Climate change is a major threat to the persistence and functioning of ecosystems globally, including wetlands (IPCC, 2013; NCADAC, 2014; NRC, 2014). Warmer climates accompanied by changes in precipitation patterns and increases in atmospheric carbon dioxide concentrations will affect wetland ecosystem functioning through changes in hydrologic conditions, biogeochemistry, and primary productivity, and alter linkages with the built environment. Increases in temperatures also will accelerate the rate of global sea-level rise, with median projected global increases of 17 to 29 inches by 2100 for two scenarios (IPCC, 2013; Figure 5-1). In this chapter the committee reviews the latest climate change and sea-level-rise projections and discusses their implications for the Everglades and restoration planning.
CLIMATE AND SEA-LEVEL CHANGE IN FLORIDA OVER THE PAST CENTURY
Global change effects on land surface temperature and precipitation are manifested most clearly and strongly at northern latitudes, but in other regions, patterns of global change are more complex and can be masked by other factors. This is particularly true of the southeastern United States, which has generally shown decreasing rather than increasing trends in land surface temperature in the second half of the 20th century (DeGaetano and Allen, 2002; Portmann et al., 2009; Trenberth et al., 2007). In Florida, Obeysekera et al. (2011b) investigated trends in air temperature and precipitation at 32 meteorological stations (1950-2008) and observed no consistent trends in either air temperature or precipitation.
There are several components of precipitation in South Florida that contribute to the complexity and variability of rainfall, including tropical cyclones and less intense tropical storms, which can be a substantial and variable contributor of precipitation. Adding to the complexity in precipitation patterns, sea surface temperatures undergo slow oscillations between relative warm and cold conditions (the Atlantic Multidecadal Oscillation [AMO] and the Pacific Decadal
FIGURE 5-1 Projected global rise in sea level for two emission scenarios in comparison with historical records. Historical and paleorecords from salt marshes are shown in purple. The green, blue, and red lines between 1900 and 2010 represent yearly average global mean sea level reconstructed from tide gages using three different methods, while the light blue line represents satellite altimetry data. The future projections show median estimates and likely ranges for future sea-level rise for a low-emissions scenario (RCP2.6; blue) and a high-emissions scenario (RCP8.5; red). The Intergovernmental Panel on Climate Change did not assess the likelihood of the specific scenarios, but they should not be assumed to be equally probable.
SOURCE: IPCC (2013).
Oscillation [PDO]), which have been shown to influence precipitation quantity, distribution, and interannual variability in South Florida (Enfield et al., 2011; Moses et al., 2013; Shin and Lee, 2011). For example, for periods of two to three decades, the AMO follows a warm-water phase of the North Atlantic, which is characterized by more hurricanes and precipitation in South Florida, and then shifts to a cold-water phase with fewer hurricanes and less rainfall (Enfield et al., 2001; Kelly, 2004) although the PDO can interfere in ways that increase or decrease these changes. The AMO has been in the warm-water phase since the mid-1990s and will likely shift to the cold-water phase in the future, likely decreasing precipitation inputs irrespective of the effects of greenhouse gases. These oscillations may mask long-term trends in precipitation in Florida.
In contrast to temperature and precipitation, there is little uncertainty about trends in sea level. Currently, sea level is rising almost an order of magnitude faster than the long-term rate of 0.35 mm/yr that prevailed for the past 4,000 years (Scholl and Stuiver, 1967; Scholl et al., 1969; Wanless et al., 1994). Using long-term measurements at Key West, NOAA1 calculated the average sea-level rise to be 8.8 inches (22 cm) over the past century (or 2.2 mm/yr). This value is more than 30 percent higher than the global average of 6.7 inches (17 cm) for the 20th century (Figure 5-1) and is consistent with relatively rapid rates observed along the Atlantic and Gulf coasts of North America (IPCC, 2013).
CLIMATE AND SEA-LEVEL PROJECTIONS FOR SOUTH FLORIDA
Given that the Comprehensive Everglades Restoration Plan (CERP) is a multidecadal restoration effort, it is important to understand how anticipated changes in climate and sea level could impact restoration outcomes.
Accurate projection of climate change and its effects is a major challenge under the best of circumstances, but these challenges are amplified in the complex meteorological environment and landscape of South Florida. Climate change projections are derived through a complex, multistep process from general circulation models (GCMs), which are large numerical models that simulate land-ocean-atmosphere exchanges of energy, water, and other characteristics within and across coarse grid cells. Dozens of different GCMs are used in climate projections, which are driven by storylines that integrate economic, demographic, and technological drivers to estimate potential future human-caused emissions and land cover change.
There are several sources of uncertainty in projecting global climate changes (Hawkins and Sutton, 2009; Kirtman et al., 2011), including uncertainty in initial conditions, external forcings that drive model scenarios (e.g., changes in future carbon dioxide emissions), and model uncertainty. The relative contribution of these categories of uncertainty shift with the timescale of projections. For South Florida, there are a number of specific issues that add to uncertainty of GCM projections of changing temperature and precipitation. Peninsular Florida’s proximity to the warm ocean and flat terrain create additional uncertainty in GCM projections. South Florida is also positioned along a discontinuity in rainfall projections. Although the position of this discontinuity is uncertain, the Intergovernmental Panel on Climate Change (IPCC) AR4 simulations suggest that eastern North America will experience increases in precipitation, while for the Caribbean there will be a marked decrease in precipitation (Enfield et al., 2011).
GCMs generally produce outputs on a relatively coarse grid scale (hundreds of kilometers), which limits local-scale assessments of climate change. Two broad downscaling approaches are used to translate coarse-scale GCM output to local-scale conditions. Statistical downscaling uses empirical relationships between past grid-based or station-based meteorological observations and comparable values from GCM hindcast simulations and relies on these relationships to tune future GCM projections of climate output (e.g., surface air temperature, precipitation) to local-scale grid or site conditions. In contrast, dynamical downscaling uses GCM meteorological output as input to a mesoscale climate model to simulate potential future climate regionally or locally. Although investigations have shown that both statistical and dynamically downscaled data are able to reproduce historical temperature and precipitation patterns for Florida, there are biases in these relationships which challenge the accuracy of future downscaled projections. For example, Obeysekera et al. (2011a) showed that various GCMs typically underpredict historical wet-season precipitation in central and southern Florida and do not represent the extremes in observed events. This bias stems from an inability in the models to depict sea-breeze-driven convective thunderstorms. There is considerable variability in projections across different GCMs and under different future scenarios (Figure 5-2). However, ensembles of GCMs that show similar results provide more confidence in outputs. More consistent patterns are evident for changes in temperature than precipitation.
Obeysekera et al. (in press) summarized the general range of GCM downscaled climate change projections for South Florida for 2060 (Table 5-1). Results suggest that South Florida will experience modest increases in temperature (Figure 5-3, top). Precipitation projections are variable for different GCMs (Figure 5-2) and more uncertain than temperature projections (Figure 5-3, bottom). Projections generally indicate increased precipitation in the fall and early winter and decreases in late winter through early summer. Moreover, precipitation is
FIGURE 5-2 Spatial patterns of specific downscaled GCM projections of precipitation change for Florida.
SOURCE: J. Obeysekera, SFWMD, personal communication, 2014.
TABLE 5-1 Summary of Climate Change Projections for South Florida for 2060
|Variable||GCMs||Statistically Downscaled||Dynamically Downscaled|
|Average temperature (°C)||1 to 1.5||1 to 2||1.8 to 2.1|
|Precipitation||−10% to +10%||−5% to +5%||−76 to +50 mm (−3 to +2 inches)|
|Reference crop evapotranspiration (in.)||76 to 15 mm (3 to 6 inches)|
SOURCE: Data from Obeysekera et al. (in press).
FIGURE 5-3 Box and whisker plots showing magnitude and variability of different downscaled GCM-projected changes in (top) temperature and (bottom) precipitation from 1970-1999 to 2041-2070 for meteorological stations in Florida under the IPCC A2 scenario sorted by latitude (after Obeysekera et al., 2014). GCM data are from the World Climate Research Programme [WCRP] Coupled Model Intercomparison Project 3 [CMIP3] multimodel dataset. Note that the A2 scenario depicts a world of independently operating, self-reliant nations, with continuously increasing population, and regionally oriented economic development.
more likely to decrease with latitude through the Florida peninsula (Figures 5-2 and 5-3, bottom). Statistically downscaled projections for the Everglades also show increases in annual temperature and decreases in annual precipitation (Obeysekera et al., in press). As a result of this considerable uncertainty, rather than evaluating specific projections, Obeysekera et al. (in press) developed scenarios to probe the hydrologic response of the Everglades to hypothetical changes in temperature, precipitation, and sea-level rise based on the results of GCM projections (discussed below in Implications for Everglades Hydrology).
Potential changes in tropical cyclone activity also have important implications for the CERP. Although there is no evidence that climate change has altered hurricane activity to date (Bender et al., 2010), the number of intense (category 4 and 5) hurricanes is projected to increase over the next century, while the total number of hurricanes is expected to decrease (Bender et al., 2010; Enfield et al., 2011). These projections are sensitive to the particular GCM models that are used in the downscaling experiments, and hence should be interpreted cautiously.
Sea-level rise is already impacting South Florida. Sea level is certain to continue to rise, although the rate of the increase depends on global factors such as future greenhouse gas emissions, thermal expansion of the ocean, and the extent of melting from glaciers and ice sheets (IPCC, 2013). The vulnerability of the Everglades to sea-level rise will depend on local factors, including isostatic uplift rates, which are generally low in South Florida2 (Adams et al., 2010), and accretion rates of peat and inorganic sediments (discussed later in this chapter).
The IPCC (2013) recently increased its estimates of global sea-level rise (IPCC, 2007a) by 60 percent based on improved process models depicting thermal expansion of the ocean, ice-sheet dynamics, and glacial melting. Model simulations of future sea-level rise were run under four different scenarios for greenhouse gas emissions called representative concentration pathways (RCPs). The models project a likely rise in global sea level between 11 and 24 inches by 2100 under the low-emissions scenario (RCP2.6, which requires technology for CO2 capture that does not exist today) and a likely increase between 21 and 38 inches under a regime of continued high emissions (RCP8.5) (Figure 5-1). The IPCC did not assess the likelihood of the RCP scenarios themselves, but these scenarios should not be considered equally probable. Although the IPCC remains
2 South Florida rests on a relatively stable tectonic platform, located too far south to be affected by glacial isostatic adjustment. However, Adams et al. (2010) suggested that Florida’s land surface may be rising isostatically, driven by dissolution of the limestone bedrock. Their predicted uplift rate of 0.047 mm/yr for northern Florida would be equivalent to a total rise of only 0.38 cm by 2100.
confident in these scenario-specific projections, some degree of uncertainty remains with regard to (1) the climate models that are used to simulate thermal expansion of the ocean; (2) modeling ice-sheet dynamics; and (3) modeling the timing and magnitude of ice-sheet collapse. The stability of the Greenland and Antarctic ice sheets has been a major element of uncertainty (IPCC, 2013), and recent research in West Antarctica has reported more rapid rates of glacial melting than previously anticipated (Rignot et al., 2014).
Following USACE guidance (USACE, 2011e), which was based on NRC (1987), the USACE Jacksonville District Office developed projections for sea-level rise in South Florida at low, intermediate, and high scenarios through 2100. These local sea-level rise projections range from 4 to 26 inches in South Florida over the next 50 years and between 9 and 78 inches over the next century (Figures 5-4 and 5-5; USACE and SFWMD, 2013b). As previously discussed in the context of Florida’s observed sea-level rise, ocean circulation patterns can cause local sea-level changes to differ from global changes, creating more uncertainty in local sea-level-rise projections compared with global projections. Thus, it is reasonable that the local USACE projections fully encompass and, at the upper projections, exceed
FIGURE 5-4 Sea-level rise scenarios for Key West, Florida, based on USACE sea-level rise guidance EC 1165-2-212.
SOURCE: USACE and SFWMD (2013b).
FIGURE 5-5 Predicted land loss in Everglades National Park based on 2 feet of sea-level rise (the intermediate scenario for 2100 in Figure 5-4), (a) assuming existing topography and (b) assuming complete loss of peat soils, which leads to substantially greater land loss. Neither scenario considers new peat accretion.
SOURCE: USACE and SFWMD (2013b).
IMPLICATIONS FOR THE EVERGLADES
The impacts of climate change on the Everglades will depend upon the magnitude and rate of change in the physical environment (e.g., sea-level rise,
3 NRC (2012b) global sea-level rise estimates exceeded those of the IPCC (2013) because the NRC assumed higher rates of loss from ice sheets and used a different extrapolation procedure based on Meier et al. (2007).
temperature) and the ecosystem’s capacity to resist and/or be resilient to these stressors. A warmer climate in South Florida accompanied by changes in precipitation patterns will affect hydrologic regimes, biogeochemical cycling, community composition and productivity, and, hence, wetland ecosystem structure and function. Accelerated sea-level rise will likely submerge many areas, thereby increasing the salinity of freshwater wetlands, altering biotic communities and productivity, and changing the rates and decomposition pathways of organic matter (Weston et al., 2006, 2011). Alterations to natural disturbance regimes, such as fire or intense hurricanes, could also have significant ecosystem effects. These issues were explored in a recent workshop on the ecological effects of climate change in the Everglades.4 In this section, the committee describes the implications of climate change and sea-level rise on Everglades hydrology, landscapes, water quality, and biota.
Implications for Everglades Hydrology
The hydrologic responses to future climate conditions are particularly challenging to characterize and quantify in the rainfall-driven South Florida ecosystem. In addition to the uncertainties in climate projections discussed in the preceding section, South Florida water management operations may also change in response to changing climate. For example, the water level in coastal canals could be maximized to buffer the coastal groundwater system against saltwater intrusion (Obeysekera et al., 2011a). Future increases in the population of Florida will increase the demand for water resources for urban areas, and under changing climate conditions, water demand is likely to change.
As a result of these important but uncertain drivers, projections of changes in hydrologic conditions in response to a changing climate are highly uncertain. From this perspective, Obeysekera et al. (2014) conducted a preliminary (“screening level”) assessment to help understand the sensitivity of the water system to climate change drivers and the potential implications for water resources and management in South Florida. Using the South Florida Water Management Model, Obeysekera et al. (2014) evaluated the hydrologic outcomes of a series of hypothetical scenarios:
1. 2010 Baseline (2010 water demands and land use corresponding to and simulated with 1965-2005 rainfall and evapotranspiration);
2. 2010 Baseline with a 10 percent decrease in rainfall;
3. 2010 Baseline with a 10 percent increase in rainfall;
4. 2010 Baseline with a 1.5°C increase in temperature and a 1.5-ft increase in sea level with increases in coastal canal maintenance levels;
5. 2010 Baseline with 10 percent decrease in rainfall, 1.5°C increase in temperature, and a 1.5-ft increase in sea level with increases in coastal canal levels;
6. 2010 Baseline with 10 percent decrease in rainfall, 1.5°C increase in temperature, and a 1.5-ft increase in sea level with no increases in coastal canal levels; and
7. 2010 Baseline with 10 percent increase in rainfall, 1.5°C increase in temperature, and a 1.5-ft increase in sea level with increases in coastal canal levels.
These hypothetical climate scenarios are reasonable changes that might be anticipated based on statistically downscaled GCM projections for South Florida for 2060 (Table 5-1). The analysis, however, was highly simplified, because seasonal and extreme interannual variations in precipitation were not considered. Instead, changes in precipitation were applied uniformly across the year, based on 1965-2005 historical climate data, even though global climate models have projected increasing precipitation extremes over many regions (Kharin et al., 2007; O’Gorman and Schneider, 2009; Sun et al., 2007).
Results of this analysis show that water discharge and demand are sensitive to hypothetical climate change projections. The hypothetical 10 percent increases in precipitation results in increases in water stage and discharge throughout the South Florida ecosystem (Figure 5-6). The scenarios of increases in temperature (i.e., evapotranspiration) and decreases in rainfall are projected to increase water demand and decrease runoff, which results in particularly acute water shortages (Figure 5-6). Simulations show up to 1.7-ft decreases in the stage of Lake Okeechobee with increasing temperature only (Scenario 4), and simulations of 10 percent decrease in rainfall combined with increasing temperature (Scenario 7) resulted in up to 6-ft decreases in lake stage (Table 5-2; Obeysekera et al., 2014). This direst scenario resulted in unmet agricultural water supply demand of 40 to 58 percent (up from 7 to 8 percent in the 2010 base), highlighting the potential water supply pressures under future scenarios. Such decreases in precipitation would impact both surface-water and groundwater levels, reducing freshwater flows to estuaries and increasing the extent of saline intrusion of coastal wetlands and aquifers (Saha et al., 2011).
This analysis suggests that for conditions that are likely occur in the future, water quantity challenges could become a critical issue in South Florida. Under scenarios of increased precipitation, the CERP as currently designed could produce desired hydrologic outcomes, but scenarios of decreased precipitation or increased temperature (or both) result in large decreases in flow that would undermine restoration as currently planned.
FIGURE 5-6 Hypothetical simulations showing mean annual changes in water stage for (a) 2010 baseline with 10 percent decrease in rainfall, 1.5°C increase in temperature, and 1.5-ft increase in sea level with increases in coastal canal levels (Scenario 5) and (b) 2010 baseline with a 10 percent increase in rainfall, 1.5°C increase in temperature, and 1.5-ft increase in sea level (Scenario 7).
SOURCE: J. Obeysekera, SFWMD, personal communication, 2014.
Implications for Everglades Landscapes
The Everglades landscape is especially sensitive to rising sea level because it has low topographic relief of porous limestone bedrock and is in close proximity to the ocean. The topography of the Everglades is shaped by two components: a dynamic surficial layer of wetland soil and the stable floor of the underlying bedrock basin (Gleason and Stone, 1994; Parker and Cooke, 1944; Petuch and Roberts, 2007). The bedrock rises less than 10 ft above mean sea level around
TABLE 5-2 Changes in Hydrologic Conditions Relative to 2010 Baseline with Three Climate Change Scenarios
|Scenario 4: No change Precip., +1.5°C, 1.5-ft SLR||Scenario 5: 10% Decrease Precip., +1.5°C, 1.5-ft SLR||Scenario 7: 10% Increase Precip., +1.5°C, 1.5-ft SLR|
|Lake Okeechobee stage||Up to 1.7-ft decrease||Up to 6-ft decrease||Minimal change|
|Structural inflow to WCA-3||−247 million m3/yr (−15%)||−704 million m3/yr (-43%)||+245 million m3/yr (+15%)|
|Structural inflow to Everglades National Park||−337 million m3/yr (−24%)||−820 million m3/yr (−58%)||+314 million m3/yr (+22%)|
NOTE: SLR = sea-level rise; WCA = Water Conservation Area.
SOURCE: Adapted from Obeysekera et al. (in press), Havens and Steinman (2013).
Lake Okeechobee, while the bedrock underlying the Shark River Slough rises less than 3.3 ft above mean sea level (Parker and Cooke, 1944). Everglades freshwater wetland soils, consisting mostly of organic-rich peat, are generally less than 3.3 ft deep across large portions of the central and southern Everglades, with thicker peats in some areas (e.g., northeastern Water Conservation Area 3 [WCA-3], localized depressions) (Richardson, 2008; Scheidt and Kalla, 2007). Freshwater peat provides essential structure and slope that influence the direction and velocity of water flow in the Everglades as the peat itself is shaped by the distribution and velocity of water across the landscape. These peat soils also support the ridge-and-slough landscape and many tree islands (Box 5-1). Soils within coastal wetlands (e.g., salt marshes, mangroves) contain substantial organic matter along with varying amounts of inorganic sediment washed in by tides, waves, or storm surges and trapped by plant structures (Castañeda-Moya et al., 2013; Krauss et al., 2013).
Freshwater peat in the Everglades represents a dynamic surface that will continue to change in the future through accretion and/or subsidence. Peat accumulates when plant materials are only partially decomposed prior to burial and compaction. Net accretion requires submerged, anaerobic conditions that allow the accumulation of plant material to outpace decomposition and compaction (see NRC, 2012a, for more detail on freshwater peat accretion rates). However, peat is highly susceptible to subsidence under dry conditions. When the water table falls, pore space collapses and oxygen penetrates more deeply into the peat profile, driving more rapid decomposition (NRC, 2012a). A future scenario of decreases or no change in precipitation coupled with increased temperature and evapotranspiration would reduce hydroperiods, accelerating rates of peat decomposition. Fire regimes are also likely to shift under such conditions. While
Potential Effects of Reduced Water Inflows on the Ridge-and-Slough Landscape
In the central and southern Everglades, the ridge-and-slough landscape consists of linear ridges that alternate with deeper sloughs; these patterns generally run north-south, parallel to pre-drainage flows (Gaiser et al., 2012). The ridges have characteristically short hydroperiods dominated by sawgrass, while the sloughs have longer hydroperiods dominated by water lilies (McVoy et al., 2011). Tree islands are among the highest and driest habitats in the Everglades (Wetzel et al., 2011), and irregularly punctuate the ridge-and-slough habitat matrix. These islands are floristically diverse, provide critical habitat for many wildlife species, and are sites where nutrients are concentrated (Ross et al., 2006; Wetzel et al., 2011) and sequestered by the dominant tree species (Lejeune et al., 2004).
Flow patterns that initially built and maintained these features, through controls on peat formation and sediment movement (Brandt et al., 2000), have been highly modified, with a resultant compression of the once variable topography (Sklar et al., 2001). Drainage and compartmentalization of the Everglades have led to peat subsidence and conversion to marl prairie habitat on the wet prairie ridges (Davis et al., 2005b), and degradation of tree island communities (NRC, 2012a). Compositional shifts away from tree dominance on the islands have disrupted their capacity to concentrate and store nutrients, with attendant release/leakage of nutrients into adjacent oligotrophic habitats and displacement of sawgrass assemblages by cattails (Wetzel et al., 2009). Without appropriate hydrologic restoration, the future of these features is in jeopardy because water surface levels are currently inadequate to move sediment from slough to ridge (Larsen et al., 2009). Continued disruption of flows, and potentially more severe water deficits with climate change, will drive further deterioration of habitat heterogeneity and increased homogenization of vegetation. In the face of climate change, implementing hydrologic restoration in the central Everglades (see Chapter 3) would help protect the remaining features of this iconic, patterned landscape that provides critical habitat in support of Everglades diversity.
low-intensity surface fires generally have only ephemeral impacts on Everglades vegetation, highly intense fires can result in large losses of inland peat over a short period (Loveless, 1959; Sklar et al., 2001). With existing water management, a scenario of reduced future precipitation would therefore increase rates of freshwater peat loss, further altering the slope and microtopography of the landscape and impacting water depth and flow (Nungesser et al., 2014; see Box 5-1). However, increased precipitation in South Florida would increase mean water depths in the freshwater Everglades wetlands (Figure 5-6), reducing microbial decomposition rates (DeBusk and Reddy, 1998) and thereby promoting peat accretion.
Rates of coastal peat and inorganic sediment accretion or subsidence will directly influence the rate of coastal wetland retreat and other impacts of sea-
level rise on the Everglades landscape. Most coastal wetlands possess a limited capacity to keep pace with rising sea level through accretion of organic matter and storm-derived sediment. In coastal wetlands, accretion and subsidence rates vary widely among different depositional settings and with the extent of human impacts (Cahoon and Lynch, 1997; Kirwan and Megonigal, 2013). Sediment cores indicate that the average accretion rate in mangroves is about 1 mm/yr over millennial timescales, with a range of 1-3 mm/yr from Florida and adjacent regions (McKee et al., 2007; Parkinson et al., 1994). More rapid accretion rates are possible over shorter time intervals—accretion rates of 6 mm/yr over several years and even higher rates associated with single storm events have been reported (see Box 5-2). However, the implications of these short-term, local elevation changes in the context of sea-level rise remain poorly understood (Kirwan and Megonigal, 2013). Continual monitoring of surface elevations is, therefore, needed over extended time periods to determine the response of wetland deposits to rising sea level.
Climate and Sea-Level Rise Effects on Mangrove Swamps
Mangrove swamps occupy the marine-terrestrial interface and are therefore among the “first responders” to sea-level rise. These communities typically have distinct spatial zonation patterns, which are governed by gradients in salinity and soil conditions (Chen and Twilley, 1999; Egler, 1952). Marine forces are clearly important, but the timing and quantity of freshwater flows from the upper parts of the watershed also influence salinity levels. This interplay can influence water budgets and ecosystem productivity, as elevated salinities during the dry season lead to decreased evapotranspiration and carbon assimilation rates (Barr et al., 2014). Thus, mangrove community distribution on the landscape is shaped bidirectionally through the interplay between freshwater flows and tidal regimes (Davis et al., 2005a), making them excellent indicators of climate change because they are highly vulnerable to marine forces and hydrologic changes in the watershed.
With increasing sea-level rise and water management practices during the 20th century, mangroves have been declining in coverage on the southern Everglades landscape (Wanless et al., 2000), despite inland migration in many areas. A readily visible indicator of this migration is the inland shift in the upper edge of the mangrove/marl prairie ecotone, also known as the “white zone” (Ross et al., 2002; Figure 5-7). These shifts often coincide with displacement and sometimes concurrent inland movement of adjacent freshwater sawgrass communities (Ross et al., 2000) and appear to be facilitated in some cases by fire (Smith et al., 2013). With rising seas, potentially drier conditions that heighten the likelihood of fire at the mangrove-marsh ecotone and increased salinity levels in the estuaries are likely to continue.
The seaward fringes of the mangrove landscape are maintained, in part, through peat accretion, which occurs at the upper end of their tidal range (Scholl, 1964). The ability of mangroves to keep pace with sea-level rise and persist in situ is also uncertain because accretion rates are highly variable and dependent not only on sufficient freshwater inflows to prevent oxidation of existing peat but also factors that control productivity of the vegetation and rates of organic matter inputs that drive accretion rates. In a recent review of mangrove adjustments to sea-level rise across the globe, Krauss et al. (2013) reported soil surface elevation changes ranging from −3.7 to 6.2 mm/yr over several years. Accretion rates as high as 20.8 mm/yr were reported, although subsurface subsidence reduced the total surface elevation change. Storm events were generally responsible for the upper limit of this range. Smoak et al. (2013) reported accretion rates of 5.9 and 6.5 mm/yr in Everglades mangrove forests produced by a single storm-surge deposit, whereas long-term rates (averaged over a 130-year period) of 2.5 to 3.6 mm/yr were measured at the same sites. After Hurricane Wilma, Casteñeda-Moya et al. (2010) reported 5 to 450 mm of sediment deposition in the Shark River mangrove forests—up to 17 times greater than average annual accretion rates of approximately 3 mm/yr. Although storm surges can provide sizeable deposits of inorganic sediment, part of this elevation gain will subsequently be lost through compaction and erosion (Whelan et al., 2009). The challenge for interpreting these short-term accretion rates is to determine their implications for accretion rates over multidecadal timescales or longer in the context of projections of sea-level rise.
In some areas, these systems can keep pace with current rates of sea-level rise, but in other places where accretion rates are low, saltwater encroaches and the swamps succumb to the sea (Lodge, 2010). Continued acceleration of sea-level rise will increase their vulnerability, as elevated salinity levels limit productivity and can lead to peat collapse (Chambers et al., 2013a,b). The mangrove zone in Taylor Slough, for example, is highly threatened due to low productivity and, hence, low accretion rates (Gaiser et al., 2006). Future rates of sea-level rise that are sufficiently rapid to impede inland migration may threaten their persistence in the broader landscape.
FIGURE 5-7 Images of the coastal gradient from 1940 (left) and 1994 (right) between U.S. Highway 1 and Card Sound Road, illustrating shifts in the “white zone.”
SOURCE: Ross et al. (2000).
The USACE scenarios for Key West, Florida (Figure 5-4) describe rates of sea-level rise that increase from historic rates of 2.24 mm/yr to between 8 and 27 mm/yr under low and high scenarios by the end of the 21st century (G. Landers, USACE, personal communication, 2014). Thus, it remains highly questionable whether accretion rates in coastal wetlands will be sufficient to prevent inundation and retreating shorelines in the future or to what extent accretion could at least mitigate the impacts. Assessing current accretion rates in both the coastal and freshwater wetlands of the Everglades and understanding the factors that contribute to their variability are high priorities for research. Efforts are currently under way in the Everglades to monitor changes in surface elevation across a network of control points using customized elevation gauges to assess accretion rates in the context of sea-level rise (Box 5-3).
The phenomenon of “peat collapse” in coastal wetlands (Cahoon et al., 2003; Day et al., 2011; DeLaune et al., 1994) poses significant concerns for Everglades management and restoration in the face of climate change. Peat collapse has been used to describe the conversion of coastal marshes to open water as well as sudden land subsidence in salt marshes and mangroves (Figure 5-10). The peat deterioration can release a large amount of sequestered carbon (as carbon dioxide and methane) and nutrients, such as phosphorus, stored in the soil profile (Bouillon et al., 2008; Nichols et al., 2007). An important suspected mechanism for peat collapse is increasing saltwater intrusion from sea-level rise and tropical storm surges and associated high sulfate concentrations that alter microbial organic matter decomposition pathways and rates (Chambers et al., 2014; Erickson et al., 2007; Weston et al., 2006). However, additional agents for lowering peat surface elevations could include mechanical damage to the vegetation or peat skeleton by high winds or storm surges (e.g., Doyle et al., 1995; Kirwan and Guntenspergen, 2010; Smith et al., 1994, 2009), nitrogen inputs that enhance microbial decomposition of root structures (Deegan et al., 2012; McKee et al., 2007), and loss of groundwater inputs (Whelan et al., 2005).
Implications for Water Quality
Changes in climate can alter linkages between coupled hydrologic and biogeochemical cycles that are critical to the functioning and persistence of wetland ecosystems (Reddy and Delaune, 2008; Reddy et al., 2010; Rivera-Monroy et al., 2007). Shifts in the frequency, timing, and intensity of rainfall events can affect the transport of sediments, nutrients, and other constituents from wetlands to downstream aquatic ecosystems. Perturbations in hydroperiod and hydrologic and pollutant loading rates can significantly affect vegetation, algae, microbial and animal communities in native and constructed wetlands
Measuring Peat Accretion and Subsidence
Wetlands have a dynamic land surface that continually rises and falls through the interplay of physical, chemical, and biological processes. Key questions remain as to what extent rates of peat and sediment accretion can keep pace with the rapid rise in sea level projected for the 21st century.
Two different approaches have been used to measure accretion and subsidence rates over contrasting timescales. The traditional method is based on the analysis of sediment cores that can be dated into discrete time slices of 0-50 years by 137Cs, 0-150 years by 210Pb, and 500-40,000 years by 14C. Accretion rates can then be calculated by dividing the length of each section by its total age, although much finer age resolution is often possible for a 210Pb chronology. The alternative method directly measures shorter-term changes in surface elevation by means of custom gauges (e.g., the sediment-erosion table-marker horizon [SET]-MH system of Cahoon et al., 1995; Webb et al., 2013; Figures 5-8 and 5-9). Although the different approaches are complementary, they
FIGURE 5-8 A sediment elevation table (SET) used to measure changes in the elevation of the soil surface in a mangrove forest in Everglades National Park.
SOURCE: U.S. Geological Survey, http://fl.biology.usgs.gov/Science_Feature_Archive/2010/monitoring_enp/monitoring_enp_gallery.html.
provide different measures of accretion rates that are specific to a discrete timescale. Any comparison of accretion rates needs to consider the general tendency for these rates to decline over longer time spans because of the continual loss of pore waters (by compression) and organic matter (by decomposition). These processes are most rapid in the upper portion of a sedimentary profile (e.g., Bemer, 1980; Glaser et al., 2012), and therefore, caution should be exercised in extrapolating short-term rates to longer timescales.
FIGURE 5-9 Measurement of soil accretion using the marker horizon method in Everglades National Park. In this method, researchers place a layer of feldspar clay (visible as a white layer) on the surface of the marsh and later return to measure the soil that has accumulated. The marker horizon method is often used in conjunction with a sediment elevation table (SET) to measure total soil accretion or erosion.
SOURCE: U.S. Geological Survey, http://fl.biology.usgs.gov/Science_Feature_Archive/2010/monitoring_enp/monitoring_enp_gallery.html.
FIGURE 5-10 Peat collapse at northern Cape Sable, Everglades National Park.
SOURCE: Wanless and Vlaswinkel (2005).
(stormwater treatment areas), and associated biogeochemical processes that ultimately have significant effects on water quality. Reduced precipitation and increased evapotranspiration will decrease the water content of wetland soils. Dry conditions promote the oxidation of soil organic matter, which results in the mineralization of associated chemical elements (Holden et al., 2004; Reddy et al., 2006). Also, oxidation of sulfides can occur, which can decrease soil pH and facilitate the mobilization of phosphorus bound to calcium carbonate (Reddy and DeLaune, 2008). When these dry areas are rehydrated, dissolved and particulate forms of carbon, phosphorus, nitrogen, sulfur, and mercury are released, increasing nutrient and contaminant loads to downstream habitats (Bates et al., 2000; Strober et al., 1995). Increases in wet-dry cycles accelerate biogeochemical cycling, and element availability and loss, which could lead to exceedences of Everglades nutrient criteria.
Climate-change-induced temperature increases influence several biogeochemical processes of wetlands and water quality. For example, increased temperature can increase primary productivity, organic matter decomposition, nutrient regeneration, and greenhouse gas emissions, and alter the composition and diversity of biotic communities (Carney et al., 2007; Watts et al., 2010). An increase in rates of these biogeochemical processes is likely to increase overall export of nutrients, dissolved organic matter, and associated contaminants and impact downstream water bodies (Qualls and Richardson, 2003; Reddy et al., 1999).
Sea-level rise that exceeds the rate of vertical soil and sediment accretion causes increased salinity stress in freshwater wetland communities and shifts ecosystems from freshwater to brackish (Koch et al., 2012; Saha et al., 2011; discussed in the next section). Increased sulfate inputs can potentially increase organic carbon mineralization and carbon dioxide emissions while decreasing methane emissions (Chambers et al., 2011, 2013a,b, 2014; Weston et al., 2011). These biogeochemical changes can increase the release of bioavailable nutrients (e.g., nitrogen, phosphorus), ultimately degrading water quality (Chambers et al., 2013a, 2014). In coastal phosphorus-limited wetlands, additional inputs of phosphorus from storm surge deposits can actually enhance the productivity of mangrove forests (Castañeda-Moya et al., 2010).
Implications for Everglades Biota
The effects of climate change and human-driven alterations of freshwater flows are already unfolding in the Everglades (Gaiser et al., 2012). Increased rates of sea-level rise have decreased the areal extent of several Everglades habitats, changed the distributions of many species, and driven inland migration of coastal vegetation (Box 5-2; Willard and Bernhardt, 2011). The rate and nature of future change remain unclear, however, because of uncertainty in downscaled climate change forecasts for the Everglades (discussed previously in this chapter), and the poor understanding of the capacity of ecological systems to respond to these impacts. Additionally, the multiple, interacting factors (e.g., increases in temperature, sea-level rise, changes in the quantity and distribution of precipitation, increases in atmospheric carbon dioxide and associated responses in biogeochemistry and ecology) are likely to generate complex effects that are difficult to fully predict.
Changes in precipitation, temperature, sea-level rise, and atmospheric carbon dioxide, in conjunction with anthropogenic alterations to hydrology, will collectively dictate the future environmental templates to which species respond. In climate change scenarios where sea-level rise is marked, temperature is elevated, and precipitation is reduced (Figure 5-6a), shortening of hydroperiods
should occur, with concomitant shifts toward less flood-tolerant vegetation and peat decomposition. The past century of Everglades water management offers numerous lessons about the adverse ecological impacts of reduced water flows (see Box 5-1 for one example). In scenarios with increasing rainfall (Figure 5-6b), freshwater flows can continue to maintain the diverse array of habitats in the Everglades and abate saltwater intrusion of coastal wetlands, essentially holding the sea at bay (Gaiser et al., 2012, Saha et al., 2012).
Changes in the composition and structure of Everglades communities are expected as species respond to changing climate. Species exposed to a warmer and perhaps drier Everglades subject to sea-level rise are likely to shift in distribution across the landscape in accordance with their climatic envelopes. For example, where salt marsh assemblages interface with mangroves, the northern edge of the mangrove distribution is controlled by the lack of cold tolerance. With increasing temperatures, mangroves are likely to advance northward with the freeze line (Cavanaugh et al., 2014), perhaps at the expense of transitional, brackish marsh assemblages (Stevens et al., 2006). Species with broad physiological tolerances will be the slowest to respond to increases in temperature, whereas those with narrow physiological ranges will be impacted more immediately. Drier conditions in the Everglades are also likely to reduce the densities of aquatic species that rely upon refugia during the dry season (e.g., fish and invertebrates; Catano et al., 2014), with consequent negative impacts to wading birds and other species that depend upon this prey base. With continued environmental changes, species eventually reach tipping points, beyond which they will either shift spatially on the landscape or gradually decline in abundance.
Low-lying coastal wetlands are sentinels of climate change impacts (Brinson et al., 1995; Scavia et al., 2002). They may be initially capable of coping by adjusting physiologically or vertically through biophysical processes to escape submergence (Cherry et al., 2009; McKee and Cherry, 2009; Morris et al., 2002). Where coastal species cannot keep pace with sea-level rise through vertical adjustment, their distributions contract at the seaward edge, and upslope expansion of species distributions must occur or their populations will gradually decline and disappear from the landscape (Brinson et al., 1995; Craft et al., 2009; Donnelly and Bertness, 2001; Williams et al., 1999). Thus, upgradient freshwater wetlands may be gradually converted to brackish marshes and finally to salt marshes in response to increased salinity. The rate and direction of response will be determined by both stress tolerance at their seaward edge and competitive ability at the inland edge of their distributions (Crain et al., 2004; Ervin and Wetzel, 2002; Kim et al., 2011). Among the biogeochemical processes affected by saltwater intrusion are increased sulfate inputs, which can increase the potential for sulfide toxicity to plants. Bidirectional compression could result if species are increasingly limited by environmental stress at the lower or upper
ends of their distributions, leading to coastal “squeezing” (Shirley and Battaglia, 2006, 2008).
Increased hurricane intensity (Bender et al., 2010; Blake et al., 2011) could also have important implications for Everglades biota. Intensified storms would drive changes in light and water availability to plants (Bianchette et al., 2009; Guntenspergen et al., 1995) and increase storm surges and associated wrack deposition (Blake et al., 2011; Tate and Battaglia, 2013), salt burning (Cahoon, 2006; Lam et al., 2011), and wind-driven damage to forest canopies (Lam et al., 2011; Rodgers et al., 2009).
A poorly understood but potentially important aspect of global change is the fertilization effects of increases in atmospheric carbon dioxide concentrations on wetland vegetation (Rasse et al., 2005). This process can enhance primary productivity and peat accretion (Erickson et al., 2013; Krauss et al., 2013) and lead to heightened sequestration of carbon, nitrogen, phosphorus, and mercury. Carbon dioxide fertilization effects could ameliorate several of the adverse consequences of climate change. Some plants, for example, reduce the size of pores that allow CO2 to enter leaves for photosynthesis, while still increasing carbon assimilation. This adjustment leads to reduced evapotranspiration (de Boer et al., 2011) and increased water-use efficiency (Li et al., 2010), potentially offsetting some effects of rising temperature. However, this trend is unlikely to increase indefinitely because the responses of individual plant species will be bounded by their genetic capacity to adapt structurally to future atmospheric carbon dioxide levels (Lammertsma et al., 2011). Shifts in plant community composition are also expected because CO2 fertilization can influence the timing of life-cycle events (e.g., flowering) (Springer and Ward, 2007), germination patterns (Mohan et al., 2004), and salinity tolerance of plants (Rozema et al., 1991). Grasses and sedges with a photosynthetic pathway that can better utilize increased CO2 and photosynthesize faster (e.g., sawgrass) may increase in abundance over similar species that use alternative pathways (Drake et al., 1996; Pearlstine et al., 2010). The effects of elevated CO2 are complicated, however, by temperature and precipitation regimes (Bjorkman et al., 1974; Raven, 2001) and may be relatively short-lived in some species as they plateau in their responses due to nutrient limitations (Reich et al., 2006). An improved quantitative understanding of carbon dioxide fertilization effects on wetland and marine ecosystems of South Florida would help refine predictions of the impacts of changing climate.
IMPLICATIONS FOR THE CERP AS ORIGINALLY DEVELOPED
Rising sea level and changes in evapotranspiration and precipitation could have significant effects on the success of the CERP.
Implications of Sea-Level Rise
Sea-level rise is already impacting shallow coastal marsh habitats (Figure 5-7), altering the salinities of surface waters and groundwaters, and changing the structural and operational requirements of coastal water management infrastructure (see Figure 5-11). To consider how future sea-level rise might affect the CERP, the committee considered three projects or areas targeted for CERP restoration: Picayune Strand, Biscayne Bay Coastal Wetlands, and Florida Bay. Each illustrates a different aspect of how sea-level rise may affect restoration.
In the Picayune Strand area in southwest Florida, drainage for a failed housing development caused the broad-scale conversion of freshwater wetland forests and marshes to communities dominated by species better adapted to drier conditions. The canal system increased the incidence of wildfires and oxidation of peat, led to proliferation of invasive species, and caused some inland expansion of mangroves (Chuirazzi and Duever, 2008). The objective of the Picayune Strand CERP project is to plug canals, rehydrate the area, and restore freshwater wetland habitat (see Chapter 4). The potential effects of sea-level rise stem from the fact that the project is a low-lying freshwater wetland, with ground surface elevations ranging from 3 to 10 ft NAVD, with several sloughs 0.5-2 ft lower in elevation (USACE, 2013e). The groundwater table can be as low as 2 ft (0.6 m) above sea level (Chuirazzi et al., 2012). USACE (2013e) determined that with 2 ft of sea-level rise (approximately the USACE intermediate local sea-level rise scenario in 2100), 9 percent of the project area would be inundated. Thus, the Picayune Strand project is likely to be minimally impacted by intermediate sea-level rise projections, but the extent to which project goals are affected remains unknown.
Shoreward portions of the soils in Picayune Strand will become increasingly impacted by saline intrusions with sea-level rise. As sea level rises, the groundwater salinity gradient would move inshore along with associated plant and thus animal communities. These effects have not yet been assessed (USACE, 2013e), although they could be determined using a coupled surface-water-flow variable-density groundwater model (e.g., Langevin et al., 2005). However, elevated groundwater stages resulting from the project will likely reduce the rate of salinity intrusion (compared with a future scenario without the project). The Picayune Strand project is therefore likely to delay ecological transitions from native freshwater wetland vegetation (e.g., cypress forest, sawgrass marshes) to brackish marshes and enhance the resilience capacity of coastal wetlands to cope with sea-level rise.
FIGURE 5-11 Vulnerability of SFWMD coastal structures to sea-level rise. High-vulnerability structures are red, medium-vulnerability structures are orange, and low-vulnerability structures are green. Those that are vulnerable to sea-level rise may require the addition of pump stations in place of gravity-driven control structures.
SOURCE: SFWMD (2009a).
Biscayne Bay Coastal Wetlands, Phase 1
The Biscayne Bay Coastal Wetlands Phase 1 project is designed to rehydrate coastal wetlands impacted by canal drainage and thereby improve salinity distributions in nearshore regions of Biscayne Bay (see Chapter 4). The impacts of various sea-level-rise scenarios on project benefits are shown in Table 5-3. At 2 ft of sea-level rise (the high scenario for 50 years and intermediate scenario for 100 years), less than 50 percent of the overall project benefits to freshwater and saltwater benefits are projected, although 88 percent of the nearshore salinity benefits remain (USACE and SFWMD, 2012b). On the basis of these analyses, planners concluded that project benefits over the 50-year planning horizon were sufficient to recommend the project, noting that the project would “delay future degradation of coastal wetland habitat caused by increased sea level conditions by redirecting freshwater flows into critical habitat” (USACE and SFWMD, 2012b). However, at the highest levels of sea-level rise considered over a 100-year time frame, all of the project benefits are lost (Table 5-3). USACE and SFWMD (2012b) state:
The effects of SLR on project benefits that occur after the 50-year project lifespan should be treated the same as benefits that occur after the project lifespan. In other words, effects that occur after the 50 year project lifespan should not be considered for plan selection or determination of project viability.
While consistency of planning constraints seems reasonable, the project highlights the limitations of 50-year planning horizons in the context of climate change.
Compared to the Picayune Strand Project, which represents a large area that is likely to gradually transition from freshwater to brackish wetlands, the Biscayne Bay Coastal Wetlands, Phase 1 project represents a narrow strip of coastal wetlands that are restricted from migrating landward by the L-31E levee and existing development (Figure 4-13). Thus, unlike Picayune Strand, all project benefits are likely to be lost at extreme levels of sea-level rise, and significant benefits are lost at likely levels of sea-level rise over the 21st century (2 ft; Table 5-3). However, these findings represent rather simplistic analysis of increments of sea-level rise overlain upon geographic information system maps, with no modeling of salinity changes expected in groundwater or nearshore areas of Biscayne Bay. Assessing the value of the project in the context of sea-level rise necessitates a rigorous analysis of existing ecosystem conditions and trends, the impacts of various sea-level-rise scenarios on project performance measures, and the extent to which the project could mitigate the impacts of sea-level rise.
TABLE 5-3 Projected Reduction in Biscayne Bay Coastal Wetlands Benefits by Component and Ecozone Under Several Sea-Level-Rise Scenarios
|Estimated Percent Benefit Reduction at 3" of SLR||Percent Reduction in Freshwater Wetland Benefits||Percent Reduction in Saltwater Wetland Benefits||Percent Reduction in Nearshore Salinity Benefits|
|Estimated Percent Reduction in Benefits with 3" of SLR *|
|Estimated Percent Reduction in Benefits with 7" of SLR *|
|Estimated Percent Reduction in Benefits with 9" of SLR *|
|Estimated Percent Reduction in Benefits with 24" of SLR|
|Estimated Percent Reduction in Benefits with at 68" of SLR|
* Reduction in benefits for SLR less than 1 ft were estimated by interpolating between the estimated losses at 0 ft of SLR and 1 ft of SLR.
SOURCE: USACE and SFWMD (2012b).
Florida Bay provides a large-scale example of the implications of sea-level rise for restoration. Florida Bay is a unique estuarine system with salinity determined by evaporation and precipitation as well as freshwater inputs. Water management changes in the South Florida ecosystem over the past 60 years have reduced freshwater inflows to the bay such that it can be seasonally hypersaline in the middle parts of the bay (Figure 5-12; Kelble et al., 2007; Nuttle et al.,
FIGURE 5-12 Mean salinity distributions in Florida Bay, showing conditions of hypersalinity (c).
SOURCE: Kelble et al. (2007).
2000). In 1987, a widespread collapse of seagrasses occurred, which is generally attributed to hypersalinity (Deis, 2011).
In general, the large-scale increases in sea level will cause Florida Bay to become deeper and incorporate portions of the southern Everglades. Increases in sea level of 2 ft (roughly the intermediate USACE local sea-level rise projection for 2100) would change the average depth of Florida Bay from 3 to 5 ft, presumably causing a significant change in salinity (e.g., Monismith et al., 2002). Sea-level rise could increase salinities throughout much of present-day Florida Bay during the wet season but could decrease occurrences of hypersalinity in central and northern Florida Bay through dilution. Any potential increase in salinity is of concern to the restoration, given that one of the objectives of the CERP is to reduce salinities in Florida Bay through increased freshwater inflow
to more closely mimic pre-drainage hydrology. However, the effect of sea-level rise on Florida Bay salinity is significant enough that restoration goals will need to be revisited under various sea-level-rise scenarios.
To fully evaluate effects of sea-level rise on the conditions and restoration of Florida Bay, a fully three-dimensional (3-D) circulation model of the bay, such as that described by Zheng and Weisberg (2012), is needed, albeit one coupled with a regional hydrologic model. Given the importance of precipitation and surface flows to Florida Bay, ideally such a model would also include a regional atmospheric model (e.g., Maxwell et al., 2011). Neither existing empirical models of salinity in Florida Bay (FATHOM) nor 2-D models properly depict the physics associated with sea-level rise (see also NRC, 2002a).
Water Budget Implications
Although future climate conditions are uncertain, the seven hypothetical scenarios presented previously in the chapter (see Implications for Everglades Hydrology; Obeysekera et al., 2014) highlight the range of challenges that Everglades restoration could face. Increases in rainfall represent the best-case scenario for the ecosystem, with increases in water flow (Table 5-2, Figure 5-6b). The combination of increased coupled evapotranspiration, decreased precipitation, and rising sea level over future decades represents the worst-case scenario among those modeled. Under this scenario, water levels would decline throughout the system (Figure 5-6a) and unmet water supply demands from agriculture and urban population centers would intensify existing conflicts over water supply. Declining groundwater levels combined with sea-level rise would further exacerbate saltwater intrusion, compromising urban water supplies (Figure 5-13) and impacting coastal ecosystems. Although no modeling has been done to quantify the effects of the CERP as currently planned under such a scenario, it is possible that the benefits of the CERP would be surpassed by the negative impacts of reduced precipitation and increased evapotranspiration. However, the scenario of potential future decreases in flow associated with decreases in precipitation and increased evapotranspiration amplify the urgency to accelerate projects that increase storage and move more water southward to enhance the resilience of the ecosystem under future conditions.
Given the competing demands of water supply, environmental restoration, and flood control, it is clear that providing as much flexibility as possible to water managers will be critical to the success of the CERP. In many systems, flexibility of operation is achieved through water storage (e.g., reservoirs, aquifer storage). For example, in the western United States, large reservoirs such as Lake Mead or Lake Shasta buffer water supplies against interannual variations in precipitation as well as reducing high flows so as to prevent floods. In South Florida, existing
FIGURE 5-13 Saltwater intrusion interface in Miami-Dade County, and proximity to water supply well fields.
SOURCE: J. Obeysekera, SFWMD, personal communication, 2014.
operational flexibility is achieved by operation of the extensive network of channels and structures and through storage in Lake Okeechobee, although the lake is highly constrained by ecological and dam safety considerations. The CERP, if fully constructed, would add substantial storage via aquifer storage and recovery and several large reservoirs, but it is unknown whether this storage would be sufficient to sustain the ecosystem under the worst-case climate scenarios.
In the face of possible changes in hydrologic conditions and sea-level rise, increasing water storage to provide more reliable flow to the Everglades as well as to water users could provide useful operational flexibility. For example, maximizing the ability to capture and store water in surface-water reservoirs during wet periods that would otherwise be discharged through the northern estuaries would also provide water for environmental and human uses during dry periods and increase groundwater recharge to mitigate salinity intrusion into South Florida aquifers.
Implications on CERP Goals
As discussed in Chapter 2, the CERP generally aims to restore the “essential hydrological and biological characteristics that defined the undisturbed South Florida ecosystem” (33 CFR § 385.3). The Natural Systems Model has played an important role in shaping restoration goals. However, under the worst-case scenarios of climate change and sea-level rise in the Everglades, some CERP goals may not be attainable, and others may need to be revisited considering substantially changed conditions under sea-level rise (e.g., Florida Bay). Although the CERP, as finalized in 2000, did not incorporate climate change effects on restoration outcomes, there is broad recognition in the research and management communities of the multiple facets of climate change and their impacts (Aumen et al., in press). Estimates of sea-level rise and downscaled climate projections and their ecological impacts will continue to be refined with future research and incorporated into management planning. Restoration planners will need to continuously revisit whether pre-drainage hydrologic and ecological targets still provide useful restoration goals, and to develop alternative goals in light of what is feasible and sustainable under future conditions. Meanwhile, changes in temperature and precipitation could reduce regional water availability, necessitating additional attention to water sustainability for built and natural systems to address potential water supply challenges unforeseen when the CERP was originally developed.
Literature on climate change is replete with studies of probable and possible impacts of climate change on water resources in various geographic regions. Work of that kind in South Florida is impressive. Although there are many recommendations to incorporate effects of climate change in water resource planning,
far fewer in-depth studies that suggest how that can be accomplished are available than the assessments of impacts of climate change. Among the more complete publications addressing adaptation is in the context of water management in California, especially the Climate Change Handbook for Regional Water Planning (CDM, 2011) developed for the California Department of Water Resources, USACE, EPA, and Resources Legacy Fund and the 2009 California Climate Adaptation Strategy (California Natural Resources Agency, 2009). Hanak and Lund (2012) and the National Research Council (NRC, 2012c) discussed progress toward incorporating climate change in managing California’s water resources.
Those documents describe an array of management options to adapt to climate change. Broad categories such as aggressive pursuit of water use efficiency, enhancement and restoration of ecosystems, increased storage, improved conveyance and transfers, management of land use, and optimization of system operations are followed by more detailed measures. Many of those actions have already been taken in South Florida, some of which are directly targeted by CERP and non-CERP projects. Miami-Dade, West Palm Beach, and other urban areas have adopted progressive increasing block-rate pricing and other conservation measures to manage demand for public water supplies. Much of the degradation of the Everglades and demands on its services are due to external forces. Although development of a comprehensive strategy to adapt to climate change in South Florida is beyond the scope of the CERP, a more complete strategy for restoration of the Everglades and adaptation to climate change would need to address management of demand and supply for water and related land resources within and external to the Everglades ecosystem. Ongoing research through the 5-year South Florida Water, Sustainability, and Climate Project5 led by Florida International University and funded by the National Science Foundation may help inform such planning.
PLANNING TO ADDRESS CLIMATE CHANGE
CERP planners are just beginning to address climate change impacts. USACE project planning guidance (USACE, 2011e) specifically includes a method for estimating sea-level rise in project design, and the USACE requires analysis of three sea-level rise scenarios for all Civil Works projects (discussed earlier for Biscayne Bay Coastal Wetlands and Picayune Strand). To date, however, such analysis has been limited to the project level, where sea-level rise is used to design coastal structures and adjust project benefits through simple analyses of land lost due to inundation under different sea-level rise scenarios (Figure 5-14). Early analyses primarily examined changes in benefits over 50 years (USACE
FIGURE 5-14 Projected impact of sea-level rise on overall habitat improvements (in habitat units) provided by the Central Everglades Planning Project under different scenarios (see also Chapter 3 and Figure 5-4). The analysis considers the reduction in overall project-derived benefits due to seawater inundation of freshwater wetlands in the project area.
SOURCE: USACE and SFWMD (2013b).
and SFWMD, 2012b), but more recent analyses considered changes in benefits over 100 years—a more useful time horizon in the context of climate change, sea-level rise, and restoration investments of the magnitude of the CERP (USACE, 2013e; USACE and SFWMD, 2013b). To the best of the committee’s knowledge, models capable of computing salinity fields in surface waters and groundwater have not been used in the CERP to assess the effects of sea-level rise on salinities. Nonetheless, suitable models (e.g., Langevin et al., 2005) currently exist that could be coupled to surface-water models.
The USACE-required sea-level-rise analyses typically are performed at the end of the planning process, after the desired project alternative has been selected. However, CERP project planning would benefit from broader incorporation of climate and sea-level-rise scenarios during project development. Despite uncertainty associated with climate projections, more rigorous scenario planning (NPS, 2013; Peterson et al., 2003; Polasky et al., 2011) provides a framework for project-level decision making. Such planning would consider
the effects of climate change on performance measures for various alternatives, quantify the specific benefits associated with mitigating the impacts of climate change, and identify the costs and benefits of additional flexibility to address climate change uncertainties (see also Chapter 3).
NRC (2008) emphasized that in light of climate change, restoration efforts “are even more essential to improve the condition of the South Florida ecosystem and strengthen its resiliency as it faces additional stresses in the future.” However, this perspective does not imply that restoration should continue unchanged. To date, as far as the committee is aware, CERP planners have not made any major adjustments at the systemwide level in light of climate change and sea-level rise. Accurate precipitation or temperature projections are not available that can serve as a basis for major changes in the CERP because of uncertainty associated with future storylines driving greenhouse gas emissions and land cover, GCM simulations, and regional downscaling. However, sea-level-rise projections are available that should form the basis for project prioritization, considering project benefits in the context of sea-level-rise and the potential for the CERP to delay or mitigate sea-level-rise impacts. Existing scheduling and prioritization as reflected in the Integrated Delivery Schedule are largely driven by project authorizations (see Chapter 4), but wise expenditures of funds necessitates periodic reassessment of the priority of previously authorized projects in light of improved understanding of future conditions.
Considering the uncertain projections of future precipitation and temperature, systemwide analysis across an array of future scenarios is critical. Current hydrologic and ecosystem modeling for the CERP is based solely on historical hydrology and does not address potential effects of long-term changes in temperature, precipitation, evapotranspiration, and sea-level rise. Additional Everglades climate-sensitivity analyses are needed to anticipate challenges, identify potential contingencies and system flexibilities to mitigate climate-related changes, and target additional research efforts toward critical uncertainties. The recent scenario analysis by Obeysekera et al. (2014) provides a useful first step, although the scenarios selected were purposefully simplistic. More recently, Swain et al. (2014) used a hydrodynamics model to examine the effects of changes in sea level and precipitation on freshwater flows, surface-water salinity, and inundation within Everglades National Park and coastal areas to the east. Additional modeling of this kind is needed to analyze the sensitivity of the South Florida ecosystem to seasonal and long-term precipitation and temperature variability to enhance the understanding of possible climate impacts. This modeling should be used to examine the possible effects of climate change on Everglades
hydrology and ecology with and without the implementation of CERP. Scenario-based modeling could also be used to explore the impacts of specific projects to identify which projects are most resilient to climate change and/or which projects mitigate the impacts of climate change under a range of scenarios. These model simulations should be refined as new data become available that reduce uncertainties in model forcings and hence model predictions.
CERP planning in the context of sea-level rise and climate change may require the development of new modeling tools or the improvement and application of existing tools. Improved salinity modeling tools are needed, such as 3-D circulation models for the major estuaries coupled to regional hydrologic models, as discussed previously in this chapter. Understanding salinity intrusion in coastal wetlands and aquifers used for urban water supply requires a surface-water flow model coupled with a variable-density groundwater flow model (e.g., Tides and Inflows in the Mangroves of the Everglades [TIME] v. 2.0). The TIME model domain includes the terrestrial areas of Everglades National Park, its coastal mangrove zones to the south and east of the park, and the northern edge of Florida Bay, but it does not include Biscayne Bay. Currently, the model is being refined for assessing sea-level rise and restoration alternatives in Everglades National Park (Bahm and Fennema, 2013). For variable density modeling of saltwater intrusion, the SFWMD uses a coupled MODFLOW/SEAWAT model (Harbaugh et al., 2000; Langevin et al., 2003; Restrepo and Montoya, 2008).
Sea-level rise will lead to landward shifts of the interface between fresh groundwater and saltwater, thereby increasing the potential of saltwater intrusion (Figure 5-13). Sea-level rise will also increase groundwater levels, as the aquifer system responds to new conditions along its seaward boundary. This water-table adjustment will be greatest along the coast, but will propagate inland. Because groundwater and surface-water systems are tightly connected in South Florida, rising water tables will alter the rates and volumes at which excess water can be conveyed through canals and natural waterways of the Everglades. The ability to forecast these sea-level-induced changes in groundwater flow and groundwater/surface-water interactions is requisite to informing the design and operation of water control infrastructure under the CERP.
IMPORTANCE OF EVERGLADES RESTORATION IN CONTEXT OF CLIMATE CHANGE
Systems such as the Everglades that are highly vulnerable to the impacts of climate change are likely to benefit from management strategies that incorporate
maximizing ecological resilience (or the capacity to respond to environmental changes) as a goal (Millar et al., 2007; Tompkins and Adger, 2004). The ability of these systems to respond to the impacts of climate change depends upon their capacity to rebound from disturbance as well as to respond to chronic and gradual environmental changes (Gunderson, 2000). Long-term persistence of ecological functioning depends upon maintenance of adequate source populations in the landscape. For example, unimpaired coastal wetland ecosystems can respond to sea-level rise through a combination of biological (e.g., organic-matter accumulation) and physical processes (e.g., sediment accretion from storm surges). Where rate of environmental change exceeds the capacity for species to remain in place, lateral migration, regulated by dispersal, and availability of suitable habitat, is necessary. In the Everglades, restoration of hydrologic regimes will support such processes, helping to perpetuate diversity, ecosystem functioning, and fluidity of the landscape as climate envelopes of species shift (Manning et al., 2009).
In the face of climate change, Everglades restoration will increase the resilience of the ecosystem and the water management system and decrease their vulnerability. From the perspective of water resources management, the CERP may offer substantial benefits. In particular, increasing surface-water flows through water conservation areas and into Everglades National Park may help mitigate the sea-level-rise-induced salinization of the aquifers that provide water supply for Dade, Broward, and adjoining counties. This issue would benefit from modeling to better characterize and quantify the scope of potential benefits.
CONCLUSIONS AND RECOMMENDATIONS
Climate change provides a strong incentive for accelerating restoration. Current impacts of rising sea levels are a harbinger of future climate change effects on the functioning and structure of the Everglades ecosystem and the ecosystem services on which South Florida depends. Sea-level rise in South Florida is already increasing saltwater intrusion into Everglades freshwater habitats and urban water supplies, and future climate changes are likely to be manifested through changes in the timing, volume, and quality of freshwaters; distributions of species; and the extent of wetland habitats. Climate change is also expected to increase agricultural water demands, which when paired with anticipated population growth, highlights the potential regional water supply challenges in South Florida under future scenarios. Everglades restoration enhances the ability of the ecosystem to withstand and adapt to future changes, and increases water availability to the ecosystem and to urban and agricultural users. Improvements in Everglades water depths promote higher rates of peat accretion that could
help mitigate the effects of sea-level rise and reduce the impacts of saltwater intrusion on urban water supplies.
Although the projections are uncertain, significant changes in precipitation and temperature coupled with increasing sea level have important implications for the CERP. The Everglades landscape is especially sensitive to sea-level rise, and rates of sea-level rise in South Florida are predicted to increase. A scenario of 1.5-degree increase in temperature and a 10 percent decrease in precipitation together with anticipated sea-level rise results in significant changes in coastal ecosystems and insufficient freshwater to sustain the natural and built systems. To decrease uncertainty associated with precipitation projections and clarify future risk, global climate model projections of intra-annual, annual, and interannual variability in precipitation and temperature need to be improved and refined. These improved climate projections should, in turn, be used by CERP planners as input to drive Everglades hydrologic models suitable for making inferences on year-to-year and seasonal variations in freshwater availability.
Climate change is not adequately considered in the CERP planning process and should be integrated into future ongoing analysis and monitoring. CERP projects are designed based on historical hydrology and have not been assessed in the context of future precipitation and evapotranspiration scenarios. Currently, only sea-level rise is considered in CERP planning and usually only as a cursory analysis at the end of the process to assess loss of benefits through 2050 with wetland inundation resulting from sea-level rise. The lack of consideration of the effects of climate change paints an incomplete picture of hydrologic and ecosystem response to the alternatives examined and ignores the potential benefits of the projects to help mitigate the impacts of climate change. Additionally, hydrologic restoration goals are based on the natural systems model, which reflects the past 50 years rather than any likely future. Depending on future climate change, some hydrologic or ecological restoration goals may be unattainable or prove to be not cost-effective. Urban and agricultural water demands unmet under dire climate scenarios highlight the need for additional analysis of water sustainability for the natural and built systems.
CERP planners should consider the implications of sea-level rise and potential hydrologic changes in systemwide planning and project prioritization. Likely sea-level-rise projections can be used to evaluate future project benefits, considering uncertainties regarding the potential for accretion in coastal and inland wetlands to mitigate these effects. Sea-level-rise scenarios should also be coupled with hydrologic change scenarios to characterize systemwide response to global change. The outcome of these analyses would inform future systemwide decisions of project prioritization. Re-prioritization should include consideration of both those rendered less important and effective in light of reduced benefits in the context of climate change and sea-level rise and those projects
that become more essential to enhance the ability of the ecosystem and the built environment to adapt to changes and mitigate the effects of changing climate.
Anticipating future changes in temperature, precipitation, and sea-level rise, CERP planners should, where feasible, design for flexibility. Climate change needs to be incorporated into adaptive management planning, at the project scale as well as when considering systemwide goals. It is likely that additional water storage will be needed to address anticipated future increases in variability of meteorological conditions. As new knowledge becomes available, it needs to be incorporated into the CERP adaptive management framework so that managers can adjust future restoration efforts appropriately as the nature of changes in climate become more evident. In addition, the current monitoring program should be evaluated to ensure that important effects of climate change will be characterized and quantified.
The committee identified several high-priority research needs related to climate change and Everglades restoration:
• Assess the rates of peat/sediment accretion and subsidence in coastal and inland freshwater wetlands in the context of sea-level rise;
• Improve modeling tools that can be used to assess the effects of projected sea-level rise on groundwater supplies and coastal ecosystem functioning, and examine the potential for the CERP to mitigate these effects;
• Improve, refine, and evaluate downscaled climate model projections in the context of South Florida water resources and Everglades restoration;
• Improve the understanding of factors that could help maintain the diverse mosaic of Everglades habitats and increase their resilience amid changes in climate and sea level; and
• With improved climate and sea-level projections, reevaluate the goals for Everglades restoration and develop alternate goals as appropriate.