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Suggested Citation:"2 Coastal Blue Carbon." National Academies of Sciences, Engineering, and Medicine. 2018. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. doi: 10.17226/25259.
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Suggested Citation:"2 Coastal Blue Carbon." National Academies of Sciences, Engineering, and Medicine. 2018. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. doi: 10.17226/25259.
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Suggested Citation:"2 Coastal Blue Carbon." National Academies of Sciences, Engineering, and Medicine. 2018. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. doi: 10.17226/25259.
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Suggested Citation:"2 Coastal Blue Carbon." National Academies of Sciences, Engineering, and Medicine. 2018. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. doi: 10.17226/25259.
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Suggested Citation:"2 Coastal Blue Carbon." National Academies of Sciences, Engineering, and Medicine. 2018. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. doi: 10.17226/25259.
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Suggested Citation:"2 Coastal Blue Carbon." National Academies of Sciences, Engineering, and Medicine. 2018. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. doi: 10.17226/25259.
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Suggested Citation:"2 Coastal Blue Carbon." National Academies of Sciences, Engineering, and Medicine. 2018. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. doi: 10.17226/25259.
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Suggested Citation:"2 Coastal Blue Carbon." National Academies of Sciences, Engineering, and Medicine. 2018. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. doi: 10.17226/25259.
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Suggested Citation:"2 Coastal Blue Carbon." National Academies of Sciences, Engineering, and Medicine. 2018. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. doi: 10.17226/25259.
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Page 39
Suggested Citation:"2 Coastal Blue Carbon." National Academies of Sciences, Engineering, and Medicine. 2018. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. doi: 10.17226/25259.
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Page 40
Suggested Citation:"2 Coastal Blue Carbon." National Academies of Sciences, Engineering, and Medicine. 2018. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. doi: 10.17226/25259.
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Page 41
Suggested Citation:"2 Coastal Blue Carbon." National Academies of Sciences, Engineering, and Medicine. 2018. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. doi: 10.17226/25259.
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Page 42
Suggested Citation:"2 Coastal Blue Carbon." National Academies of Sciences, Engineering, and Medicine. 2018. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. doi: 10.17226/25259.
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Page 43
Suggested Citation:"2 Coastal Blue Carbon." National Academies of Sciences, Engineering, and Medicine. 2018. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. doi: 10.17226/25259.
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Page 44
Suggested Citation:"2 Coastal Blue Carbon." National Academies of Sciences, Engineering, and Medicine. 2018. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. doi: 10.17226/25259.
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Page 45
Suggested Citation:"2 Coastal Blue Carbon." National Academies of Sciences, Engineering, and Medicine. 2018. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. doi: 10.17226/25259.
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Page 46
Suggested Citation:"2 Coastal Blue Carbon." National Academies of Sciences, Engineering, and Medicine. 2018. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. doi: 10.17226/25259.
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Page 47
Suggested Citation:"2 Coastal Blue Carbon." National Academies of Sciences, Engineering, and Medicine. 2018. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. doi: 10.17226/25259.
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Suggested Citation:"2 Coastal Blue Carbon." National Academies of Sciences, Engineering, and Medicine. 2018. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. doi: 10.17226/25259.
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Page 49
Suggested Citation:"2 Coastal Blue Carbon." National Academies of Sciences, Engineering, and Medicine. 2018. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. doi: 10.17226/25259.
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Page 50
Suggested Citation:"2 Coastal Blue Carbon." National Academies of Sciences, Engineering, and Medicine. 2018. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. doi: 10.17226/25259.
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Page 51
Suggested Citation:"2 Coastal Blue Carbon." National Academies of Sciences, Engineering, and Medicine. 2018. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. doi: 10.17226/25259.
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Suggested Citation:"2 Coastal Blue Carbon." National Academies of Sciences, Engineering, and Medicine. 2018. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. doi: 10.17226/25259.
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Page 53
Suggested Citation:"2 Coastal Blue Carbon." National Academies of Sciences, Engineering, and Medicine. 2018. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. doi: 10.17226/25259.
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Page 54
Suggested Citation:"2 Coastal Blue Carbon." National Academies of Sciences, Engineering, and Medicine. 2018. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. doi: 10.17226/25259.
×
Page 55
Suggested Citation:"2 Coastal Blue Carbon." National Academies of Sciences, Engineering, and Medicine. 2018. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. doi: 10.17226/25259.
×
Page 56
Suggested Citation:"2 Coastal Blue Carbon." National Academies of Sciences, Engineering, and Medicine. 2018. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. doi: 10.17226/25259.
×
Page 57
Suggested Citation:"2 Coastal Blue Carbon." National Academies of Sciences, Engineering, and Medicine. 2018. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. doi: 10.17226/25259.
×
Page 58
Suggested Citation:"2 Coastal Blue Carbon." National Academies of Sciences, Engineering, and Medicine. 2018. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. doi: 10.17226/25259.
×
Page 59
Suggested Citation:"2 Coastal Blue Carbon." National Academies of Sciences, Engineering, and Medicine. 2018. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. doi: 10.17226/25259.
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2 Coastal Blue Carbon INTRODUCTION Overview of Coastal Blue Carbon—Definition and Motivation Coastal carbon sequestration, in this report, refers to CO2 removal from the atmosphere in conjunction with plant growth and the accumulation and burial of plant organic carbon (OC) residue in the soil of tidal wetland and seagrass ecosystems. Tidal wetlands, including salt marshes and mangroves, thrive in soft-sediment, shallow regions of estuaries between high and mean sea level, while seagrasses inhabit adjacent soft-sediment estuarine bottoms with adequate light penetration. Macroalgal systems such as kelp forests, while extremely productive, do not have root systems and soils to accumulate carbon (Howard et al., 2017). Little is known about the fate of macroalgal OC, a portion of which could potentially be sequestered by other means when exported from the macroalgal bed (Krause-Jensen et al., 2018). Further understanding macroalgal transport processes or other farming approaches that could permanently remove CO2 from the atmosphere is needed in order to assess if there is potential for globally significant levels of CO2 sequestration. Given the state of research on this topic, which is summarized in Appendix C, the CO2 removal capacity of macroalgal storage is not a focus of this chapter. Other ocean-based NETs, such as sequestration in microbial or planktonic biomass (Zhang et al., 2017b) and ocean alkalization (Rau et al., 2013; Renforth and Henderson, 2017), may be conducted in the coastal environment but are essentially open ocean approaches. They remove carbon through very different mechanisms from coastal wetlands, which bear closer similarity to terrestrial-based negative emissions technologies (NETs). As described in Chapter 1, these approaches are not included in this report and are worth further investigation due to the potentially large role ocean sinks can play in CO2 removal. Tidal wetlands and seagrasses are among the most productive regions on earth, sequestering CO2 at a rate of 7.98 t CO2 ha-1 yr-1 for tidal wetlands and 1.58 t CO2 ha-1 yr-1 for seagrass meadows (EPA, 2017; IPCC, 2014b). Scaled to their current global areal extent, they are an important component of the global carbon (C) cycle. Tidal wetlands grew to their current areal extent only in the past 4-6k years, once the rate of sea level rise (SLR) fell to less than a couple millimeters per year. The organic carbon that has accumulated over this time is deep and vast, ranging from 2 to 25 Gt C (best-estimate of 7 Gt C; Bauer et al., 2013; Donato et al., 2011; Fourqurean et al., 2012; Pendleton et al., 2012; Regnier et al., 2013). The recent State of the Carbon Cycle Report (SOCCR-2) estimates that tidal marsh wetland soils and estuarine sediments of North America store roughly 1.323 Gt C as the top 1 m of soil and sediment. However, coastal soil profiles are known to reach greater depths and thus carbon storage is higher than this estimate (Jonathan et al., 2018). Variation in soil and sediment depths have become an increasingly recognized source of uncertainty in tidal wetland coastal carbon stocks, in addition to other sources of uncertainty (Holmquist et al., 2018). Although seagrasses have lower OC burial rates than tidal wetlands per unit area, they potentially cover a much larger area and thus could have higher total carbon rates and sequestration capacity. However, substantial difficulties in large-scale estimation persist as there is large variability in measurements of seagrass organic carbon stocks and burial rates. The mapping of seagrass areas in U.S. water is also limited; less than 60% of meadows are mapped and existing maps have varying degrees of accuracy due to difficulties in remote sensing of underwater habitat (Oreska et al. In press). Annual rates of coastal carbon sequestration are also high. Globally, the total carbon sequestration rates are estimated at 31–34 Mt C yr–1 for mangrove, 5–87 Mt C yr–1 for salt marshes, and 48–112 MtC yr–1 for seagrass beds, or 226 ± 39 gCm-2 y-1 for mangrove, 218 ± 24 gC m-2 y-1 for salt marshes, 138 ± 38 gCm-2 y-1 for seagrasses, summing up to a global annual rate of 0.84 Gt CO2 yr-1 (Mcleod et al. 2011). According to the U.S. Inventory of Greenhouse Gas Emissions and Sinks, U.S. tidal PREPUBLICATION COPY 31

32 Negative Emissions Technologies and Reliable Sequestration: A Research Agenda wetlands (marshes and mangroves) sequester over 12 Mt CO2 yr-1 and a net of about 8 Mt CO2 equivalents/year when both CO2 and CH4 fluxes are considered (EPA, 2017). CH4 fluxes introduce significant uncertainty in large-scale estimates given difficulties with detecting salinity boundaries that determine CH4 flux rates (Poffenbarger et al., 2011). For North America, soil OC accretion rate (sediment burial) was estimated at 5±2 Mt yr-1 for tidal marshes, 2±1 Mt yr-1 for mangroves and 1±1 Mt yr-1 for seagrass meadows (30 Mt CO2 in total; SOCCR-2). The U.S. EPA estimate was only for the continental United States and included fewer wetland areas and tidal wetland types than the North American SOCCR-2 report. The U.S. EPA report also did not include seagrass meadows. Tidal wetlands, particularly mangroves, also sequester CO2 in aboveground biomass via long-term storage in wood and woody stems. While this contributes to negative CO2 emissions, this CO2 removal flux is omitted from estimates presented in this report as most recent U.S. estimates focus on soils only as it is the proportionally larger sink for tidal marshes ecosystems that represent the largest area of coastal carbon ecosystems in the U.S. The motivation for including coastal blue carbon as a potential NET is that there is the potential to more than double the current rate through several CO2 removal approaches that restore and create coastal wetlands. Further, there is concern that the current rate of sequestration will drop substantially because of expected changes in factors that contribute to sequestration, especially those that affect the current areal extent and carbon burial rate per unit area. Sequestration is vulnerable to impacts from climate change, including increasing rates of SLR (Figure 2.1) and rising temperatures, and human activities in the coastal zone. While conversion of U.S. coastal wetlands has slowed, it is estimated that globally, drainage and excavation of mangrove, tidal marsh, and seagrass soils release 450 million tons of CO2, annually (range 150-1005 Mt CO2 yr-1; Pendleton et al., 2012). There are also indirect threats to tidal wetlands posed by declining sediment supplies and groundwater/oil and gas extraction (Megonigal et al., 2016). Reversing historic loss and degradation through restoration, incorporation of wetland creation into coastal adaptation projects, and managing wetland area and carbon accumulation rates provide an opportunity for increased carbon removal and storage through the 21st century. FIGURE 2.1. Relative sea level over the last 2000 years, reconstructed using analysis of microfossils in sediment from salt marshes in North Carolina and recent acceleration. SOURCE: Kemp et al., 2011. PREPUBLICATION COPY

Coastal Blue Carbon 33 FIGURE 2.2. Redfield model of tidal wetland development over the past several thousand years as a result of bay infilling and marsh progradation, sediment and organic carbon burial and elevation gain, and flooding of uplands as sea level rises and wetlands transgress upland habitats. Example from Barnstable Harbor, Massachusetts. SOURCE: Redfield, 1972. Coastal wetlands and seagrasses are already the targets of restoration and management for the broad range of ecosystem services they provide beyond CO2 removal, including coastal storm protection and wave attenuation, water quality improvement, wildlife habitat, and support of fisheries (Alongi, 2011; Barbier et al., 2011; Lee et al., 2014; Nagelkerken et al., 2008; Zhang et al., 2012). These activities and investments, which are not included in this report as NET costs, can be leveraged to provide CO2 removal advantages at marginal costs. COASTAL BLUE CARBON PROCESSES Coastal carbon sequestration is calculated for each ecosystem as the product of their areal extent (horizontal dimension) and their vertical OC accumulation rate (vertical dimension) (Hopkinson et al., 2012). Over the past 4-6k years, the rate of SLR slowed enough to allow tidal wetlands to maintain or expand their areal extent and their vertical elevation relative to SLR (Figure 2.2). The survival of existing tidal wetlands requires that vertical elevation gain at least match the rate of SLR. Tidal wetlands form with salt-tolerant plants in soft-sediment intertidal regions above MSL. Once established, the elevation of wetlands increases relative to SL through the accretion of particles trapped by wetland vegetation from tidal waters (watershed, oceanic or local sources) and the accumulation of undecomposed wetland plant organic matter. The rate of accretion and carbon burial varies hyperbolically with flooding frequency and depth and thus the rate of SLR (see Figure 2.3; Morris, 2016). With adequate sediment supply relative to the rate of SLR and local subsidence (or rise), sedimentation leads to the shallowing of estuaries. Tidal wetlands prograde into formerly open water regions, once depths reach MSL, thereby increasing their horizontal extent. Tidal wetlands will also expand horizontally through transgression when rising seas flood adjacent upland areas and wetland plants invade. In seagrass meadows, lateral expansion occurs through asexual clonal growth. Seagrasses also disperse seeds to colonize new areas. Within seagrass meadows, organic carbon is accreted vertically when wave attenuation from the plant canopy causes sediment to settle from the water column and bury plant detritus as well as allochthonous carbon. About half of the carbon buried in seagrass beds is produced by non-seagrass sources (Oreska et al., 2018). Carbon burial varies spatially within a meadow; wave attenuation is more effective in inner parts of the meadow, with greater erosion occurring at the edges (Oreska et al., 2017). Seagrass bed productivity is controlled by different factors including nutrient and light availability (Apostolaki et al., 2011; Hendriks et al., 2017). Light availability is controlled by a variety of factors including water depth and turbidity. Turbidity is related to local geomorphic drivers as well as eutrophication (which controls phytoplankton density) and growth of epiphytes on leaves (a factor contributing to reduced photosynthesis of submerged macrophytes). Critical to coastal carbon sequestration is the rate of vertical elevation gain and specifically the relative contribution of undecomposed plant organic carbon to mineral sediments. In tidal wetlands, most of the organic carbon accumulating is that produced in situ (autochthonous), while in seagrass meadows PREPUBLICATION COPY

34 Negative Emissions Technologies and Reliable Sequestration: A Research Agenda trapping of organic carbon from external sources (allochthonous) can be important as well. The fate of wetland plant biomass production varies tremendously from system to system depending on the rates of net primary production (NPP), respiration of microbes and larger animals living on and in tidal wetlands (Re) (i.e., decomposition and consumption of the OC), and the tidal export of undecomposed plant material (Hopkinson, 1988), which is mostly above-ground plant material. The balance between NPP, Re, and export is the organic matter that is buried and preserved. The continued surface deposition acts to increase over time the depth of OC within the sediment, where conditions reduce organic matter decomposition, thus preserving it for longer and longer periods of time (Redfield, 1972). The dynamic exchange and export of OC to adjacent systems is a unique characteristic of tidal and estuarine systems compared with terrestrial ecosystems. Carbon that is exported laterally may exit the coastal ocean through CO2 outgassing or dissolved organic carbon and particle carbon export to the open ocean, with carbon import across these interfaces also possible. The balance of these processes results in annual OC storage at higher rates than long-term OC sequestration (Breithaupt et al., 2012). To simplify the Committee’s treatment of negative carbon emissions in this study, the focus in this Chapter is on long- term (50-100 years), buried soil carbon. COASTAL BLUE CARBON IN THE FUTURE—THE IMPACTS OF CHANGING ECOSYSTEM DRIVERS The baseline carbon sequestration capacity for coastal wetlands and seagrass meadows is the predicted changes in areal extent and OC burial rates of these ecosystems in the absence of human intervention. Without any human intervention, this baseline is possibly decreasing over time, as compared with the current burial rate, under the stress of climate change and human activity. It is upon this expected baseline that CO2 removal approaches to increase future CO2 removal trajectories can be evaluated. Extrapolating past and current rates of CO2 removal to predict CO2 removal in the future will not likely provide accurate estimates, as drivers are changing rapidly in many coastal areas. The heart of a research agenda is to fill knowledge gaps on the response to these changing drivers, and to constrain uncertainties in coastal blue carbon to better predict and manage future trajectories and accelerate new opportunities for CO2 removal. The drivers of concern are those most likely to change as a result of climate change or other anthropogenic impacts over the next hundred years, including: • relative SLR (which affects extent, depth, and duration of tidal flooding), • sediment availability (from watershed inputs, tidal flooding and/or storms), • temperature (and growing season length), light availability, • salinity (related to river flow, local climate, and sea level), • inorganic N and P availability and enrichment, and • development of wetland area or uplands adjacent to wetlands. Partially or fully driven by the above abiotic factors, biotic factors include: • Plant productivity and species composition, • Plant migration rates, and • Organic matter decomposition rates. Expansion of tidal wetlands into open water areas of estuaries has slowed and in some places reversed over the past 100 years or so, primarily as a result of decreased sediment input due to watershed management activities compounded by increased rates of relative SLR. Increased rates of SLR have also increased transgression (the expansion of wetlands into terrestrial uplands). SLR interacts with PREPUBLICATION COPY

Coastal Blue Carbon 35 FIGURE 2.3. Carbon sequestration is a function of the rate of relative sea level rise (RSLR) and maximum marsh biomass (Bmax) as depicted by simulations of the Marsh Equilibrium Model. SOURCE: James Morris, presentation to committee. anthropogenic stressors and can result in accelerated erosion and subsidence in certain conditions. Several models have been developed to provide a predictive understanding of both the areal extent and OC burial components of tidal wetlands (French et al., 2008; Kirwan et al., 2010, 2016b; Morris et al., 2002; Morris, 2016; Mudd, 2011). Models show a simple bell-shaped relationship between elevation and productivity characterizes tidal biomass production and a linear elevation- decomposition relationship characterizes belowground organic carbon degradation (Figure 2.3). Current models suggest there is a threshold level of sea level where the vertical elevation and lateral migration of tidal wetlands may not keep up with the water level resulting in wetland drowning and a sudden decrease the OC burial (Figure 2.4). In addition to driving the vertical response, SLR can also drive the migration of tidal wetlands inland (transgression) and either into open water (progradation) or into interior wetlands (redeposition of eroded carbon) (Figure 2.5). Without adequate sediment supply to maintain a critical depth of tidal flats adjacent to tidal wetlands, erosion of the edge of existing wetlands can result in decreased areal extent (Mariotti and Fagherazzi, 2010; Mariotti and Carr, 2014, 2013). Thus, the future expanse of tidal wetlands will reflect the balance between positive or negative progradation/erosion and upland transgression. Despite the value of these predictive models, knowledge gaps remain that affect the ability to predict future CO2 removal capacity. A key knowledge gap is the fate of eroded OC. When a wetland erodes or is drowned, the fate (i.e., whether it will decompose and contribute to carbon emissions, be deposited and buried long term, or be redeposited on the marsh platform) depends on geomorphological processes controlling erosion, deposition, and resuspension (Hopkinson et al., 2018). Improvements to our understanding of what could happen to eroded carbon will require improvements in our knowledge of OC preservation and its refractory nature, and its transport depositional fate. Additionally, while there is apparent agreement between current models of tidal wetland organic carbon burial and selected field observations, reliance on old paradigms of organic carbon preservation in soils (Lehmann and Kleber, 2015; Schmidt et al., 2011), and reliance on microcosm results as evidence for a hyperbolic response of tidal wetland platform vegetation to flooding do not lend confidence to predictions of marsh survival and OC burial if sea level increases by 1-2 m by century end (Morris, 2016). Regional and local drivers and recent changes, including human impacts, can limit the broad predictive ability of these models. Nevertheless, experimental manipulations coupled with hierarchical approaches to scaling, and better integration of field-validated remote sensing have greatly improved integration of plot-based drivers of PREPUBLICATION COPY

36 Negative Emissions Technologies and Reliable Sequestration: A Research Agenda 400 65 Marsh height (left axis) Sea level (left axis) 300 Ideal C burial (right axis) 60 Baseline C burial (right axis) C burial rate (gC m-2 y-1) Elevation (mm) 200 55 100 50 0 45 -100 40 2000 2050 2100 2150 2200 Year FIGURE 2.4. Hypothetical projection of marsh height (blue), sea level (yellow), ideal carbon burial rates (green), and baseline carbon burial rates without human intervention (red). Impact of relative rate of SLR exceeds the growth in elevation of tidal wetlands until the year 2150 (in this example) when the marsh collapses. Without human intervention, the projected baseline of carbon burial rates decreases from the current 50 gC m-2 y-1 to 0 in year 2150 when the marsh collapses. FIGURE 2.5. Organic carbon balance through upland migration (transgression), shoreline erosion (or progradation), and vertical accretion, which includes OC burial. SLR and sediment availability are two of the most important factors controlling the OC balance (Modified from Kirwan et al., 2016a). OC accumulation rates and landscape-scale estimation of coastal wetland CO2 removal (Byrd et al., 2018; EPA, 2017; Holmquist et al., 2018). The future CO2 removal capacity of tidal wetlands is dependent on their ability to transgress into upland areas as SLR increases. “Coastal squeeze” and vegetation shifts may reduce the lateral space for wetland transgression into uplands. Coastal squeeze occurs when there is no more lateral space for upland migration (DOE, 2017c; Doody, 2004). The decline in available lateral space results from upland barriers, when uplands are occupied by other land uses (e.g., agriculture, urban lands) or when the slope does not support the migration of wetlands into upland areas (Doody, 2004). Predicting the trajectories of available lands is a key knowledge gap, including what factors may reduce barriers to allowing wetland transgression into uplands that are occupied by other land uses. Vegetation shifts associated with wetland PREPUBLICATION COPY

Coastal Blue Carbon 37 transgression and change in carbon uptake capacity, as in shifts to woody species or loss of vegetated wetland from inland subsidence due to marsh dieback, can result in overall changes in carbon burial rates. Warming of the air and sea water will impact the coastal carbon cycling and sequestration driven by complex interaction of plants, microbes, and physical processes (Megonigal et al., 2016). Both primary production and OC decomposition could increase with warming. While theory and mesocosm studies of estuarine water show that warming will differentially increase Re relative to NPP, which would thus decrease NEP and the potential OC available for either burial or export, there are insufficient data to extrapolate these results to tidal wetland and seagrass ecosystems (Yvon-Durocher et al., 2010). Warming could also drive the displacement of salt marsh by mangrove (Megonigal et al., 2016). The final carbon burial rate is determined by the rate and sensitivity of these processes to warming. APPROACHES FOR COASTAL BLUE CARBON The overall goal for a research agenda on coastal blue carbon is to be able to quantitatively evaluate the enhanced organic carbon burial for a variety of management and engineering approaches under a changing suite of social barriers, human activities, and climate scenarios. These approaches build on our current understanding of the baseline of annual carbon burial rates and estimation of incremental carbon burial induced from these projects. There are knowledge gaps that are both research and technological for each of these approaches. The Committee focuses on five approaches by which coastal blue carbon can be accelerated through 2060 and potentially for the remainder of the century. They vary in terms of potential cost, degree of human intervention required, technological readiness, and social barriers (likelihood of enactment). 1. Actively manage coastal wetlands and seagrass meadows to increase CO2 removal against the decreasing baseline of carbon burial rates. 2. Restore coastal wetlands and seagrass meadows where they have been degraded or lost. 3. Convert hardened and eroding shorelines to natural shorelines consisting of wetland area as part of coastal adaptation. 4. Manage wetland transgression into uplands with change in SLR and human drivers/impacts. 5. Increase the carbon storage capacity of coastal wetlands and shorelines by augmentation with carbon-rich materials. Active Ecosystem Management As a result of increasing rates of SLR and human disturbance, the baseline carbon burial rate is in decline over time (Figure 2.4), meaning the current capacity of natural negative emissions in coastal wetlands are shrinking. With active management, this trend can be reversed and carbon burial rate could be equal to the current or increase over time. We currently have reasonable estimates of OC burial in coastal macrophyte systems in the US, albeit there are large uncertainties in current areal extent of seagrasses and medium confidence in their appropriate OC burial rates. Key to maintaining existing areas of natural tidal wetlands and seagrass meadows is management to reduce the impacts of human drivers that result in coastal change. As described earlier, progress has been made in developing models to predict the future areal extent and OC burial rate for extant wetlands of the US. Some of the biggest uncertainties are the interplay between sediment availability and OC accumulation, the effects of climate drivers and SLR on NEP and organic carbon burial, the factors controlling edge erosion and the importance of released sediment on wetland platform survival, and the effects of other human activities, such as pollutant runoff, on NEP and organic carbon burial. A Habitat Evolution Model (HEM) that incorporated future sea level rise scenarios applied to the Tampa Bay PREPUBLICATION COPY

38 Negative Emissions Technologies and Reliable Sequestration: A Research Agenda Estuary indicated that, if managed, coastal habitats would remove about 74 Mt CO2 from the atmosphere by 2100 (ESA, 2016). The HEM can be used to prioritize coastal wetland areas for active management. Given the projected decline in the baseline of carbon burial, any management practices and projects that can reverse the trend is a coastal NET. For example, enhancing coastal nitrogen management by building sewage treatment systems could significantly reduce nitrogen leaching to salt marshes, enhance rooting depth and marsh productivity (Deegan et al., 2012). Sediments are also required to maintain elevation of tidal flats proximal to wetlands to prevent shoreline erosion (Bilkovic et al., 2017; Fagherazzi et al., 2012). Edge erosion may be controlled by either direct (e.g., dredging) addition of sediments or indirect addition (e.g., river diversions or removal of dams on rivers). It can also be controlled directly by living shorelines or breakwaters immediately adjacent to wetland shorelines. The potential problem with preventing shoreline erosion via breakwaters is that it prevents the liberation of sediments internal to the estuarine system, which may be a critical source of sediment contributing to elevation gain of wetlands interior to their edge. Management strategies to conserve carbon at the edge of eroding shorelines or creating breakwaters that protect the CO2 sequestration of inland coastal marshes before barrier islands degrade are strategies to maintain the negative CO2 emissions achieved by these areas of coastal wetlands (Bilkovic et al., 2017). Protecting shorelines from erosion has been taken up by some states as an active management effort with associated policy and regulatory framework (Bridges et al., 2015). Further, protecting inland coastal marshes not only sustain areas for OC burial but also can protect significant stores of peat. Initial saltwater intrusion may influence CH4 emissions (Neubauer et al., 2013) while old peat deposits may be vulnerable to decomposition upon salinization (Wilson et al., 2018). Design of breakwater-type living shoreline projects can slow edge erosion, but introduce uncertainties about whether they contribute to sediment needed for the platform elevation gain, as edge erosion may be important to providing sediment to maintain elevation of the remaining marsh platform. For example, at Plum Island Ecosystems LTER, marsh erosion liberates sufficient sediment to meet almost 30% of the annual rate of sediment accumulation, with rivers only contributing 9% (Hopkinson et al. 2018). Restoration of Lost or Degraded Coastal Wetlands and Wetland Creation Wetlands are drained, excavated and tidally-restricted as a result of human activities that reduce their area or their capacity to sequester CO2 and confer other ecosystem services (Kroeger et al., 2017b). The goal of restoration is to return or improve wetland functions and provision of ecosystem services. OC sequestration accompanies ongoing restoration activities (Kroeger et al., 2017b), but often is not the primary or even secondary objective. However, reversing the effects of anthropogenic activities on coastal wetlands to both reduce the GHG burden on the atmosphere and reinitiate processes that promote CO2 removal has now been taken up as part of national and international policy actions [e.g., recent IPCC GHG Inventory Guidance (IPCC, 2014b) and as a new component of the US EPA National Greenhouse Gas Inventory (EPA, 2017)]. Tidal marsh restoration sites in the Snohomish Estuary have measured annual accumulation rates between 0.9 t C ha-1 yr-1 to 3.52 tC ha-1 yr-1 (3.3 tCO2 to 12.9 tCO2), depending on characteristics of the sites including project age and elevation (Crooks et al., 2014). Restoration sites in the Tampa Bay Estuary are estimated to have accumulated 217,000 t CO2e1 between 2006 and 2016 (ESA, 2016). While restoration of tidal wetlands and seagrass meadows is occurring, the potential for increasing the areas for restoration is significant (EPA, 2017). In the United States, there are approximately 1.3 million ha of tidal wetlands and seagrasses that have been converted to other land uses or otherwise lost and currently potentially available for restoration. Notably, the different land uses in which these former coastal wetlands occur may pose significant barriers to restoring them to coastal 1 For any concentration and type of greenhouse gas (e.g. methane, perfluorocarbons, and nitrous oxide) CO2e signifies the concentration of CO2 which would have the same amount of radiative forcing. PREPUBLICATION COPY

Coastal Blue Carbon 39 wetlands. These land uses within the coastal boundary (upper limit MHHW) are characterized by NOAA’s Coastal Change Analysis Program (C-CAP) as: developed (low to medium-high intensity); cultivated (including pasture/hay and grassland), including some portion that are tidally-restricted (Kroeger et al., 2017b); and open water/unconsolidated shore (and possibly older eroded areas) due to erosion of tidal wetlands or loss of seagrass meadows (Waycott et al., 2009). To restore wetlands within developed and cultivated lands within the coastal zone, restoration of hydrology either through reversing drainage or removing tidal restrictions are likely the most important restoration activities in these coastal settings. Restoring saltwater flow to a tidally restricted wetland allows for both a reduction in methane emissions and a reconnection to the sea level rise processes that promote soil accumulation (Kroeger et al., 2017a). In submerged areas where elevation needs to be increased, a variety of sediment management activities can be applied: (1) direct addition to the marsh surface, e.g., through thin layer deposition, and (2) indirect addition to the estuary and then tidal current conveyance to the wetland surface (e.g., river diversions, and dam removal along rivers). OC burial can start again once intertidal elevations are reached and wetland vegetation established (Osland et al., 2014). For example, reversing coastal wetland loss to subsidence and SLR is the primary focus of the Louisiana Master Plan (CPRA, 2017). Dredged material has proven to be a valuable sediment source for wetland creation—and hence raising sequestration above a baseline. Approximately 194 million cubic yards of sediment are dredged annually from the Nation’s navigable waters (USACE, 2015). Mississippi River diversions are a smaller scale and perhaps be more influential in reversing wetland loss. However, there is significant uncertainty as to whether these approaches applied in all subsided or eroded coastal wetlands will achieve similar results given the interacting effects of other drivers of coastal change. Only a fraction of coastal water has been surveyed for seagrass. Globally, the seagrass meadow was estimated to cover 30-60 million ha (Duarte et al., 2005; Fourqurean et al., 2012; Kennedy et al., 2010; Mcleod et al., 2011). Based on current extent and fraction of reported loss, current area of US seagrass meadows is estimated at 0.6 million ha (Waycott et al., 2009). For seagrass meadows, watershed management to improve water quality and clarity, control sediment loads and resuspension and replanting of degraded seagrass cover are well proven technologies that can increase areal extent, productivity and OC burial. Although not widely applied in the U.S., there has been a strong push in developing carbon markets for wetlands, particularly tidal wetlands. Despite the absence of a market, a methodology for crediting tidal wetland and seagrass restoration with carbon sequestration has been developed (Verified Carbon Standard VM0033). Conversion of Hardened and Eroding Shorelines to Natural Shorelines Conversion of hardened and eroding shorelines to natural and nature-based shorelines that can keep up with SLR is a growing environmental risk reduction strategy (van Wesenbeeck et al., 2014). Such a strategy can also serve as an effective CO2 removal approach when projects enhance wetland area or performance (Bilkovic and Mitchell, 2017; Bridges et al., 2015; Davis et al., 2015; Saleh and Weinstein, 2016). Risks from flooding and storm surge are increasing, impacting coastal inhabitants in the United States and globally. Hauer et al., 2016 report that flooding of upland areas due to future SLR is projected to put between 2.2 and 13.1 million people at risk, depending on SLR and population projections. Given the reported costs of relocation at $1M per resident (Huntington et al., 2012), extensive adaptation for coastal risk reduction is anticipated (Brody et al., 2007). Failure and costly maintenance of infrastructure that extends to coastal areas portend a significant increase in new nature-based, coastal infrastructure as a means to avert risk but also reduce cost of coastal adaptation. Further, since shoreline armoring is prohibited or significantly restricted in several US states (O'Connell, 2010), nature-based, living shoreline approaches are expected to be more frequently used. There is considerable activity around the world in coastal adaptation projects that employ natural and nature-based features (NBBF), which can enhance CO2 removal value of coastal adaptation efforts PREPUBLICATION COPY

40 Negative Emissions Technologies and Reliable Sequestration: A Research Agenda (Bridges et al., 2015). Nature-based features are those that may mimic characteristics of natural features, but are created by human design, engineering, and construction to provide specific services such as coastal risk reduction. The built components of the system include nature-based and other structures that support a range of objectives, including erosion control and storm risk reduction (e.g., seawalls, levees), as well as infrastructure providing economic and social functions (e.g., navigation channels, ports, harbors, residential housing). This is a rapidly growing area of research with case studies of projects demonstrated from around the United States, Europe, Mexico, and China (See Bilkovic et al., 2017; Bridges et al., 2015; Saleh and Weinstein, 2016; and Zanuttigh and Nicholls, 2015 for reviews). In the United States, extensive coastal armoring has already been implemented, with about 14% (22,000 km (Gittman et al., 2015) of the shoreline armored. Converting these armored shorelines to natural shorelines with plants, sediments, and tidal flooding features will have a significant benefit as a NET. However, while research and practical implementation of nature-based approaches to tidal wetland creation has grown to deployment level, the CO2 removal potential has mostly been estimated from assumptions that these projects attain similar OC accumulation rates as natural or restored wetlands. Management of Wetland Transgression into Uplands Wetland transgression allows the area of tidal wetlands to expand as sea levels rise and increase annual CO2 removal capacity, especially if erosion of existing tidal wetlands can also be prevented. Coastal mapping and SLR projections of inundation allow for identification of potential opportunities for transgression. Schuerch et al., 2018 modeled that wetland gains of up to 60% would be possible if transgression is able to occur in just 37% of wetland areas globally, compared to an expected loss of up to 30% due to sea level rise without transgression. However, knowledge is more limited with regard to how current land cover, ownership, and economic value of upland areas will influence the practical potential for transgression. For example, commercially valuable upland is being armored once the flooding risk becomes apparent. Haer et al., 2013 report that inundated land area will increase between 2.6 and 7.6 million ha by 2100, depending on SLR projection and method for extrapolation applied. Assuming 1ft of vertical accretion by 2100, 2, 4, and 6ft of SLR above MHHW correspond with 1.5, 0.87, and 0.93 million ha of net upland (excluding developed land) inundated (NOAA SLR viewer2). Managed shoreline retreat strategies that discourage development and decrease population in areas with increasing flood risks and allowing them to flood with coastal waters (Kousky, 2014), not only reduces coastal risks but increases land area for CO2 removal. Barriers to managed transgression for wetlands into uplands may vary by upland land use type. Approximately 43% of upland land area between MHHWS and MHHWS +2ft is cultivated, with another 20% is pasture/hay and grassland (NOAA Office for Coastal Management, 2018). Change in areas of inundated developed areas were not available. However, because economic costs of inaction increase over time (Hauer et al., 2016; Reed et al., 2016), coastal adaptation policies for coastal risk reduction could foster a predictable trend for not only identifying new areas available for inland migration and capacity for its management, but also additional areas for other NETs. In other words, similar socioeconomic issues may exist for converting other coastal land uses to coastal wetlands (i.e., restoration). Management strategies would be needed to maximize the potential expansion of tidal wetlands into uplands. Management of wetland transgression could also include policy measures that both reduce flood risk (Brody et al., 2007) and increase CO2 removal capacity, however we lack typologies and rigorous methodologies for predicting where transgression will occur and how likely it will be allowed by existing land uses, nor what it would cost to protect some potentially large expanses of low-lying coast for wetland expansion. There is limited knowledge of the OC burial rate trajectory associated with upland land transgression to wetlands as upland soils can begin burying OC at greater rates but productivity of 2 See https://coast.noaa.gov/digitalcoast/tools/slr.html. PREPUBLICATION COPY

Coastal Blue Carbon 41 upland species may decline while wetland species invade, with a period of net lower productivity and lower OC burial rates than “natural” coastal wetlands. Augmentation of Shorelines with Carbon-rich Materials Augmenting coastal projects with carbon-rich materials has the potential to enhance carbon storage by burying externally-produced carbon-rich materials (e.g., burying wood or biochar) and increase rates of OC burial by bioengineering wetland species toward lower rates of decomposition (e.g., manipulating lignin content). These approaches could occur in tandem with restoration, coastal adaptation and shoreline protection, and management of wetland transgression. Direct addition of slow-decomposition forms of organic carbon such as logs and biochar can augment carbon burial. A number of studies have evaluated wood burial as means to increase CO2 sequestration capacity (Freeman et al., 2012) and biochar addition to reduce nitrogen mineralization of peat and coastal wetlands (Luo et al., 2016; Zheng et al., 2018). The scientific literature also supports the use of carbon-rich materials to increase CO2 removal potential of peatlands by ‘injecting’ timber (Freeman et al., 2012). Similarly, decay resistant conditions of tidal wetlands and seagrass meadows could also be harnessed, but further study is needed to evaluate decay rates of different materials and demonstration projects are needed to achieve CO2 removal at scale. Evidence indicates that waterlogged wood of ships that has been buried in sediment is often in a very good state of in-situ preservation in marine environments (Gregory et al., 2012). Macchioni et al., 2016 found that wood foundations of Venice, Italy showed all samples with at least 30% residual bulk density, with the earliest known construction in 1854. The state of wood preservation related to several factors, including: thickness of the element, depth of burial, horizontal/vertical position, and wood species (Macchioni et al., 2016). A 3-year wood degradation study showed that wood buried below 43cm was highly protected from decay, with erosion bacteria promoting the loss of surficial wood (<0.5mm; Bjordal and Nilsson, 2008). Wood burial is but one example nature-based example. Constructing revetments or breakwaters with concrete composed of carbon-rich water, aggregate or with embedded wood is another example of a more engineered approach. In addition to burying new sources of atmospheric carbon, afforestation management strategies for planting mangroves in marsh areas have also been considered. SLR and warming are leading to increased expansion of mangroves in some areas indicating that these or other tree species could be introduced in other areas of transgressing wetlands. Accounting methods based on new remote sensing and field validation research would be needed to avoid double-counting afforestation gains in wetlands with terrestrial areas, identified as an area of improvement for future U.S. EPA GHG inventory compilation (EPA, 2017). Genetically engineering wetland macrophytes to increase their lignin content is another option to enhance wetland OC burial. There has been considerable interest in lignin degradation and altering lignin content of vegetation to improve the efficiency of cellulosic biofuels production (Wei et al., 2001). While the interest for biofuels is in decreasing lignin content to increase accessibility of plant polysaccharides to microbial and enzymatic digestion (e.g., Ragauskas et al., 2006), the interest for coastal blue carbon would be in increasing lignin content of roots and rhizomes and reducing OC decomposition. Biofuels research has identified genes encoding the enzymes leading to the building blocks of lignin (Hoffmann et al., 2003) and there has been success in downregulating some of these genes and hence lignin biosynthesis (Chen and Dixon, 2007; O'Connell et al., 2002; Reddy et al., 2005). The predominantly anoxic conditions in coastal peat sediments allow an accumulation of phenolic compounds from lignin, inhibiting decay by suppressing phenol oxidase enzyme activity and microbial enzymatic decomposition of senescent vegetation (Appel, 1993; McLatchey and Reddy, 1998). Promoting production of phenols and decay inhibitors by genetic modification of plants like Sphagnum that produce phenols has been considered for freshwater environments (Freeman et al., 2012). Physiochemical enhancement to suppress phenol oxidase activity by manipulating oxygen availability, acidification by addition of sulfates, addition of phenolic compounds (peat leachates and polyphenolic waste materials), lowering pH, labile carbon and inorganic nutrient supply has also been proposed (Freeman et al., 2012). PREPUBLICATION COPY

42 Negative Emissions Technologies and Reliable Sequestration: A Research Agenda This is a new area of consideration and therefore at a research development and demonstration phase. These types of projects will still require increased technological capacity to assess the permanence of materials used and could employ adaptive management-based approaches to “learn while doing” and as well as designed experiments, to understand the conditions where they are best applied, and accelerate progress to deployment at scale. With this approach, ecological concerns and off-site negative impacts associated with hard coastal defenses will persist unless the design is such that wetland functions are enhanced and shoreline processes improved, as shoreline armoring is prohibited or significantly restricted in several US states (O'Connell, 2010). Other opportunities include using species or phenotypes with higher lignin content or afforestation (in. mangrove forests) with fewer technological barriers but other potential ecological implications. IMPACT POTENTIAL Total Carbon Fluxes under Coastal Blue Carbon Coastal blue carbon is an approach with near-term readiness and low cost when coastal ecosystems are maintained, restored, created, or engineered with minimal hard infrastructure and created for other purposes (e.g., coastal risk reduction, fisheries production). Through coastal restoration, adaptation, and management, there is the potential to maintain and accelerate the rate of negative CO2 emissions at a scale of 0.02-0.08 Gt CO2 yr-1 (Table 2.1). The committee expects the timeframe for readiness for each approach to vary. By 2030, the committee estimates that implementation of several of the management approaches described above can result in annual flux of 0.037 Gt CO2 yr-1. By 2060, annual flux could reach 0.077 Gt CO2 yr-1 depending on technological developments, improved scientific understanding, and overcoming potential societal barriers. Technological advancements that enable successful demonstration and deployment of carbon-rich projects supply about 36% of annual flux by 2030 and 43% by 2060. Wetland transgression becomes a more important flux over time, increasing to 32% by 2100. The source of these estimates is described in this section. The total carbon flux per year, and potential impact of coastal blue carbon is most influenced by the total area of coastal carbon ecosystems, the rate at which they bury OC, and the potential for augmenting projects and strategies for management of wetland transgression. The key coastal blue carbon approaches the committee considered are as described above: (1) Active ecosystem Management; (2) Restore coastal wetlands where they have been degraded or lost; (3) Convert hardened and eroding shorelines to natural shorelines as part of coastal adaptation; (4) Manage wetland transgression into uplands with change in SLR and human drivers/impacts; and (5) Increase CO2 removal capacity by augmentation with carbon-rich materials. The total carbon flux potential is based on the maximum area available for implementation of each approach, and the sequestration rates of 7.98 t CO2 ha-1 yr-1 for tidal wetlands and 1.58 t CO2 ha-1 yr-1 for seagrass meadows (EPA 2017; SOCCR 2). Active Ecosystem Management According to NOAA C-CAP, current estuarine wetland area is 0.22 million ha. Estimating areas of seagrass meadow is more challenging because of knowledge gaps in seagrass meadow distribution, extent, and species identity. Current estimates of seagrass meadow area in the United States is 0.24 million ha (SOCCR). The total carbon flux of coastal blue carbon can be estimated from the combined area and rate of OC burial for existing tidal wetlands and seagrass beds. For existing “natural” tidal wetlands and seagrasses, the total carbon flux is currently approximately 0.021 Gt CO2 yr-1 (Table 2.1). In order to maintain these annual rates, the rate of sequestration per unit area and total area of coastal wetlands would require different levels of management, and in many cases, proactive management. PREPUBLICATION COPY

Coastal Blue Carbon 43 Because of high uncertainty in projecting the future change to the baseline sequestration, here we did not deduct the baseline from calculating the impact potential for this NET. Instead, we use the current coastal carbon burial rate as the CO2 removal capacity of active ecosystem management to maintain this number. Restoration of Lost or Degraded Coastal Wetlands and Wetland Creation The total carbon flux from restoration of coastal wetlands can be estimated using the rate of OC burial applied to the areas within the coastal wetland boundary (under MHHW line) currently in other land uses or where wetland condition has been degraded. There are multiple approaches by which coastal wetlands can be restored depending on their type, degree of degradation, and geomorphic setting. Each of these approaches have been shown to achieve CO2 removal at similar or higher rates than existing “natural” tidal wetlands (e.g., Osland et al., 2012). The total potential annual flux was estimated from the sum of the annual sequestration for 6 types of potentially-restorable land use types: 1) medium to high intensity developed lands (53,938 ha), 2) low intensity developed lands, open space and barren land (139,171 ha), 3) cultivated, pasture/hay and grasslands, including tidally-restricted lands (317,468 ha), 4) unconsolidated shore (341,721 ha), 5) and recently lost or eroded and converted to open water (125,525 ha; EPA, 2017; NOAA Office for Coastal Management, 2011). Additionally, the area of seagrass meadow estimated to have been lost in the United States provides an opportunity for further restoration (342,943 ha; Waycott et al., 2009). To avoid double-counting of land areas, the committee assumed that: developed and cultivated lands approximated the MSL-MHHW extent of the tidal frame, cultivated lands were at approximately MSL, unconsolidated shore occupied the tidal frame from below MSL to 50ft below MLW and recent open water from 50ft below MLW to 150ft below MLW. The Committee estimated an annual flux of 0.008 Gt CO2 yr-1 if all former coastal wetlands were restored and potentially suitable areas used for wetland creation. Because not all former coastal areas could be restored immediately, the Committee estimated an annual rate of 0.004 Gt CO2 yr-1 by 2030 (with 50% of the available land area restored) and 0.008 Gt CO2 yr-1 by 2060 with all potential area restored. The Committee held this rate constant into 2100, however this rate depends on maintaining this area of coastal wetlands. TABLE 2.1. Total U.S. annual carbon flux for tidal wetlands and seagrass meadows for key coastal blue carbon approaches evaluated as part of this report. 2018 2030 2060 2100e Flux Flux Flux Flux (Gt CO2 yr- (Gt CO2 yr- (Gt CO2 yr- (Gt CO2 yr- 1) 1) 1) 1) Active Ecosystem Management* 0.021 0.021 0.021 0.021 Restoreda 0.002 0.008 0.008 N-B adaptationb 0.001 0.002 0.002 Managed wetland transgressiond 0-2ft 0.012 0.012 2-4ft 0.007 Carbon-rich projectsc 0.013 0.034 0.008 Total 0.021 0.037 0.077 0.058 a 25% of potential area restored by 2030, full area restored by 2060; b N-B=nature-based, 25% of potential area adapted by 2030, full potential area adapted by 2060; c Augmentation of projects with carbon-rich materials implemented at 25% potential area for restoration and adaptation projects by 2030, full potential area by 2060 (annual rate based on area of projects implemented by year indicated); d Projected 0-2ft SLR by 2060 and 2-4ft SLR by 2100, land area estimated includes assumption of 1ft of accretion through 2100; e No additional areas for any CO2 removal approach EXCEPT managed wetland transgression were included in the 2100 scenario. *Active ecosystem management is unlike other approaches in that maintaining rather than increasing wetland area constitutes negative emissions. However, proactive management is needed to maintain their rates of flux and area. Due to high uncertainty in estimating the decreasing baseline of the current carbon burial rate, we have not deducted the baseline from calculating this CO2 removal number. PREPUBLICATION COPY

44 Negative Emissions Technologies and Reliable Sequestration: A Research Agenda The ability to achieve the CO2 removal potential of these lands will depend on site conditions, elevation, and degree of disturbance. For instance, utilizing lands in low intensity or non-human areas may pose the lowest risks to increasing available land for coastal blue carbon because those lands are not in high demand or with otherwise high productive use for humans. Kroeger et al., 2017b report that on the Atlantic Coast of the US, 27% of former tidal wetlands are currently tidally-restricted. Much of this area is likely in agricultural use. Converting from agricultural uses to a coastal wetland may pose greater societal and economic consequences than converting from lands that are of relatively lower intensity of human use (see chapter on terrestrial carbon for more detail on implications of significant shifts in land use). For unconsolidated shore area, while some of these areas will be areas of eroded or subsided tidal wetlands, they also may include valuable near-shore habitat that may not be suitable for tidal wetland restoration or creation. Some areas may be considered more appropriate for seagrass restoration. Other considerations include changes in coastal policy; should existing National Flood Insurance Program (NFIP) change to a risk-based model, existing areas of developed land with high frequency of repetitive losses due to flooding or storm surge damage may become more readily available for other uses that can sequester OC. These considerations introduce significant knowledge gaps due to potential variability in the annual carbon flux of restored and created coastal wetlands. Conversion of Hardened to Natural Shorelines and Stabilization of Eroding Shorelines The Committee used the length of armored shoreline for the U.S. (22,000km) and a width of 61m (used as an approximate tidal range from MSL to 50ft below MLW) to compute the potential area that could be converted to living shorelines and bury OC. To avoid double-counting with potential restoration areas currently in cultivation and development, the Committee used the approximated area occupied by the tidal range below MSL. Replacing the existing length of hardened shorelines (22,000km) with living shorelines, we estimated an annual rate of 0.001 Gt CO2 yr-1 by 2030. Employing these natural and nature- based measures to prevent erosion of another 22,000km of existing shoreline would result in an annual rate of 0.002 Gt CO2 yr-1 by 2060. Management of Wetland Transgression into Uplands The committee approximated the following SLR scenarios: 0.68m by 2060, 1.12m by 2100, and 1.68m by 2130 (NOAA Office for Coastal Management, 2011). If the existing tidal wetland areas are maintained in their existing state (keeping pace with SLR), and areas of tidal wetland increase as uplands become inundated by regular tidal flooding (either through assisted management or other means), total tidal wetland area will increase. With 0.68m of SLR, approximately 1.5 million ha of new tidal estuarine wetlands are estimated to develop (NOAA Office for Coastal Management, 2018). At an annual rate of 7.98 t CO2 ha-1 yr-1 for tidal marsh, this equates to 0.012 Gt CO2 yr-1. Under 1.12m of SLR, an additional area of 0.87 million ha is projected (0.007 Gt CO2 yr-1), for a total annual flux increase to 0.019 Gt CO2 yr-1. However, significant uncertainties persist with regard to the fate of tidal wetlands under SLR and future coastal management (Kirwan and Megonigal, 2013). As the leading edge of upland migrating inland becomes marsh, the coastal edge may be submerged and/or eroded, depending on coastal management. The carbon lost from submerged and/or eroding front may result in a permanent loss of carbon, be emitted to the atmosphere, or be redeposited on the marsh or mangrove shoreline or bank. There has already been observed significant erosion of coastal wetlands in areas, and projected SLR suggests new areas will be eroded without management interventions (Figure 2.4). Using an accretion rate of 1ft by 2100, “open water” area is projected to increase on the order of 1.5-2 million acres with each foot of SLR (NOAA Office for Coastal Management, 2018). PREPUBLICATION COPY

Coastal Blue Carbon 45 Augmentation of NET projects with Carbon-rich Materials An area of approximately 467,246 ha (the sum of unconsolidated shore and recently eroded wetland areas) is available for restoration that could possible incorporate carbon-rich materials as a way to increase the area to an elevation appropriate for persistence of wetland vegetation. If carbon-rich materials were supplied at the same rate that restoration and adaptation projects were implemented, and included as part of managed wetland transgression strategies (50% by 2030 and full area by 2060), the annual flux would reach 0.007 Gt CO2 yr-1 and 0.027 Gt CO2 yr-1 by 2030 and 2060, respectively (based on augmenting with carbon-rich materials to 1.5 ft for restoration projects in unconsolidated shore areas and 3 ft in recently eroded wetlands, 3 ft for coastal adaptation projects, and equivalent of 1ft stocks in strategies for managed wetland transgression). Addition of carbon-rich materials is merely an opportunity for carbon storage and does not result in an increase in annual carbon removal. Thus, annual rates were obtained by dividing the total storage capacity of this level of project implementation by the number of years the projects will occur (up to 2100 in this scenario). Summary of Coastal Blue Carbon Estimates Combining the annual fluxes for each approach based on the potential rate at which they could be implemented, we estimated an annual carbon flux rate for the four time horizons: current (2018), 2030, 2060 and 2100. Total potential annual flux was estimated as 0.021, 0.037, 0.077, and 0.058 Gt CO2 yr-1, respectively (Table 2.2; Figure 2.6). Total U.S. potential carbon removal capacity is the magnitude of OC burial rates, should all potential activities identified by the committee be implemented and rates of OC sequestration maintained over the specified time horizons. While global areas of coastal tidal wetland and seagrass vegetation have been estimated and annual rates applied to derive global carbon capacity of existing areas, little is known about the global capacity if restoration, creation, and nature-based engineering approaches considered were applied at a global scale. FIGURE 2.6. Annual CO2 flux (Gt yr-1) for coastal wetland CO2 removal approaches: a) natural, restoration and nature-based coastal adaptation, b) restoration and nature-based coastal adaptation augmented with carbon-rich materials, c) managed wetland transgression for 0-2ft and 2-4ft of SLR, and d) Total cumulative annual flux of all coastal blue carbon approaches combined. PREPUBLICATION COPY

46 Negative Emissions Technologies and Reliable Sequestration: A Research Agenda Existing coastal ecosystem management areas yield an estimated potential carbon capacity of 0.233, 0.868 and 1.714 Gt CO2 by 2030, 2060 and 2100, respectively. Potential tidal wetland and seagrass areas that could be restored yield another 0.023, 0.265, and 0.591 Gt CO2 by 2030, 2060 and 2100, respectively. These estimates assume that areas that could be potentially restored within the coastal boundary (below MHHW) are implemented at 25% total area by 2030 and 100% total area by 2060. The estimate does not include potential new projects beyond 2060, which would increase the total 2100 capacity. Nature-based adaptation projects, conversion to or creation of living shorelines, yield additional 0.006, 0.068, and 0.152 Gt CO2 by 2030, 2060, and 2100, respectively. Similarly, the estimate does not include potential new projects beyond 2060 which would increase the total 2100 capacity. Managed wetland transgression adds another 0.496 Gt CO2 by 2060 (with 0.68m SLR) and 0.980 Gt CO2 by 2100 (with 1.12m of SLR). Finally, augmenting restoration and creation, nature-based adaptation (engineering) approaches and wetland transgression areas with carbon-rich materials, implemented at the same rate as described, yields another 0.148, 1.123, 1.426 Gt CO2 by 2030, 2060 and 2100, respectively. Thus, the total potential capacity for coastal carbon is 0.410, 2.820, 5.424 Gt CO2 by 2030, 2060 and 2100, respectively. Other Radiative Impacts Methane Restoration of tidal wetlands from drained upland and impounded fresh wetlands has an extra benefit to negative carbon emissions, which is the reduction of methane (CH4) emissions from ditches these areas. Generally, tidal restrictions such as roads or undersized culverts increase greenhouse gas emissions because they either drain the wetland, causing emissions of carbon dioxide, or they cut off the input of the saline water needed to inhibit methane emissions. The supply of tidal salty water is often associated TABLE 2.2. Total U.S. (cumulative) potential carbon capacity for tidal wetlands and seagrass meadows 2018 2030 2060 2100e Capacity Capacity Capacity Capacity (Gt CO2) (Gt CO2) (Gt CO2) (Gt CO2) Active Ecosystem 0.021 0.233 0.868 1.714 Management Restoreda 0.023 0.265 0.591 b N-B adaptation 0.006 0.068 0.152 Managed wetland 0-2ft 0.496 0.980 transgressiond 2-4ft 0.561 Carbon-rich projectsc 0.148 1.123 1.426 Total 0.021 0.410 2.820 5.424 a 25% of potential area restored by 2030, full area restored by 2060; bN-B=nature-based, 25% of potential area adapted by 2030, full potential area adapted by 2060; cAugmentation of projects with carbon-rich materials implemented at 25% potential area for restoration and adaptation projects by 2030, full potential area by 2060 - values are cumulative; dProjected 0-2ft SLR by 2060 and 2-4ft SLR by 2100, land area estimated includes assumption of 1ft of accretion through 2100; eNo additional areas for any coastal blue carbon approach EXCEPT managed wetland transgression were included in the 2100 scenario. *Active ecosystem management is a critical CO2 removal approach unlike other approaches in that maintaining rather than increasing wetland area constitutes negative emissions. However, proactive management is needed to maintain their rates of flux and area. PREPUBLICATION COPY

Coastal Blue Carbon 47 associated with sulfate that will inhibit methanogenesis as sulfate reduction overpasses carbon reduction as electron donors (Poffenbarger et al., 2011). Therefore, salinity has been frequently used as a proxy for estimating methane emissions. Restoration of tidal connections to currently impounded wetlands could decrease CH4 emissions of 7.9-41.1 (g CH4 m−2 y−1) (Kroeger et al., 2017b), or 739-3843 g CO2 m-2y-1 when converted to CO2 equivalent (CO2e) by a factor of 34 (CH4 global warming potential, GWP, IPCC, 2013). In the U.S. Atlantic coast, if 2,650 km2 wetlands could be restored, the CH4 benefit would be 2.0- 10.2 Mt CO2 y-1. Nitrous Oxide Nitrous oxide (N2O) is another potent greenhouse gas with a GWP of 298 (IPCC, 2013). N2O is produced largely by the denitrification process in wetlands and saturated uplands. The salinity and sulfate in tidal wetlands suppress the production of N2O. Therefore, the N2O emissions in tidal wetlands are very small compared with CO2 and CH4 fluxes, about an order of magnitude less than CH4 fluxes (Martin et al., 2018; Murray et al., 2015). Albedo Land use and land-cover changes have the potential to change the albedo, or reflectivity, of the Earth’s surface, and thereby change the radiation balance. There is little difference in albedo for various wetland plants. The albedo of flooded salt marsh is ~0.089 (Moffett et al., 2010), slightly higher than ocean water. Considering the small total area of coastal wetlands not only for the United States, but for the entire world, coastal carbon approaches would have a trivial effect on the overall global radiation balance, considering albedo alone. SECONDARY IMPACTS Ecosystem Services Tidal wetlands and seagrass meadows provide ecosystem services (Barbier et al., 2011), which are broadly defined as “the benefits people obtain from ecosystems” (MEA, 2005b). Examples of ecosystem services that coastal ecosystems provide include important recreational and tourism opportunities, key fishery habitats, water quality improvements, and flood and erosion mitigation. Each have monetary and non-monetary value, which can reduce risks to life, property, and economies (Barbier et al., 2011; Duarte, 2000; Lovelock et al., 2017; Mcleod et al., 2011). As described above, these coastal ecosystems are maintained, restored, used for nature-based adaptation, and managed to enable wetland transgression into uplands for many of these ecosystem services despite the additional benefit of coastal CO2 removal. While ecosystem services in coastal ecosystems are well-documented, there is less data available on their monetary and non-monetary values. Where monetary values are available, it has proven difficult to bring these services into the marketplace, with the exception of carbon in mangroves (Jerath et al., 2016). Tidal marshes ecosystem service values have also been extensively studied but less is known about the value seagrass meadows provide (Table 2.3; For a review, see Barbier et al., 2011; seagrass (Orth et al., 2006; Waycott et al., 2009), tidal marsh (Craft et al., 2009; Gedan et al., 2009). Even before consideration of their market value for carbon removal, watershed restoration provides a 3:1 (CPRA) to 8:1 (Sklar, presentation to committee) return on investment. The National Science and Technology PREPUBLICATION COPY

48 Negative Emissions Technologies and Reliable Sequestration: A Research Agenda TABLE 2.3. Ecosystem service value examples for coastal blue carbon (modified from Barbier et al. 2011). Units - $/ha/yr Ecosystem service Ecosystem process or function Ecosystem service value example Mangrove Seagrass Coastal Meadow Marsh Raw materials and Generates biological productivity and $484-595/ha/yr N/A 15.27/ha/yr food provisioning diversity (2007 USD) (1995 GBPa) Natural hazard Attenuates and/or dissipates waves $8966-10821/ha N/A $8236/ha/yr regulation (2007 USD) (2008 USD) Regulation of Provides sediment stabilization and $3679/ha/yr N/A N/A erosion soil retention in vegetation root (2001 USD) structure Regulation of Provides nutrient and pollution N/A N/A $785- pollution and uptake, as well as retention, particle 15000/acre detoxification deposition (1995 USD) Maintenance of Provides sustainable reproductive $708-987/ha $18.50/ha $981- fisheries habitat and nursery grounds, sheltered (2007 USD) (2006 6471/acre living space AUDb) (1997 USD) Organic matter Generates biogeochemical activity, $30.5/ha/yr N/A $30.5/ha/yr accumulation sedimentation, biological productivity (2011 USD) (2011 USD) Recreation & Provides unique and aesthetic N/A N/A 32.80/person Aesthetics submerged vegetated landscape, (2007 GBP) suitable habitat for diverse flora and fauna a GBP: Great Britain Pounds; bAUD: Australian Dollars Council released a report on research needs for coastal green infrastructure in order to improve assessments of ecosystem services (NSTC, 2015). Risks of Coastal Blue Carbon There are potential risks associated with some of the coastal blue carbon approaches considered that will drive where they are deployed and how (e.g., large fill volumes, subtidal areas, coastal landscape processes). These include the potential for sediment contaminants, toxicity, and bioaccumulation and biomagnification in organisms, issues related to altering degradability of coastal plants, potential risks of using subtidal areas for tidal wetland carbon removal, the effect of shoreline modifications on sediment redeposition and natural marsh accretion, and safeguarding against abusive use of coastal blue carbon as means to reclaim land for purposes that degrade capacity for carbon removal. To address potential contaminants of dredge material, the USACE already uses Ocean and Inland Testing Manuals (OTM and ITM) and tiered approaches to evaluating toxicity issues associated with dredged materials in accordance with the Food and Drug Administration Action Levels for Poisonous and Deleterious Substances in Fish and Shellfish for Human Food and the Water Resources Development Act of 1999. The USACE follows Section 103(b) of the Marine Protection, Research and Sanctuaries Act in choosing placement sites designated by the EPA to reduce human and environmental impacts to the maximum extent possible (EPA, 1991). The alteration of wetland plant community composition and/or its degradability also raises significant ecological questions. Most coastal fisheries of the U.S. east and Gulf of Mexico coasts are estuarine-dependent relying on tidal wetlands as nursery grounds for juvenile development. At some PREPUBLICATION COPY

Coastal Blue Carbon 49 stage, most of these organisms are dependent on wetland vegetation-derived detritus. Changing the lignin content of the detrital feedstock could negatively impact secondary production of the entire coastal zone. The social and economic impacts of introducing genetically-modified plants into the coastal zone would need to be evaluated before or at least simultaneously with any research agenda on lignin modification to investigate at what point, if any, does marsh survival based on increased lignin content balance the alteration of coastal fisheries and dependent human livelihood. Societal Impacts There may also be significant societal barriers to allowing flood-prone lands to be converted to wetlands despite increasing coastal risks. This introduces coastal management considerations of how to plan and design the coast to enable human development to persist but also enhance carbon removal capacity by increasing areas of coastal wetlands (Stark et al. 2016). However, due to increasing risks of flooding and storm damage in the coastal zone, regulatory frameworks may change to reduce the financial burden on the federal government, which may disincentivize coastal development. Coastal hazards significantly increase risks to people and infrastructure. For example, the cumulative cost of the 16 separate billion-dollar weather events in the United States in 2017 was $306.2 billion (Insurance Information Institute, 2018). Superstorm Sandy, Hurricane Katrina and the events of 2017 have put FEMA at over $24 billion in debt (CBO, 2017) with additional borrowing expected. As restoration and nature-based adaptation projects become a strategy better enabled by the regulatory agencies, safeguarding against potential negative consequences of land reclamation may become a socioeconomic barrier (Chee et al., 2017). The difference between wetland creation for coastal adaptation (wetland reclamation) and land reclamation is distinct but may be blurred with uninformed policy or unenforced policies regulating or guiding best practices. To ensure that coastal restoration and nature-based adaptation projects build wetlands for effective carbon removal rather than land development in the coastal zone can be built into the regulatory framework. Other Barriers There are existing permitting mechanisms to deploy NETs addressed in the report. However, permitting process will need to be improved (with potential basis for revisions to existing NW54). For example, USACE permitting process averages over 300 days (USACE, 2017). In 2016, a NW54 was approved to expedite living shoreline or coastal bioengineering projects. Minimum requirements limit projects eligible for this expedited permit review to 30ft depth from shoreline and 500ft length (USACE, 2017). Discovering ways to accelerate regulatory approval of projects that also safeguards robust and effective carbon removal is a key need. ESTIMATED COSTS OF IMPLEMENTING COASTAL BLUE CARBON Costs for implementing different types of coastal blue carbon approaches described vary widely, and depend mostly on project size, type of intervention, design and construction costs, materials costs and costs associated with transport of materials and equipment and monitoring carbon removal. However, if such projects occur regardless of carbon removal potential, due to the multiple ecosystem services and coastal adaptation functions they confer, then only the incremental costs for monitoring carbon removal needs to considered. PREPUBLICATION COPY

50 Negative Emissions Technologies and Reliable Sequestration: A Research Agenda Incremental Costs for Monitoring Coastal Blue Carbon If the projects are implemented for purposes other than or in addition to carbon removal , costs are reduced to the incremental cost of monitoring coastal blue carbon. Such monitoring costs approximate $0.75/ton CO2 for tidal wetlands and $4/ton CO2 for seagrass meadows) for all coastal blue carbon approaches except where projects are augmented with carbon-rich materials (estimated at $1-30/ton CO2) depending on material and construction method used. To estimate these monitoring costs, the Committee considered costs associated with existing coastal and terrestrial monitoring networks that include coupled remote sensing and plot-based measurements. For example, the California Forest Change Detection Program costs are $0.004 ha-1 achieved through agency efficiencies and leveraging staff across programs (Fisher et al., 2007a). Monitoring needs for coastal blue carbon would not be limited to land change detection, and this would represent the minimum cost. A relevant example of a comprehensive monitoring program is the Coastwide Reference Monitoring System (CRMS), a mechanism to monitor and evaluate the effectiveness of projects conducted under the Coastal Wetlands Planning, Protection, and Restoration Act (CWPPRA) in Louisiana at the project, region, and coastwide levels (Steyer et al., 2003). The network provides multiple forms of data and research for a variety of user groups, including resource managers, academics, landowners, and researchers. The estimated cost of this research and monitoring program is $80 ha-1 or $6 ha-1 yr-1 based on projects funded thus far.3 Monitoring system costs reported for national level programs were reported as $0.50 to $5.50 ha-1, and presumed annual costs (Böttcher et al., 2009). Recognizing that the CRMS program includes research, the incremental cost of monitoring may be lower. RESEARCH AGENDA The Committee’s recommended research agenda has the overarching goal to preserve and enhance the high rates of organic carbon sequestration in existing tidal wetlands and seagrasses in the coastal zone and to expand the area covered by these ecosystems. As discussed earlier, rates of carbon sequestration can be enhanced through a combination of management activities that depend on: • increasing the OC density in soils of coastal systems, • retarding edge erosion of existing wetlands, • increasing aerial expanse of wetlands through transgression into upland areas as these areas become flooded by the sea, • augmenting mineral sediment availability to insure wetland elevation remains in balance with increasing rates of SLR, • hybrid, “engineering” (restoration, creation, coastal adaptation) approaches that enhance carbon removal and maintain or improve coastal ecosystem services and • augmenting soils with high concentrations of slowly degrading OC such as biochar or logs. The research agenda also examines technological needs and feasibility of carbon removal as part of coastal protection projects designed to minimize exposure of human systems to the risk of storms and floods. The research is recommended through the lens of barriers to reaching 1 GT CO2 scale deployment in the U.S. and focused on reducing the largest uncertainties associated with processes most affecting annual carbon removal and capacity. 3 Based on projects listed on the CWPPRA website for the time period of 2005-2019. See https://www.lacoast.gov/new/Projects/List.aspx. PREPUBLICATION COPY

Coastal Blue Carbon 51 Activities requiring new research include basic science research on carbon removal potential, carbon transformation, and permanence under different approaches as well as socioeconomic and policy research associated with converting existing land to lands for carbon removal. A proposed framework to accelerate deployment of approaches that increase wetland area through restoration, creation, nature- based adaptation, and managed wetland transgression at scale is a combination of designed experimental research, demonstration sites, and adaptive management of engineered projects. Designed research enables rigorous tests of approaches, demonstration sites offer tests of augmenting projects with carbon- rich materials, while adaptive management enables modification of technology and engineering or management parameters when projects do not meet expected performance criteria. Some proposed approaches will have implications for species diversity and productivity in the coastal zone to be measured. Lastly, social science research is recommended to further investigate both the implications of these projects and social barriers to achieving them at scale. These components of the research agenda are described in detail below and costs for each component are summarized in Table 2.4. TABLE 2.4. Costs and components of a coastal blue carbon research agenda Recommended Research Estimated Time Justification Research Frame (yr) Budget Basic research in understanding and 6M 5-10 5 projects at $2M/y for 10 using coastal ecosystems as a NET years to address fate of organic carbon produced and buried in soils/sediments of coastal ecosystems; 5 projects at $2M/y for 10 years to address change in area coastal Basic Research blue carbon ecosystems in response to change in major climate change or sea level rise and management drivers, 5 projects at $2M/y for 5 years to address selection of materials and coastal plants/phenotypes producing high organic carbon density materials with slow decay rates buried in coastal sediments carbon. Mapping current and future (i.e. after 2M 20 Former NASA CMS projects sea level rise) coastal wetlands. (wetland: $1.5M/yr; seagrass Development $500K/yr) National Coastal Wetland Data 2M 20 Scale of NSF Sustainability Center, including data on all Research Networks restoration and carbon removal projects. PREPUBLICATION COPY

52 Negative Emissions Technologies and Reliable Sequestration: A Research Agenda TABLE 2.4 continued Recommended Research Estimated Time Justification Research Frame (yr) Budget Carbon-rich NET demonstration 10M 20 Carbon-rich NET projects & field experiment network demonstration projects & field experiment network (15 sites funded at $670K/site/yr) Demonstration/Deployment Integrated network of coastal sites for 40M 20 15 engineered sites at a cost scientific and experimental work on of $1M/yr per site carbon removal and storage. (approximate funding for an LTER); 20 augmented managed & engineered sites at a cost of $500K/yr; 8 new managed sites at $500K/yr (wetland transgression – 0-2ft and seagrass); 5 U.S. scale synthesis activities (wetland: 3; seagrass: 2) at a cost of $200k/yr per activity Coastal blue carbon project 5M 10 Policies, incentives & deployment (social science, economic barriers will change as & policy research on incentives and coastal risk increases Deployment barriers) Basic Research Research is needed that will address some of the key uncertainties in understanding and using coastal ecosystems as a NET, including but not limiting to: 1) the fate of OC fixed in coastal ecosystems; 2) the changes in areal extent of coastal ecosystems in response to climate change, sea-level rise and human disturbance, and 3) genetic engineering or selection of high OC density materials and coastal plants that decay slowly in coastal sediments. The committee envisions a comprehensive research program at a similar scale to NOAA’s national Sea Grant program budget. The fate of OC produced and buried in soils/sediment of coastal ecosystems. Basic research is required to reduce uncertainties in how changes in sea level, climate, and human activities will impact the primary production, ecosystem respiration, and long-term burial of OC in coastal PREPUBLICATION COPY

Coastal Blue Carbon 53 wetland ecosystems. Our current understanding and ability to predict carbon burial rates in existing coastal wetland ecosystems is limited, and predicting how rates will change in the future is an important challenge. Research should be designed to take advantage of existing strong gradients in OC burial rates, tidal amplitude, biogeographic province/wetland species composition, climate, sediment availability, direct local human activities (e.g., N-enrichment) in order to develop models that apply universally. A combination of field, lab experiments, and modeling activities would be appropriate in conducting this research. Potential funding agencies: NSF DEB, Chemistry & Materials Science, USACE, DOE, Industry R&D, A&E firms, foundations. Research budget: $2M per year over 10 years. The change in areal extent of coastal ecosystems through the remainder of the 21st century in response to changes in major controlling drivers, such as climate change, sediment availability, sea- level rise, and human disturbances. Coastal wetland ecosystems are being heavily impacted and their areal extent is changing rapidly as a result of rapidly changing rates of SLR, sediment availability and other factors. The fate of existing systems and whether they will decrease in areal expanse because of edge erosion or drowning or increase in expanse in conjunction with transgression into upland areas as they flood with rising seas is poorly understood. Predicting areal expanse in the future is of paramount importance due to the effect on the future carbon removal trend. Research should be conducted to develop mechanistic and predictive understanding of these dynamics in the future. A research program should be developed to improve understanding and prediction of the extent of the coastal wetlands under multiple stresses. Potential funding agencies: NSF DEB, ocean science, DOE, NASA. Research budget: $2M per year over 10 years. Selection of plants / phenotypes able to produce high OC-density tissues as well as other OC materials that resist decay in coastal ecosystem sediment carbon and slow-decay species/phenotypes used in enhancing coastal carbon. There is little knowledge about the preservation of OC-rich materials in coastal sediments and the capacity to increase their production by new strains / phenotypes of coastal plants. Research is needed to better understand the decomposition and preservation capacity of OC-rich materials such as wood logs and biochar. Research should be conducted to investigate the feasibility and ecological costs and benefits of introducing new plants / phenotypes / genotypes able to produce greater amounts of less degradable tissues. Genetic research has the potential to increase lignin content of existing wetland plants. A program to improve technological readiness of using carbon-rich materials, in some cases with new lab-scale experiments, including materials science is needed. Multi-omics research should also be supported in coastal wetlands in order to better understand how lignin decomposition is related and controlled by microorganisms (Billings et al., 2015) and environmental variation. Potential funding agencies: NSF Engineering & Infrastructure programs, DEB, Chemistry & Materials Science, USACE, DOE, Industry R&D, A&E firms, foundations. Research budget: $2M per year over 5 years. Development Mapping Transitions in Wetland and Seagrass Land Cover and Land Use Due to SLR and other drivers More work is needed to develop and refine remote sensing approaches to estimate potential areas for restoration, nature-based adaptation, and wetland elevation gain, wetland productivity, wetland PREPUBLICATION COPY

54 Negative Emissions Technologies and Reliable Sequestration: A Research Agenda organic carbon burial, edge erosion, and transgression into uplands in the future. Current approaches are labor intensive in the field and lab. Remote sensing offers the opportunity to scale efficiently and to improve accuracy and be able to better predict and map existing and potentially available coastal wetland area for carbon removal . For existing, transgressing, and restored tidal wetlands, knowledge gaps persist in mapping salinity boundaries, which reduces our ability to predict CH4 and CO2 emissions and uptake. Similarly, for seagrass meadows, OC accumulation rates are roughly understood, but our ability to map and monitor their areal extent is limited. Other key research needs include development of robust typologies that coincide with scenario modeling to identify and project vulnerable land areas where management and restoration should be focused. Finally, developing new methods to account for forested/afforested wetlands along tidal boundaries (i.e., differentiating them from terrestrial forest) in areas where wetlands are transgressing is another key research need. Further technological development of mapping and remote sensing tools, and field-validated modeling to derive more robust relationships and constrain variability in detecting coastal classifications fundamental for carbon removal (e.g., project types, salinity, soil accretion) are recommended. Research efforts should prioritize long-term research sites. Research sites should be augmented with long-term data to support remote sensing applications. Potential funding agencies: NASA, DOE, NOAA, USFS, EPA Research budget: $2M/yr (tidal wetland: $1.5M; seagrass: $500K) for 5 years. Development of a Comprehensive Coastal Blue Carbon Projects Database A strategy is needed to develop a robust means to verify and catalogue number, types, size, cost, engineering specifications, and performance of coastal blue carbon approaches. Best management practices may include design criteria and performance functions associated with technological and ecological specifications of projects. The database will be useful for developing and testing predictive models of coastal blue carbon ecosystem extent and CO2 sequestration rate. It will also be critical for evaluating the conditions under which carbon removal is optimized, to facilitate adaptive management. The Committee recommends development of an integrated project data repository for comprehensive databases, design guidelines for coastal blue carbon with emphasis on coastal restoration, nature-based adaptation, and management of wetland transgression approaches that are most cost- effective, including levels of technical readiness, and best management practices, and policy instruments for implementation, monitoring and assessment. This would include a catalogue of the number, types, size, ecological and geophysical site conditions, and performance of coastal blue carbon approaches and assessment of associated resource requirements. There is an existing “Blue Carbon Research Coordination Network” developing a comprehensive database of carbon data and there are also a few online resources that catalogue living shoreline projects, with only limited quantitative data and no comprehensive project and data repositories of this type. Potential funding agencies: NSF, EPA, USFWS, USACE, NOAA, state agencies. Research budget: $2M/year for 20 years, with an interagency-academia- NGO-industry program work group. Plus, on the scale of the information management for NEON network, LTER network, etc. Demonstration/Deployment Coastal blue carbon demonstration projects & field experiment network Demonstration of carbon-rich projects are needed to justify and prioritize approaches that are the most cost-effective (minimal infrastructure and proactive CO2 removal approaches that minimize erosion and wetland loss) for increasing CO2 removal capacity of coastal projects. Field-scale experiments should PREPUBLICATION COPY

Coastal Blue Carbon 55 take advantage of the extreme range in environmental conditions and settings across which coastal carbon ecosystems exist in the United States to facilitate our understanding of basic processes, such as OC burial rates and long-term decay rates of carbon-rich materials under waterlogged, saline conditions or multi- year studies of low risk, high carbon species or phenotypes. This includes augmenting a subset of existing natural, restoration, eroding, and adaptation demonstration sites, to test carbon-rich approaches in mesoscale field projects. Potential funding agencies: NSF Engineering & Infrastructure programs, DEB, NOAA, USACE, DOE, Industry R&D, A&E firms, foundations. Research budget: new field experimental network of demonstration projects that accelerate carbon removal: $10M per year for 20 years. Carbon removal systems field experiments & adaptively managed site network (“managed” & “engineered” sites), modeling and synthesis For “engineered” projects (wetland restoration, creation, and nature-based coastal adaptation projects), knowledge gaps exist in whether the various management actions implemented will sustain the rates of OC accumulation known to occur in case studies. While number and area of wetland restoration projects are increasing, there are less studies on long-term OC rates under different settings and project types, typologies guiding implementation (Dürr et al., 2011), nor best management practices for monitoring and evaluating their carbon removal performance or response to SLR. Such projects have been implemented since the 1950’s, but few studies have answered the suite of research questions that have been so rigorously applied for “natural” coastal systems. For example, intervention projects that integrate artificial and natural features for coastal defense often focus on biological habitat or diversity rather than carbon sequestration (Firth et al., 2014). Given the likelihood of increasing numbers of projects to protect coastlines, this research infrastructure serves to address both technological issues with engineering coastlines for increasing carbon removal capacity and testing and demonstrating performance of existing and new projects. Further an understanding of how adaptation projects that reduce risks to people and infrastructure can increase and accelerate carbon removal over time while also preserving and enhancing other ecosystem services; an area of uncertainty is the effect of large-scale shoreline modifications, in areas reliant on sediment resuspension and redeposition for marsh accretion, to maintain this critical function for maintaining the marsh platform and wetland function in the long-term. Further, the Committee is unaware of any SET or long-term estimates of OC accumulation from projects that have applied nature-based approaches to coastal risk reduction, and few from created wetlands. Research is also needed to better understand how to manage wetland transgression and erosion. Such research should include increasing understanding of underlying processes for enhancing OC burial with different management activities as or that enable wetland transgression to occur, as well as costs, benefits and socioecological system scenarios and responses to better predict trajectories and variability in OC burial over time. While management of wetland transgression offers immense new areas of wetlands to bury OC and provide good potential for carbon removal, less is known about OC accumulation rates achieved under these conditions. In an adaptive management framework, if management activities do not maintain OC accumulation rates, corrective action is applied. Uncertainties about predictions of engineered wetland performance and wetland survival may develop as a barrier to any coastal blue carbon approach without sustained and new research efforts, including adaptive management research approaches that enable planning, trial and error, and corrective actions if performance criteria are not met (Zedler, 2017). While soil OC burial was considered an incremental cost of coastal blue carbon, a monitoring network would be implemented as part of landscape-level research of management interventions, coupled soil and decay models with modeled scenarios to forecast outcomes, and validated with empirical measurements. All such research should be designed to maximize transfer of technologies and approaches to multiple coastal settings. Objectives targeted for such research include: 1) optimization of OC burial in transgressing wetlands with a suite of structural and non-structural management approaches, 2) effects of SLR and sediment availability on trajectories and thresholds of wetland elevation gain and OC burial, and 3) radiative gas emissions in new PREPUBLICATION COPY

56 Negative Emissions Technologies and Reliable Sequestration: A Research Agenda marshes created by SLR-induced transgression. Knowledge is also required to better understand the OC balance of uplands as they transition to tidal wetlands. Little is known of the trajectory of upland plant productivity, OC accumulation, and OC fate during the onset of tidal flooding, exposure to saline seawater, and the conversion to tidal wetland. Demonstrations / deployments should be conducted across the major upland land use and land-cover categories likely to be inundated by the sea over the 21st century. Development of research infrastructure and network across new engineering and managed sites and augment research infrastructure at existing engineering and managed sites for with a combination of field experiments, network of adaptively managed projects for accelerated deployment, coupled with modeling and synthesis efforts, is needed. Designed research and field experiments and observations across networked sites with statistical sampling applied across gradients of project type, coastal biogeomorphic conditions, watershed management, SLR and climate change exposure. Specific research infrastructure includes plot-to-atmosphere measurements of CO2 and CH4 fluxes coupled with remote sensing, eddy flux measurement sites for CO2 and CH4, micrometeorological station, water level and salinity instrumentation, array of sediment elevation tables, and vegetation plots, at a minimum. An integrated research network (infrastructure) across existing NSF/LTER coastal sites, NOAA/NERRA sites, and NSF/NEON’s coastal sites is recommended with consistent and comparable monitoring of carbon removal rates across sites, augment existing sites with some research infrastructure already in place, and prioritizing long-term research sites in managed areas anticipated for wetland transgression with up to 2ft SLR, near-shore submerged aquatic areas (i.e. seagrass), and engineered projects. A synthesis effort would develop a data/site gap analysis based on a rigorous typology. Integrate existing projects and augment with new projects to demonstrate and accelerate near-term deployment of restoration and nature-based coastal adaptation (living shorelines), and apply an adaptive management research framework. Efforts should leverage known examples of adaptive management research infrastructure like CRMS in coastal Louisiana. This research should justify and prioritize approaches that are most cost-effective (minimal infrastructure and proactive CO2 removal approaches that minimize erosion and wetland loss) for increasing CO2 removal capacity and other ecosystem services. Synthesis activities are needed to update the state of knowledge and evaluate knowledge gaps as new research becomes available, and for scaling up to the regional levels. Synthesis products would inform a multi- region, inter-organization (agency-academic-NGO-industry) work group be convened to develop a framework for an adaptive management plan, identify projects and common research protocols for carbon removal and other ecosystem services, and identify a common set of SLR and climate change scenarios and models. Potential funding agencies: NOAA – SG, NSF multiple directorates, DOE, NASA, USFWS, USACE, state agencies, foundations. Research budget: $30M/yr for 20 years - 15 new engineered sites at a cost of $1M/yr per site (approx. LTER); 20 augmented managed & engineered sites at a cost of $500K/yr; 8 new managed sites at $500K/yr (wetland transgression – 0-2ft and seagrass); 5 U.S. scale synthesis activities (wetland: 3; seagrass: 2) at a cost of $1M/yr ($175k/yr per activity) and support staff for synthesis and inter-organization work group ($125K/yr). Deployment Socio-Ecological and Economic Research to Quantify the Costs and Benefits of Coastal Blue Carbon Numerous social and policy aspects emerge related to coastal blue carbon, particularly related to the need for land cover and land use change. Social barriers are not well understood, particularly in the PREPUBLICATION COPY

Coastal Blue Carbon 57 context of new policy initiatives to reduce costs incurred with increasing coastal risk. Social acceptance of allowing wetlands to transgress into upland areas is highly uncertain (Brody et al., 2012). Other socio- ecological knowledge gaps include how anticipatory decision-making and managed shoreline retreat strategies could be incentivized (e.g., “true” value of risk reduction rather than insurance based) for coastal risk reduction. As restoration and nature-based adaptation projects become a strategy better enabled by the regulatory agencies, safeguarding against potential negative consequences of land reclamation (Chee et al., 2017) may become a socioeconomic barrier to implementation. The difference between wetland creation for coastal adaptation (wetland reclamation) and land reclamation is distinct but may be blurred with uninformed or unenforced policies regulating or guiding best practices. To ensure that coastal restoration and nature-based adaptation projects build wetlands for effective carbon removal rather than land development in the coastal zone can be built into the regulatory framework. Research is needed on incentives and barriers to increasing available area for coastal blue carbon and potential impacts of coastal wetland reclamation as a type of carbon removal (e.g., there are numerous studies illustrating biodiversity risks associated with sea walls that reduce tidal mudflat area). Such social science and policy research could address questions, such as: • If the ecosystem services value of coastal wetland reclamation is added, what are trade-offs and what governance/policies are necessary to manage against unintended outcomes? • What would the added costs be for permitting (and to USACE/state/local agencies)? As coastal restoration projects are implemented for ecosystem services other than carbon removal, the $/ton CO2 cost for carbon removal is merely the incremental cost of monitoring. However, there may be cost differences associated with coastal projects with carbon removal as a consideration compared to those that disregard carbon removal. For example, there may be additional costs of implementing a coastal adaptation structural strategy that sequesters CO2 (natural or nature-based infrastructure) compared to a typical coastal adaptation structural strategy (seawalls). Continued, regional- to locally- specific research is needed to develop approaches for projecting coastal vulnerability that integrates robust information based on modeling and field measurements of OC accumulation on lands made available through managed shoreline retreat. Building and promoting coastal carbon as a NET does not depend on a carbon price, because most wetland restoration and coastal adaptation projects are conducted without concern for CO2 mitigation. Research that quantifies cost-benefits of coastal blue carbon may incentivize government actions to convert lands exposed to coastal flooding to wetlands or private property owner willingness to abandon vulnerable properties (e.g., NFIP repetitive loss properties) and other flood-vulnerable coastal land uses (e.g., agriculture, low density lands) in lieu of other ecosystem services, and ultimately releasing those lands for carbon removal. Further, scientists need to evaluate the economic co-benefits of wetland restoration/creation/protection. Scientists also need to better understand ways to align the timing of expenditures on wetland actions to decrease vulnerability with the benefits accrued to local governments and citizen taxpayers. Given the paucity of social science, economic and policy research to date, numerous socioeconomic issues pose barriers to coastal blue carbon deployment at scale. Research is needed to explore socio-ecological and economic linkages between actions taken to protect, restore, and expand tidal wetlands and human well-being: • What combinations of federal, state and local decision-making are most effective, cost efficient and protective of local community property rights and values while decreasing community vulnerability to SLR and storm damages? • How will market forces influence land use in coastal areas? What are the costs and benefits of regulatory vs market-based approaches to wetland management? PREPUBLICATION COPY

58 Negative Emissions Technologies and Reliable Sequestration: A Research Agenda Research is needed to develop a typology to better predict the likelihood of individual parcels of property county by county becoming wetland (actively and passively) as sea level rises. The typology will likely include factors such as • trends in local land use change, in property values, in the major sources of local government revenue, extent to which marsh management can reduce property damage from storms and SLR, • current extent of development, existing open space, future cost of maintaining infrastructure under a scenario of climate change and SLR, and • politics or people’s attitudes. Research is needed on how: • people’s attitudes about risk and decisions on risk avoidance are made and how that varies in relation to the risk timeline. • availability of resources influences community/local adaptive opportunity and capacity • adaptive options involving wetlands impact local government revenue and finances Potential funding agencies: NSF multiple directorates, NOAA, EPA, state agencies, foundations. Research budget: $5M per year for 10 years. Monitoring & Verification and Research Management The cost of coastal blue carbon ($/ton CO2) includes the incremental cost of monitoring (described above). Time delays in raising money, planning, review of plans and construction need to be taken into consideration when planning timelines are estimated. Research oversight and coordination is embedded within the research agenda cost. Recommendations include developing: 1) a multi-agency (federal, state, local agencies and academic institutions) working group charged and funded to oversee and integrate research efforts and 2) wetland mitigation-type policies that assess performance based on criteria for best implementation/management practices as opposed to. Type of carbon accounting. SUMMARY Coastal wetlands are extremely productive ecosystems; they act as long-term carbon sinks by removing carbon from the atmosphere through photosynthesis and storing it in their soils for long periods of time. Compared to other ecosystems, coastal wetlands sequester a very high amount of carbon per unit area: 7.98 t CO2 ha-1 yr-1 for tidal wetlands and 1.58 t CO2 ha-1 yr-1 for seagrass meadows. This chapter identifies approaches for tidal wetland and seagrass ecosystem management that have the potential to contribute to carbon removal and reliable sequestration gains against the expected future baseline of natural sequestration. These include restoration of former wetlands, the use of nature- based features in coastal resilience projects, managed migration as sea levels rise, augmentation of engineered projects with carbon rich materials, and management to prevent expected future losses in carbon capacity. By implementing these five approaches, the committee identified the opportunity for coastal wetlands to store an additional 5.424 Gt CO2 by 2100. Carbon sequestration is one among many ecosystem services coastal wetlands provide that drive interest in their conservation and restoration. Ongoing restoration activities and the expected growth of coastal resilience efforts provide an opportunity for leveraging carbon benefits at no or low marginal cost. While coastal wetland restoration is among those activities underway now, the projected capacity for coastal carbon removal is uncertain and thus immature as a long-term NET. Biological and geomorphic controls on the rate and permanence of carbon accumulation and sequestration are not well enough understood to predict the future impact potential with high rates of sea level rise and future PREPUBLICATION COPY

Coastal Blue Carbon 59 coastal management practices. There are significant unknowns related to how future changes in coastal watersheds will influence fundamental controls on OC burial and the influence of human interventions. In this chapter, the committee presents a research agenda of basic research, pilot deployment, and monitoring with the aim to better understand the controls on the baseline level of carbon sequestration for the remainder of the century as well as to enhance sequestration rates above the shifting baseline. Additionally, societal decisions about climate adaptation and coastal development will influence the potential for maintaining or increasing coastal wetland area (Schuerch et al., 2018). The research agenda therefore also outlines information needs regarding understanding societal responses to land use change in the coastal zone. PREPUBLICATION COPY

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To achieve goals for climate and economic growth, “negative emissions technologies” (NETs) that remove and sequester carbon dioxide from the air will need to play a significant role in mitigating climate change. Unlike carbon capture and storage technologies that remove carbon dioxide emissions directly from large point sources such as coal power plants, NETs remove carbon dioxide directly from the atmosphere or enhance natural carbon sinks. Storing the carbon dioxide from NETs has the same impact on the atmosphere and climate as simultaneously preventing an equal amount of carbon dioxide from being emitted. Recent analyses found that deploying NETs may be less expensive and less disruptive than reducing some emissions, such as a substantial portion of agricultural and land-use emissions and some transportation emissions.

In 2015, the National Academies published Climate Intervention: Carbon Dioxide Removal and Reliable Sequestration, which described and initially assessed NETs and sequestration technologies. This report acknowledged the relative paucity of research on NETs and recommended development of a research agenda that covers all aspects of NETs from fundamental science to full-scale deployment. To address this need, Negative Emissions Technologies and Reliable Sequestration: A Research Agenda assesses the benefits, risks, and “sustainable scale potential” for NETs and sequestration. This report also defines the essential components of a research and development program, including its estimated costs and potential impact.

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