19, 2017, New Orleans, Louisiana; November 15–16, 2017, St. Petersburg, Florida); one workshop with 20 invited participants (July 18–20, 2017, Houston, Texas); and an additional meeting in closed session to develop this report (January 18, 2018, Galveston, Texas). The committee heard presentations from leaders in fields related to many aspects of the SOT, including state and federal agencies, academia, nongovernmental organizations, and the energy industry. These participants are listed in Appendix B. The committee also consulted peer-reviewed research literature, community-sponsored efforts, and state and federal government reports to provide a strong scientific foundation.
The report structure is as follows: Chapter 2 summarizes the current state of the scientific knowledge about the Gulf Coast and its processes; Chapter 3 discusses the high-priority gaps in scientific knowledge associated with these processes (hereafter referred to as “research gaps”) (SOT Task 1); Chapter 4 presents barriers to communication between scientists and stakeholders (SOT Task 3); and Chapter 5 identifies critical areas of research to increase understanding of long-term coastal dynamics (SOT Task 2b), presents guidelines for a research agenda (including essential components of a research and monitoring program [SOT Task 2a]), and opportunities for more effective communication between scientists and stakeholders (SOT Task 3).
The committee hopes this study will provide valuable guidance for the GRP and an array of governmental and nongovernmental stakeholders.