Sea Level Rise
Ocean and Coastal National Park Units
Ninety-two percent of U.S. coastal national parks are affected by sea level rise or will be in the future. Because of its impact on the NPS, sea level rise is one of the seven issues prioritized by Regional Natural Resource Chiefs for the NPS Ocean and Coastal Resources Program (OCRP) to focus on over the next three years.
Infrastructure, natural resources, cultural resources, and archeological sites are vulnerable to periodic high tide flooding, such as what is depicted in this image from War in the Pacific National Historic Park in Guam. Additionally, as storms from stronger with climate change, higher storm surges will threaten coastal parks, especially those along the Atlantic and Gulf Coasts. As a result of flooding and erosion attributed to sea level rise and storm surge, valued resources managed by parks may be lost or damaged, which will impact visitor experience and safety in parks.
The OCRP sea level rise (SLR) project aims to help parks understand and adjust or adapt to changes related to sea level rise by providing parks with the best available sea level rise and storm surge information, data, tools, and projections for use in education, communication, management, and for general awareness.
What Causes Sea Level Rise?
Global sea level is rising at an accelerated rate, mainly in response to a changing climate. There are two major causes of global sea level rise:
- The ocean is absorbing around 90% of the heat trapped by additional greenhouse gases in the atmosphere associated with emissions from human activity. As it warms, ocean water expands in a process called thermal expansion, causing sea level to rise.
- Warming of the atmosphere is also causing sea level rise by melting land-based ice, such as glaciers and ice sheets. The addition of freshwater runoff from land causes sea level to rise. The photo set to the left shows the difference in the extent of Pedersen Glacier in Kenai Fjords National Park from 1909 to 2005.
Although the above reasons explain why global sea level is rising, local factors contribute to differences in relative sea level rise, which is sea level change as compared to the surface of the land:
- Vertical land motion impacts the relative height of sea level for a specific location. For example, much of the East Coast and Gulf Coasts are experiencing land subsidence, which is where the land is sinking.
In contrast, the land in much of Alaska is rising through the processes of glacial rebound and tectonic uplift. Relative sea level in Alaska is actually going down because the land is rising faster than the water in that location.
- Changes to ocean circulation can change the rate of sea level rise on a more local scale. For example, a recent slow-down of the Gulf Stream has contributed to higher sea levels on the East Coast.
- Different parts of the Earth are warming at different rates, and, as mentioned above, as the oceans warm, water expands through the process of thermal expansion. Areas that are warming faster experience greater rates of thermal expansion, contributing to higher rates of relative sea level rise. The Gulf of Maine is one location that is warming unusually fast, having spent much of the last few years at higher-than-normal temperatures and experiencing several marine heat waves. This phenomenon is largely due to changes in ocean circulation that allow more warm water from the Gulf Stream to enter the Gulf of Maine rather than the usual cooler water from currents in the North Atlantic.
Storm Impacts on Coastal Parks
In addition to sea level rise, coastal parks face periodic flooding and erosion events due to storms. Although current research indicates that it is not clear whether storms will become more frequent with climate change, it is highly likely that storms will become more intense (i.e., windier and wetter) because hurricanes gather strength when passing over warm ocean waters: as oceans get warmer, storms get stronger.
Stronger storm surge, exacerbated by sea level rise results in increased erosion of beaches, coastal wetlands, and other critical habitats and can cause extensive inland flooding.
National parks protect many irreplaceable cultural and historic resources and important natural resources that could be damaged or destroyed by such increased storm surges. Visitor safety, access, and recreational enjoyment of national parks is also threatened by the potential impact of SLR and the increased intensity of storms.
SLR and Storm Surge Challenges
Sea level rise and storm surges cause challenges for parks by threatening the natural and cultural resources intrinsic to those parks as well as the built environment that allow the park to operate and support visitors.
Threats to Infrastructure
Regular high tide flooding and intensified storms threaten infrastructure in many coastal parks. Many buildings and road systems were not designed for the current rate of SLR and may need to be reassessed in light of increased risks. Determining which areas of coastal parks are most likely to flood can help park managers prioritize which structures to adapt.
Some parks are working to replace vulnerable structures with new ones that are better suited to deal with sea level rise and storms. For example, Assateague Island National Seashore has built portable changing rooms, passive-solar vault toilets, and solar-powered showers that are designed so they can be moved before a storm is likely to hit.
Also, some parking areas at Assateague Island are now made of crushed clam shells rather than asphalt to reduce asphalt being washed into sensitive environments when the parking lots are damaged during storms.
Other times, infrastructure must be abandoned due to damage from flooding or storm surge. Infrastructure left abandoned will pollute the natural environment with debris. For example, a highway built in the mid 20th century on Assateague Island National Seashore was damaged by a three-day Nor'easter in 1962 and left abandoned when future commercial development of the island was deemed no longer feasible. The national seashore was established shortly after in 1965. Many sections of the road are still present on the island, and cleanup efforts are underway to remove the abandoned infrastructure from the natural environment.
Although similar modifications may be appropriate for some other parks, these methods will not work for every park. Creativity is important in devising park-specific sea level rise adaptation planning.
Threats to Natural Resources
A storm surge caused salt water intrusion in anchialine pools in Pu‘uhonua o Hōnaunau National Historical Park. What once was a freshwater or brackish environment became flooded with saltwater.
Sea level rise and storm surges can cause dramatic changes to the environments in our national parks as a result of coastal flooding, erosion, saltwater intrusion, habitat loss or modification, altered physical dynamics (e.g., changes to currents), and other impacts. The ramifications of these changes can be drastic. For example, many national park units provide nesting, feeding, or breeding grounds for species listed as threatened or endangered. The loss of coastal habitat from erosion or saltwater intrusion may make the habitat inhospitable, which reduces the potential suitable habitat for these species. These effects can lead to localized extinction events of these at-risk species.
Flooded sea turtle nesting habitat is one example of how rising sea levels and increased storm surges impact valuable natural resources. On Cape Hatteras National Seashore, five species of sea turtle are placed under the protection of the Endangered Species Act: the Kemp's Ridley, leatherback and hawksbill sea turtles are listed as Endangered, and the loggerhead and green sea turtles are listed as Threatened.
During the 2018 breeding season on Cape Hatteras, 19 nests were completely washed away, 8 nests completely failed, and many others were partially affected by prolonged overwash and flooding from storm events. The video on the right shows flooding of turtle nests in Sarasota FL during Tropical Storm Debby in 2012. Similar conditions occurred during the 2018 storms that flooded Cape Hatteras turtle nests.
Storms, which are becoming more intense and are possibly becoming more frequent, threaten this valuable nesting habitat because they cause sand compaction and inundation over sea turtle nests, which can completely or partially inhibit embryonic development of entire nests. If flooding during the breeding season continues to become more intense or more common, these breeding populations of sea turtles will continue to struggle, and localized extinction events are possible.
Protecting natural resources will look different for each park and each resource. For example, loss of a salt marsh might be mitigated through assisted migration upland or restoration, while a threatened native species might be protected by removing additional stressors, such as an invasive species that it must compete with. Other ways to conserve natural resources include restricting visitor access to an area where a resource is struggling or proactively directing the change occurring to an environment towards a more desirable future. Many more creative solutions are being developed and implemented by parks.
Threats to Cultural Resources
Coastal national parks manage an array of culturally important resources, from Native American and colonial archeological and historic sties, to structures such as lighthouses. Here are some of examples of what could be lost, damaged, or no longer available for visitor enjoyment due to sea level rise: culturally- and historically-important archeological sites, buried artifacts now under water, and historic structures or landscapes.
One way to protect historic structures from sea level rise is to move them farther from the coast. The Cape Hatteras Lighthouse provides a perfect example of this method. The lighthouse that stands today was built in 1870 to replace the original lighthouse built in 1799. The lighthouse has helped guide ships through a particularly treacherous section of Atlantic shipping routes for centuries and is therefore important to maritime history.
By the mid-20th century, however, erosion began seriously threatening the structure. In 1999, the lighthouse was moved 1,500 feet from shore where it is now better protected from further erosion.
Moving historic sites farther inland is, of course, not possible for every park. For example, archeological sites, such as the shell mounds in the image below, cannot be moved. Other methods of protection, such as stabilizing shorelines or nourishing beaches with dredged sediment to combat erosion may better suit parks with these types of archaeological sites.
Similar shell mound sites at Canaveral National Seashore have recently been the subject of a shoreline stabilization project. This project can serve as an example for the ways archeological sites can be managed in the face of sea level rise. Although it may be possible to protect archeological sites in some parks, accepting the loss of archeological sites may be the only feasible option for other parks. Since most archeological sites cannot be moved in the same way a building or lighthouse can be moved, some parks may need to accept that their sites will be lost to erosion or sea level rise. These parks could elect to study the site and gather as much information on the artifacts and their contexts now before the artifacts are lost to sea level rise in the future.
Impact to Visitors
Parks that are changing due to sea level rise or storm events are communicating those changes to their visitors. Many parks have already created their own sea level rise communication products and practices, and the SLR project aims to support and strengthen communication by providing parks with additional resources. For example, the SLR team is working on park-specific sea level rise briefs that can be used to educate visitors on the process of sea level rise and how it is affecting their park.
Colonial National Historical Park (COLO) is facing the loss of many historical and archeological sites to erosion and inundation. Not only does this have implications for the loss of valuable resources, but it also impacts the visitor experience by reducing accessibility of resources for visitors to enjoy.
COLO provides an example of the type of communication products being created by parks to help visitors understand changes occurring. COLO is in the process of prioritizing which resources to protect from SLR threats and which resources must be lost to the sea. Communicating why certain sites were prioritized while other sites were not is important for helping visitors of the park understand the changes at COLO they may witness over time.
Visitor safety is another concern for coastal parks. Parks need to be aware of what areas could become dangerous in storm events so they can alert visitors. Additionally, properly assessing damage after a storm event is important for parks to know when it is safe to reopen for visitor enjoyment and how to prioritize any necessary repairs.
NPS Response to SLR
The NPS has recently developed the Resist-Accept-Direct (RAD) decision framework for responding to rapid ecological change, and this framework can be applied to the issue of sea level rise as it affects various natural and cultural resources. Other agencies, such as the US Fish and Wildlife Service, also implement this decision framework and provide good examples of these methods.
- Resist change by intervening to reduce vulnerability to change and/or restore conditions where change has occurred. Jamaica Bay, which is part of Gateway National Recreation Area, is comprised of salt marsh habitat that is quickly being converted to open water habitat as sea levels rise. This salt marsh landscape stands in stark contrast to the New York City skyline behind it, and it is important as a natural area in the midst of a highly developed section of the Atlantic Coast. The marsh habitat functions as a fish and shellfish nursery and feeding ground for abundant wildlife and acts as a natural filter for improving water quality in the bay. In order to resist further detriment of this important salt marsh habitat, the NPS has been depositing thin layers of sediment over the marsh to elevate the surface and prevent the marsh from drowning.
- Accept change by allowing ecosystems to shift to new conditions with uncertain consequences. This response to sea level rise is being used on Assateague Island in Maryland and Virginia, which is co-managed by the NPS (Assateague Island National Seashore) and the Fish and Wildlife Service (Chincoteague National Wildlife Refuge). The Assateague Island National Seashore General Management Plan places a strong emphasis on adapting to sea level rise by letting natural processes be the primary force that influences the condition and evolution of natural resources. In line with this management strategy, the park stopped maintaining an artificial dune that prevented overwash. Instead, they have shifted their management plan to accepting island migration and dune overwash as a strategic retreat from rising seas. The swipe-able image to the right shows the same section of Assateague Island National Seashore before and after Hurricane Sandy, where overwash deposits provide evidence for landward sand transport. The yellow arrow provides a point of reference when comparing the two images.
- Direct change by intervening to transform ecosystems to new, more stable states. An example of this response can be found at Blackwater National Wildlife Refuge. This refuge is facilitating the conversion of forested areas into salt marsh habitat. Rather than trying to resist salt water intrusion into forests, they are working to remove trees, which will help promote the natural encroachment of salt marsh habitat upslope as sea levels continue to rise.
Ongoing OCRP SLR Projects
The goal of the OCRP SLR project is to help parks address the challenges related to sea level rise presented above. Several efforts associated with this goal are in progress, including:
Creating park-specific sea level rise briefs for use in visitor education and/or in helping park planners and managers better understand sea level rise in their park: these two-page resource sheets aim to clearly and concisely summarize relevant sea level information and data specific to each park.
Centralizing sea level rise data and tools most relevant to each ocean and coastal park: parks will be able to easily locate resources that may be helpful in discussions on sea level rise or storm impacts. The map to the right is interactive - use the "+" and "-" buttons to zoom in on your park or click the arrows in the upper right-hand corner to open the map in a new window. Explore the resources available for your park.
Identifying gaps in tide gauge site locations: Gaps in the tide gauge network lead to uncertainty regarding oceanographic processes in and near parks. Localized tide gauge data is important in understanding relative sea level rise rates, especially where tide gauges are able to measure land level changes in addition to sea level changes.
Coordinating with partners to link to new/updated SLR tools and to provide learning opportunities (e.g., webinars) focused on the use of SLR tools and data for NPS units.
We hope these products will be helpful to all NPS Ocean and Coastal Parks. We look forward to receiving your comments so we can work to improve them to provide you the most useful information to meet your needs. If you have any questions or comments on this SLR project, or if you are interested in receiving a park-specific SLR brief for your park, please do not hesitate to reach out to us at catherine_johnson@nps.gov or jamie_kilgo@nps.gov.
This StoryMap was developed by the NPS Ocean and Coastal Resources Program (ORCP). Click here to learn more about the sea level rise project or the other OCRP projects.
