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Everglades Case Study - Map displaying location of Sandfly Key (marked Sandfly Island)
Map displaying location of Sandfly Key (marked Sandfly Island)

Sandfly Key is an extensive, largely manmade shell mound island located in Everglades National Park. The island consists of multiple shell mounds composed of oyster shells, fish bones, and other refuse that prehistoric Indians discarded over time and used to build a series of mounds and features. Given its organic composition, the mound is extremely vulnerable to erosion. Factors that can accelerate the mound's erosion are rising sea level, increased precipitation, and hurricanes (characterized by storm surge, heavy precipitation, and strong winds). This section focuses on data that can be used to quantify erosion risks for the shell mound island, as well as other standard climatological variables.

The climate normals (below) for the Everglades daily station exhibits a steady increase in precipitation since 1961, although this increase appears to be part of a multidecadal precipitation cycle. Periods of high precipitation will increase the erosion risk for Sandfly Key. Although temperature might not directly affect the shell mound island, it is an indicator of climate change. Since 1961, temperature does appear to be increasing in the ENP region, although this increase also appears to be part of a multidecadal temperature cycle noted in observations over the past ~80 years.

Climate normals from daily station closest to Sandfly Key: Everglades(ID:082850)

1961–1990 1971–2000 1981–2010
Temperature 73.8°F 74.1°F 75.6°F
Precipitation 51.36 in. 52.10 in. 55.38 in.

The Palmer Drought Severity Index (PDSI) for the section of Florida that contains Sandfly Key (Climate Division 5, 1895-Present) can be found at:

Although precipitation is increasing, there seems to be no clear response from the drought index to show the increased precipitation. This can be due to one of two reasons. Either the precipitation is concentrated in a wet spell, which does not affect PDSI very much, or other factors, such as evapotranspiration, recharge, runoff, etc., compensate for the increased precipitation. From 1895 to 2005, it is unlikely that drought affected Sandfly Key because the PSDI values have not been extreme and dry periods have been mixed with periods of surplus moisture. Since 2006, there seems to be a prolonged dry period that contains some moderate droughts. Although it is not large in magnitude, the duration of the dry period may contribute to mound erosion in several ways. During a prolonged dry period, the organic composition of the shell mound deposits may become dry and brittle, which could lead to oxidation and degradation of organic components. This may include shrinking, cracking, and disintegration of ecofacts such as preserved carbon, nuts and seeds, as well as damage to artifacts such as bone tools. Another indirect but potentially severe impact caused by a dry period may be the drying, decay, and ultimately death of vegetation, such as old and large deeply rooted trees. During high winds or storms, stressed trees may be more easily uprooted and could damage the mounds in tip-overs. NPS archeologists have reported this to be a severe problem on the west edge of Sandfly Key, where severe loss of shell mound deposits was caused by large trees uprooted during Hurricane Wilma.

In addition to atmospheric influences on Sandfly Key, changes in the ocean may also impact the shell mound. Since hurricanes bring long durations of rainfall, storm surge, and increased wind speeds, hurricanes pose a significant erosion threat to the shell mound island. As major hurricanes are more intense and typically result in higher storm surge than occurs with weaker hurricanes, they pose a greater risk for erosion. The first table (below) lists the climatological probability of a hurricane landfall in the vicinity of Sandfly Key. This method of calculating probability does not take into account future climate change or sea level rise. The method only considers the mean number of hurricanes from 1944 to the present and determines landfall probabilities. To further quantify the erosion threat, storm surge maps are available for the region from NOAA's SLOSH storm surge model. This model determines how much storm surge would occur in a region on the basis of hurricane category. According to the maps, Sandfly Key is in a region that is very susceptible to storm surge, and the mound sits at an elevation of 1 m. Since Sandfly Key has such a low elevation and is very susceptible to storm surge, a rising mean sea level would increase the risk of erosion during a future hurricane.

Annual Climatological Landfall Probability of Hurricane:

1 or more 2 or more 3 or more
Hurricane (Cat. 1 and up) 0.393 0.090 0.014
Major (Cat. 3 and up) 0.221 0.026 0.0002

Storm Surge Maps (2011):

A note on future sea level changes:
Sea level changes are significant to shell mound sites because of the erosion threat. Two reliable ways to forecast future sea level changes are using a persistence forecast and using a numerical model. A persistent forecast assumes that conditions do not vary much within a given time span. In this case, a persistence forecast assumes that sea level will continue to rise/sink at the same rate it has been for the duration of the observational period. Using the rate of mean sea level rise recorded from a buoy (1965–2006), mean sea level at Sandfly Key will continue to rise 2.02 mm/year every year. At this rate, by the year 2100, the sea level will have risen by 22.2 cm at Sandfly Key. Many meteorologists believe the causes for rising sea level (melting ice, rising SST, etc.) will not remain constant. In this case, a persistent forecast would be less accurate. Dr. Gary Mitchum produced a sea level report that predicted sea level changes using a numerical model. He used global sea level predictions from the 2007 IPCC report as a good approximation for changing sea level for the southeastern U.S., despite sea level in the southeastern U.S. rising 20% more than global levels. Also, Mitchum accounted for Antarctica ice melting more than predicted in the IPCC 2007 report. The Mitchum report concluded the most likely scenario is an increase of 80 cm by the year 2100; however, Mitchum's confidence in the forecast is low because of uncertainty in the 2007 IPCC models.

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