Reason for Incorporating Sea-level Rise Transition Areas
Sea-level rise is projected to have a profound impact on the people and natural communities of the South Atlantic coast. Both the transition of ecosystems and complete loss of land are projected to occur by 2050. Rather than planning for worst-case inundation scenarios, a recent focus has been placed on managing retreat of coastal wetlands as marshes migrate inland and ecosystems transition (Amundsen et al. 2010, Nicholls and Cazenave 2010, Rogers et al. 2014). This data layer depicts areas where transitions are likely to occur, but given the great uncertainty involved, does not attempt to distinguish between particular transitions (e.g., forested wetlands to open water are treated the same as forested wetlands to salt marsh). Local conditions, such as management actions, subsidence, organic accretion, sedimentation, plant productivity zones, storms, and other factors will determine how ecosystems transition.
Sea-level Rise Projections
We used the SRES (Special Report on Emissions Scenarios) A1B scenario for guiding sea-level rise projections, as it is a balanced approach between the fossil fuel intensive (A1FI) and lowest emission scenario (B2). The 2013 report from the Intergovernmental Panel on Climate Change (IPCC) shows the A1B scenario has a likely sea-level rise range of 0.42–0.80 m from 1996 to 2100 (Church et al. 2013). Other sources report sea-level rise rates with the A1B scenario ranging from 0.32–1.56 m by 2100 (Vermeer and Rahmstorf 2009, Grinsted et al. 2010, Jevrejeva et al. 2010). The U.S. National Climate Assessment (Parris et al. 2012) projected 0.2- 2.0 m sea-level rise by 2100 with the highest rate projected with maximum possible glacier and ice sheet loss. In addition to the sea-level rise scenario presented here, others will soon be available under "additional data layers" on the Conservation Planning Atlas website.
Addressing Threats in Blueprint 2.0
Many ecosystems of the South Atlantic region are transitioning in response to sea-level rise and urbanization. As an adaptation strategy, the Blueprint represents a plan for responding to those threats. This sea-level rise transition layer, which overlays the Blueprint in the Blueprint 2.0 map, was reviewed at the March and April 2015 Blueprint 2.0 workshops. 82% of workshop attendees, representing the broader South Atlantic conservation community, wanted to be part of conservation action in areas transitioning due to sea-level rise, urbanization, or both. As a result, priority areas identified in the indicator and connectivity analysis that occur in transition areas remain in the Blueprint. This data layer was summarized to simplify more detailed data available on the Conservation Planning Atlas and provide a basic binary depiction (yes or no) of whether an area is predicted to transition due to sea-level rise by the year 2050. The simplified urbanization and sea-level rise threat threat layers allow users to identify those areas of the Blueprint likely to transition in the next 35 years.
To project sea-level rise transition areas, we used NOAA Coastal Services Center's Marsh Impacts/Migration data from the Sea Level Rise and Coastal Flooding Impacts Viewer (accessed 1 February 2015). This data is best described as a modified bathtub approach that accounts for local and regional tidal variability in mean higher high water (see website for specific details and disclaimers). For the South Atlantic LCC region, current sea-level rise rates range from a 1.76 mm/yr in Apalachicola, Florida, 3.11 mm/yr in Charleston, South Carolina, to a long-term average of 4.57 mm/yr in Duck, North Carolina (NOAA Tides and Currents, Sea Level Trends, accessed 20 April 2015). Rates are projected to accelerate over time. NOAA uses the sea-level rise acceleration curve from the A1B scenario to provide estimates of sea-levels for different time steps, and we used the projection to 2050 for consistency with the urbanization threat model to define one planning horizon. Surface elevation change may vary locally and regionally, but data is lacking throughout the region. Therefore, we used the NOAA data with a 0 mm/year accretion rate (4 mm/year scenarios will soon be available).
Land cover classifications from NOAA's marsh migration data, via the Sea Level Rise and Coastal Flooding Impacts Viewer, were simplified to correspond with the ecosystems defined by the South Atlantic LCC. The changes are also consistent with the level of detail mapped with relatively high accuracy. Developed open space, low, medium, and high intensity developments were reclassified into a single "developed" class. Scrub/shrub wetlands were included with "forested wetlands." Brackish and estuarine marshes were reclassified together into "estuarine marsh." Unconsolidated shore and open water were classified together as open water, as both land cover types are indicative of wetland loss. Classifications of uplands and freshwater emergent wetlands did not change.
Each cell was classified as a transition zone or no transition zone. Transition zones were defined as any projected change in a land cover type (see classifications above). For example, estuarine marsh to open water transitions were defined the same as a forested wetland to estuarine marsh transitions. NOAA quantified land cover transitions at a 10 m horizontal resolution with LiDAR elevation data; however, the data were converted to a 200 m resolution with a nearest neighbor method to incorporate the scenarios with the Blueprint. While vertical accuracy of elevation data are critical to determine land cover transitions, elevations are relatively consistent with horizontal distances up to 200 m. As coastal zone management is generally applied well beyond 10 m scales, a 200 m scale also characterizes a more realistic interpretation of the data.
NOAA's land cover types did not perfectly overlap with the South Atlantic LCC ecosystems due to slight differences in the USGS's NLCD (National Land Cover Database) and NOAA's CCAP (Coastal Change Analysis Program Regional Land Cover and Change). For example, forested wetlands along water bodies were sometimes classified as open water by NOAA CCAP data and transitions were not possible. To remedy the mismatch, we identified areas where NOAA classified open water was designated as a terrestrial ecosystem in NLCD. We reclassified only these particular areas based on a 5x5 focal maximum (ArcGIS-Spatial Analyst) and mosaicked these areas to the original transition areas.
NOAA states: "…model results contain inherent uncertainties that may not be evident but may alter the effectiveness of management decisions." They further advise that "to provide appropriate information for management, outputs of a model need to be interpreted based on the assumptions, simplifications, and uncertainties included in the model."
Important assumptions of the approach include no future change in coastal geomorphology and the effects of detailed hydrological characteristics are negligible (e.g., ditches, engineering structures). The modeling does not show marsh migration across developed lands, as classified by CCAP "developed" land cover classes. Great uncertainty exists in these areas because of the human decisions involved. The data do not consider complex natural processes such as freshwater influence, subsidence, sediment erosion dynamics, and storm impacts.
Most notably, many factors are associated with surface elevation change at local and regional levels, but detailed data are not available across the region. Surface elevation change includes dynamics based on organic vertical accretion / plant productivity and biological feedbacks, sediment availability and deposition, tidal range, flooding conditions, and subsidence (Reed 1995; Morris et al. 2002; Cahoon 2006; Schile et al. 2014).
The sea-level rise scenario does not incorporate beach movements and barrier island rollover. Indicators of Beaches and Dunes ecological integrity characterize areas of potential or likely barrier island rollover.
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