Proceedings of a Workshop
|Coastal Blue Carbon Approaches for Carbon Dioxide Removal and Reliable Sequestration|
|Proceedings of a Workshop—in Brief|
Carbon dioxide removal (CDR) techniques, which aim to remove and sequester excess carbon from the atmosphere, have been identified as an important part of the possible responses to climate change and have been garnering increased attention.1 The Committee on Developing a Research Agenda for Carbon Dioxide Removal and Sequestration was convened to develop a detailed research and development agenda to assess the benefits, risks, and sustainable scale potential for CDR and sequestration approaches, as well as increase their commercial viability. The CDR approaches under consideration by the committee are coastal and land ecosystem management, accelerated weathering, bioenergy with carbon capture, direct air capture, and geologic sequestration. To aid the development of the research agenda, each approach is being examined by the committee through a series of information-gathering workshops and webinars for open discussions with relevant communities about the current state of knowledge, along with the research needs for understanding the potential of each approach and for deploying them at large scales.
Coastal environments provide many valuable ecosystem services. Their role as carbon sinks has been a topic of exploration to evaluate the potential for the restoration and management of coastal habitats as a viable CDR approach. To explore the state of knowledge, technical research needs, costs, co-benefits, and societal and governance constraints of CDR in coastal ecosystems (often termed coastal blue carbon), the committee convened its first workshop on July 26, 2017, in Woods Hole, Massachusetts. Invited speakers described their relevant work in order to provide the committee with an overview of the state of knowledge and research needs related to understanding carbon capacity and flux in coastal systems, the processes driving sustainability of coastal wetland carbon storage in the future, potential incentives for coastal blue carbon, and policy and governance challenges. The workshop was preceded by an introductory webinar on July 19, 2017, where invited speakers provided an overview of the ecosystems under consideration for coastal carbon removal and sequestration, as well as the costs and other considerations of restoring them. This Proceedings of a Workshop—in Brief summarizes the presentations from both the webinar and workshop.
INTRODUCTION TO COASTAL BLUE CARBON AND COASTAL WETLAND RESTORATION
Coastal wetlands—salt marshes, mangroves, and seagrasses—have been identified as having the greatest CDR capacity of all coastal ecosystems, as explained by Jennifer Howard from Conservation International during the introductory webinar. She presented long-term soil carbon burial rates of seagrass beds (138±38 g C/m2 year), salt marshes (218±24 g C/m2 year), and mangroves (226±39 g C/m2 year), which have rates that exceed those in forests
1 National Research Council. 2015. Climate intervention: carbon dioxide removal and reliable sequestration. Washington, DC: The National Academies Press.
(see Figure 1).2 Carbon dioxide is removed from the air as a result of photosynthesis, incorporated into plant biomass, and accumulated in the soil as dead organic matter. Inundation from the tides inhibits the microbial activity that breaks down carbon. Conversely, if a coastal wetland is degraded (e.g., drained for fish ponds or agriculture, dredged, or altered for development), it will become a carbon source.
Figure 1. Mean long-term rates of carbon sequestration in soils in terrestrial forests and sediments in vegetated coastal ecosystems. SOURCES: Presentation by Jennifer Howard; figure from Blue Carbon Initiative with data from McLeod et al., 2011.2
In assessing the climate mitigation potential of coastal ecosystems, Howard identified important science questions that evaluate the carbon sequestration potential of an ecosystem over the long term and implementation questions about the potential to manage the ecosystem to raise sequestration rates above the baseline. These questions can be applied to specific ecosystems; for example, coral reefs and other marine fauna are important components of the carbon cycle, but do not provide long-term removal of carbon, and organisms with calcified skeletons may be a small net source of carbon dioxide as a product of calcification. Additionally, kelp also lack long-term carbon storage due to the absence of soil, but there are some emerging farming approaches that may resolve this issue. Phytoplankton are a very large global carbon sink where a small, yet significant percentage of carbon is sequestered when biomass sinks and is stored at the ocean bottom. However, there are concerns about the potential environmental impacts and questions about who would have jurisdiction for activities in the open ocean when considering the current approaches to raising this carbon sequestration above the natural baseline. Given what is known about the carbon storage potential of each ecosystem and the existing policy and governance frameworks for climate mitigation, Howard concluded that coastal wetlands should be a primary focus for climate mitigation efforts in coastal areas.
Wetland management, specifically wetland restoration and conservation, affects coastal wetland CDR capacity. Nicholas Wildman from the Massachusetts Division of Ecology Restoration (DER) described the wetland restoration activities undertaken by his agency. Since 1998, more than 800 hectares (ha) (about 2,000 acres) of coastal wetlands have been restored by DER; the majority of marsh restorations are tidal restriction removals, in which the connection to the saltwater tide is restored to a wetland previously located behind a barrier such as a road. The size of the projects range from 7.2 ha to 52 ha, and costs can range from $50,000 for small projects to $2 million for larger ones with more complex engineering requirements. DER does consider climate change to be an important driver of restoration (e.g., restoring natural ecosystems to improve coastal resilience to rising sea levels). However, CDR specifically has not yet been a consideration for selection, prioritization, or design due to a lack of information about these benefits. Wildman said research that can uncover the predictability, sustainability, scalability, and transferability of CDR benefits is needed in order to incorporate them into restoration planning.
Large-scale coastal and inland wetland restoration is currently occurring in the Florida Everglades. Fred Sklar from South Florida Water Management described these restoration activities and commented on their potential CDR benefits. In the Everglades, marsh areas have been drained and mangroves have been deprived of fresh water, resulting in soil oxidation. This has contributed to a significant loss of peat soil and therefore a major loss of carbon storage. South Florida has become an agricultural and urban center, which increases the complexity and cost of restoration. Sklar described the Central Everglades Planning Project and the C-111 South Dade project, which would restore hydrology in the southern Everglades, and the creation of stormwater treatment areas that incorporate wetlands under the Florida Forever Act. He has roughly calculated that completion of these three planned restoration initiatives in the Everglades would come to a combined cost of $3.1 billion over 50 years and remove more than
2 McLeod, E., G. Chmura, S. Bouillon, R. Salm, M. Björk, C. M. Duarte, C. E. Lovelock, W. H. Schlesinger, and B. R. Silliman. 2011. A blueprint for blue carbon: Toward an improved understanding of the role of vegetated coastal habitats in sequestering CO2. Frontiers in Ecology and Environment 9:552–560.
5.8 million tons of CO2/year over an area of almost 12,000 km2 (1.2 million ha).3 However, Sklar pointed out that this cost should not be solely attributed to carbon removal because there are other ecosystem services provided by the restoration, including those driving the restoration programs. Sklar identified scientific challenges associated with predicting the CDR potential of Everglades restoration, including a better understanding of how saltwater intrusion caused by restoration of hydrology will accelerate soil decomposition and how increasing water tables influence carbon dioxide and methane fluxes.
Wetland restoration also occurs within dense urban areas, and Walter Meyer from Local Office Landscape Architecture described natural infrastructure projects that make use of wetland features completed by his firm. A stormwater filtration project in Mayaguez, Puerto Rico, included 5 acres of new wetlands for water filtration at a cost of $163,000 per acre ($403,000 per ha). A second project, a new residential development in New York City, will incorporate 10 acres of wetlands to manage onsite hydrology for flood protection at a projected cost of $410,000 per acre ($1.01 million per ha). Another project reconnects the Everglades to the ocean through hydrologic connections in urban areas in Miami-Dade County, Florida, using 142 acres of new wetlands at a cost of $151,000 per acre ($373,000 per ha). While these projects were not undertaken with CDR in mind, Meyer stressed that restoration projects conducted in urban areas for CDR could be designed in concert with natural infrastructure projects, which may open cost-sharing opportunities.
APPROACHES FOR UNDERSTANDING THE CAPACITY AND FLUX OF COASTAL BLUE CARBON
The committee's workshop started with a session exploring the scientific underpinning for understanding coastal wetland CDR capacity. Some methodologies for evaluating the carbon storage capacity and flux in salt marshes are relatively well developed, said Patrick Megonigal of the Smithsonian Ecological Research Center. These include measurements of the wetland area, measurements of soil carbon stock, and assessments of changes in carbon stock. Other measurements, including accurate measurements of the trace amounts of methane and nitrous oxide emissions, need to be improved to identify when conditions in a wetland are favorable for climate mitigation. This is an issue of interest in areas of low salinity where methane emissions may be more significant than areas of higher salinity. Salinity is frequently used as a proxy for estimating methane emissions due to the challenges of measuring emissions directly.4 Megonigal noted that the sample sizes required for accurate estimates of greenhouse gases are far larger than estimates for soil carbon stocks. Alternatives that reduce sample size requirements, such as measurements of eddy fluxes, are costly to conduct without continued technological advances. Another challenge is in knowing the fate of soil carbon when a wetland converts to open water or is eroded. The fate of eroded carbon in an estuary (i.e., whether it will decompose and contribute to carbon emissions or be deposited long term) depends on geomorphological processes controlling erosion, deposition, and resuspension. Improvements to our understanding of what could happen to eroded carbon would require improvements
Seagrass. Credit: National Oceanic and Atmospheric Administration
3 Based on measured soil bulk densities in Everglades seagrass and mangroves.
4 Poffenbarger, H., B. Needelman, and J. P. Megonigal. 2011. Salinity influence on methane emissions from tidal marshes. Wetlands 31(5):831–842.
to coupled marsh hydro-biogeochemical models. A third issue is the ability to quantify the carbon that is produced offsite that settles in the marsh. This is an issue relevant to carbon crediting because wetlands are not credited for sequestering forms of carbon that originate outside the system and remain largely unreactive. This too would benefit from improved modeling of the stability of different forms of carbon. Finally, Megonigal also pointed to the need for leaders to engage in transdisciplinary research that spans wetland ecology, biogeochemistry, and geomorphology in order to advance coastal carbon science.
Unique challenges exist in quantifying the carbon storage potential of seagrass beds and Julie Simpson from the Massachusetts Institute of Technology noted particularly that their subtidal location makes collecting samples challenging. Seagrasses are an important ecosystem to investigate because although they appear to have lower sequestration rates than salt marshes and mangroves (see Figure 1), they potentially cover a much larger area worldwide and would thus have higher total global carbon storage. There is a large variation in measurements of seagrass carbon stocks and sequestration rates; potential sources of variability include plant species, plant density, nutrient availability, meadow size, wave exposure, and substrate type. Simpson has worked at eelgrass meadows along the Massachusetts coast and she stated that her samples affirm that although there is variability across sample sites, eelgrass bed sediment cores do have higher carbon densities than surrounding unvegetated areas. Isotope signals indicate that this carbon comes from both the eelgrass biomass and external sources that get trapped in the vegetated bed. Another study has shown that the variability of carbon densities within a seagrass meadow is correlated to the distance from the meadow's edge.5 She also found that restored seagrass meadows are effective at storing carbon, achieving carbon storage quantities (in the top 12 cm) similar to those of older meadows after about 15 years.
Simpson summarized the information gaps in understanding seagrass carbon storage. The mapping of seagrass areas in U.S. water is limited; less than 60% of meadows are mapped and existing maps have varying degrees of accuracy due to difficulties in remotely sensing the underwater habitat. The fate of seagrass biomass is unknown; seagrass beds capture a large amount of carbon from external sources, but exported plant biomass may either be
5 Oreska, M., K. McGlathery, and J. Porter. 2017. Seagrass blue carbon spatial patterns at the meadow-scale. PLoS ONE 12(4):e0176630.
degraded or stored elsewhere (e.g., deposited in deep sea trenches). Greenhouse gas emissions from seagrasses are also not well studied. Simpson noted that the impact of future changes in the landscape need to be assessed to evaluate the sustainability of a meadow, including changes in water quality, land use, physical disturbances, and impacts from climate change.
Robert Twilley from Louisiana State University described the carbon storage potential of mangroves. Global carbon storage numbers are challenging to determine both because of variability in carbon burial rates and uncertainty about the global extent of mangrove areas. Using an average gross carbon sequestration rate, Twilley provided an estimate that 525 g C/m2 year is sequestered in mangrove soils and biomass globally (with averages of 225 g C/m2 year in soil6 and 300 g C/m2 year in biomass). However, this rate will vary by the coastal setting in which the mangroves are located (e.g., deltas, estuaries, or lagoons). Additionally, changes in land use and hydrology and damage from storms lead to carbon emissions and erosion from mangroves. Twilley provided an estimate that mangroves lose about 450 g C/m2 annually. The net sequestration would thus be about 75 g C/m2 year, but these rates are highly uncertain. Because the emission rate is not much lower than the gross carbon sequestration rate, Twilley explained that a more accurate understanding of the carbon exchange occurring between mangroves and estuaries is needed in order to quantify the size of the mangrove carbon sink.
Sediment supply plays a role in the survival of wetlands and thus their ability to accumulate carbon, as shown by Twilley's research on the impact of Mississippi River management on carbon storage in delta wetlands.7 A comparison of the carbon storage in two basins—one receiving river sediment input and one where the connection to the river has been restricted—provides evidence that maintaining riverine sediment delivery to the delta improves wetland carbon storage capacity.
Carbon sequestration in wetlands is also influenced by their connection to tides, a process studied by Kevin Kroeger from the U.S. Geological Survey (USGS). 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. An estimated 2,650 km2 (265,000 ha), or about 27%, of wetlands along the Atlantic Ocean coast are currently artificially restricted from the tides. Kroeger identified these as potential areas for restoration, where the reintroduction of tidal connections to saline water would reduce emissions significantly and also pave the way for continued future carbon storage benefits. However, restoration projects of this type may require the development of CDR incentives in order for their value to be realized. Lastly, Kroeger explained that an accurate estimate of the area available for restoration requires improved mapping capabilities, particularly in those areas that may have lost physical wetland features and thus are difficult to identify.
Neil Ganju from USGS described the estuarine sediment processes—wave erosion, conversion to open water, and sediment supply—that affect wetland carbon storage. Waves are the main cause of edge erosion in wetlands; wave power is correlated to erosion rate, meaning the amount of area lost from a wetland can be predicted based on the local wave conditions.8 Open water ponds may form in wetlands due to animal activity, sea-level rise, sediment deficits, or road construction; if they expand due to lack of sediment supply, they will lead to marsh loss.9 Ganju noted that most tidal marshes in the United States are experiencing low sediment supply and at sea-level rise rates of 3-6 mm/year, will experience this pond collapse regime. Wetlands are rarely in a stable state and usually are either expanding due to a net sediment supply or contracting due to a net sediment loss.
Ganju and his collaborators have conducted sediment flux measurements that integrate these processes in order to model the fate of wetlands.10 The unvegetated marsh to vegetated marsh ratio (UVVR) is one metric they found that affects the sediment balance within a wetland because vegetation within the marsh captures sediment, which may prevent conversion to open water. UVVR was found to correlate with the lifespan of the marsh, because it affects the rate at which sediment is exported from the marsh. Long-term sequestration of carbon in a wetland is dependent on this lifespan. Ganju and his collaborators found that all marshes they measured had a UVVR and sediment budget that put them in a sediment deficit. Ganju described how the remaining knowledge gaps in
6 Breithaupt, J. L., J. M. Smoak, T. J. Smith, C. J. Sanders, and A. Hoare. 2012. Organic carbon burial rates in mangrove sediments: Strengthening the global budget. Global Biogeochemical Cycles 26.
7 Built on Twilley, R. R., S. J. Bentley, Q. Chen, D. A. Edmonds, S. C. Hagen, N. S.-N. Lam, C. S. Willson, K. Xu, D. Braud, R. Hampton Peele, and A. McCall. 2016. Co-evolution of wetland landscapes, flooding, and human settlement in the Mississippi River Delta Plain. Sustainability Science 11:711–731.
8 Leonardi, N., N. Ganju, and S. Fagherazzi. 2016. A linear relationship between wave power and erosion determines salt-marsh resilience to violent storms and hurricanes. Proceedings of the National Academy of Sciences 113(1):64–68.
9 Mariotti, G. 2016. Revisiting salt marsh resilience to sea level rise: Are ponds responsible for permanent land loss? Journal of Geophysical Research: Earth Surface 121(7):1391–1407.
10 Ganju, N., Z. Defne, M. Kirwan, S. Fagherazzi, A. D'Alpaos, and L. Carniello. 2017. Spatially integrative metrics reveal hidden vulnerability of microtidal salt marshes. Nature Communications 8.
determining a marsh's lifespan include the rate of change of the UVVR, fate of exported carbon in the estuary, potential for inland migration to increase marsh survival, application of UVVR, and sediment budgets of seagrasses and mangroves.
APPROACHES FOR PREDICTING CARBON STORAGE POTENTIAL ACROSS MULTIPLE SCALES
Lisamarie Windham-Myers from USGS described several synthesis efforts under way to apply plot-level data on a regional and national scale. National or international carbon storage numbers are developed by multiplying the area of coastal wetland habitat by the average carbon stock per area. Windham-Myers highlighted that it is critical to improve accuracy and precision and reduce bias in measurements so errors are not propagated to larger scales. The total estimated wetland area in the continental United States, determined through remote sensing and field surveys, is about 1.9 million ha of salt marsh, 0.3 million ha of mangroves, and 2.3 million ha of seagrasses. However, Windham-Myers noted that an accurate map of coastal wetlands with CDR potential would be improved with better identification of which wetlands are within the extent of tidal influence and, specifically, identification of where wetlands are affected by sea-level rise.
In synthesizing data on carbon stocks, Windham-Myers said that she and her collaborators found soil carbon density down to 1 m in depth to be similar across vegetation types, salinity, and regions, at about 0.026 g C/cm3.11 Biomass stocks, which contribute biomass to the soil and trap sediment, were also consistent across marshes, as measured using 30 m resolution remote sensing images. This suggests that soil accumulation rates, and the geomorphic processes that dictate these rates, are a more significant influence on carbon storage and long-term CDR potential than biomass and soil carbon density. Soil accumulation is correlated with the rate of sea-level rise, as wetlands that survive rising sea levels must be accreting soil. Using tide gauges, sea-level rise rates can be mapped, though accurate information for modeling soil accretion is dependent on having sufficient tide gauge coverage to capture the in-basin variability that has been seen in marsh responses to sea-level rise. Using the Marsh Equilibrium Model (MEM) described by the next speaker, Windham-Myers calculated that given the range of sea-level rise seen in the United States, sequestration rates of 40 to 220 g C/m2 year would be expected. However, the effect
Salt marsh. Credit: U.S. Fish & Wildlife Service
11 Holmquist, J., L. Windham-Myers, N. Bliss, S. Crooks, J. Morris, J. P. Megonigal, T. Troxler, et al. In Review. Accuracy and precision of soil carbon estimates for tidal wetlands in the conterminous United States. Targeted Journal: Nature Scientific Reports.
of methane emissions on these numbers is not well understood. Marsh elevation, which is measured using LiDAR (Light Detection and Ranging)—a technology that requires ground truthing for accuracy, Windham-Myers noted—is also a critical measurement for geomorphic model projections of wetlands susceptible to conversion to open water from sea-level rise.
This balance between sea-level rise and marsh accretion has been modeled by James Morris from the University of South Carolina, who described the MEM at the workshop. Much of the wetland soil accretion is from organic carbon stored in plant biomass, with the rigid lignin fibers of the vegetation (which makes up about 10% of the plant) acting as the prominent long-term biomass storage. The growth of wetland vegetation is dependent on its elevation relative to sea level and follows a parabolic growth curve, which shows how vegetation grows best at an intermediate optimal elevation relative to sea level. The maximum accretion rate occurs when the growth rate is also at a maximum because denser biomass contributes more to accretion and also traps more incoming sediment. Marshes with more biomass and higher accretion rates will be more resilient to sea-level rise. The MEM calculates the responses of biomass and elevation to changing rates of sea-level rise, including identification of equilibria and thresholds for marsh loss. Model parameters vary from marsh to marsh. Morris simulated a wide range of values to develop a curve that describes the mean carbon sequestration rate as a function of the relative rate of sea-level rise and maximum biomass (see Figure 2). This curve shows that if a certain rate of sea-level rise (estimated at about 1.5 cm/year) is exceeded, there will be a point when no marsh can survive unless there is the ability to migrate inland.
Figure 2. 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: Presentation by James Morris.
Several models like the MEM have been developed. Although they use different assumptions and equations, they are built on the same idea—that the rate of sea-level rise accelerates wetland accretion up to a threshold point at which the wetland is drowned. Matt Kirwan from the Virginia Institute of Marine Science described his model for vertical accretion, which he combines with models for erosion and upland migration to assess the lateral and vertical changes in wetlands in response to sea-level rise. Although a marsh may experience edge erosion, this carbon loss can be compensated for through upland migration, vertical accretion, or deposition of the eroded carbon into the marsh. The upland migration rate is dependent on sea-level rise, given that marshes expand until a threshold rate of sea-level rise causes them to drown, which varies depending on marsh slope. Similarly, as stated by earlier speakers, sea-level rise will accelerate vertical accretion up to a threshold point. Kirwan has found that up to 30% of platform marsh carbon comes from being transported from the eroded edge, compensating for some area loss. Given that future sea-level rise rates will inevitably lead to marsh erosion and/or drowning (at rates as low as 5 mm/year in some places12), Kirwan noted that marsh persistence will depend on a marsh's ability to migrate into uplands; however, in some areas, man-made barriers stand in the way of marsh migration.
Scott Hagen from Louisiana State University described the Hydro-MEM model, which couples the MEM model with a hydrodynamic model to predict marsh evolution (e.g., conversion to open water or migration) based on tidal dynamics, biomass productivity, and accretion. Hydro-MEM relies on high-resolution LiDAR data to identify the elevation of coastal marsh platforms. However, Hagen has demonstrated that LiDAR requires corrections based
12 Kirwan, M. L., D. Walters, W. Reay, and J. Carr. 2016. Sea level driven marsh expansion in a coupled model of marsh erosion and migration. Geophysical Research Letters 43:4366–4373.
on field verifications for sufficient accuracy. Hydro-MEM has been applied to marshes along the U.S. east coast and the northern Gulf of Mexico. As an example, for the Grand Bay marshes in Mississippi, Hydro-MEM demonstrates that these marshes will lose productivity with 55 cm of sea-level rise and become open water at 80 cm, if this inundation occurs by 2100 (or sooner at higher sea-level rise rates). As noted previously, in many areas, marshes will be unable to migrate inland due to the presence of roads or open water. Hagen and his colleagues are now applying the model to a spatial assessment of ecosystem service values of marshes by modeling their productivity (or loss) in order to inform coastal resource managers of the value of the natural and nature-based features in a specific watershed.
ECONOMIC CONTEXT OF COASTAL BLUE CARBON APPROACHES
Steve Crooks from Silvestrum Associates addressed the requirements for developing commercial and policy incentives for managing coastal wetlands for CDR. Crooks pointed out that the most significant climate mitigation opportunity for coastal wetlands is the avoidance of carbon emissions by preventing their loss, erosion, or drainage. According to the U.S. Inventory of Greenhouse Gas Emissions and Sinks,13 nationally, marshes and mangroves store 3,190 million metric tons of CO2 in the top 1 m of soil and sequester a net of about 8 million metric tons of CO2 equivalents/year. In comparison, current wetland restoration efforts sequester 0.02 million metric tons of CO2 equivalents/year. Restoration projects can also contribute to avoided emissions when they occur in areas of degraded wetlands. Crooks stated that the ability to determine the permanence of carbon sequestered in wetlands is one of the biggest challenges in developing financial instruments and the risk that carbon storage will not be sustained in the long term influences the economic value of investments in wetland projects. He identified several requirements for wetland projects that provide credits for carbon dioxide removal, including demonstrating (1) true reductions in carbon, (2) these reductions are in addition to what would have occurred without the incentive, (3) the reductions are permanent, (4) these projects can pass independent emission verifications, (5) the ownership of the greenhouse gas reduction is clear, (6) there are no harmful effects, and (7) the projects are practical to implement.
Mississippi River Delta salt marsh. Credit: USGS
13 U.S. Environmental Protection Agency. 2017. Inventory of U.S. Greenhouse Gas Emissions and Sinks 1990–2015. https://www.epa.gov/ghgemissions/inventory-us-greenhouse-gas-emissions-and-sinks-1990-2015 (accessed October 12, 2017).
Crooks emphasized the importance of considering the landscape context of wetland projects, particularly considering how the landscape will change, and how wetlands fit into this context. However, if upslope area is set aside and expected to allow for future wetland migration, it is necessary to also account for potential future land uses and associated emissions on the upslope site. Additionally, a better understanding of the fate of exported carbon from the site (e.g., from erosion) is required. According to Crooks, scientific needs for improving the capability to finance wetland carbon sequestration projects include developing better maps of wetland extent and conversion, modeling relationships between productivity and carbon sequestration in mangroves, projecting the response to climate change along the coasts, tracking the fate of carbon exported from wetlands, quantifying methane emissions, and improving understanding of seagrass and kelp ecosystems.
The value of coastal wetlands is enhanced by their range of ecosystem services, as stated by Katie Arkema from Stanford University. She described how her work connects science and practice to value the ecosystem benefits of wetlands and other marine ecosystems, benefits such as fishery habitats, coastal protection, and recreation and tourism. Arkema described the use of ecosystem service assessments for understanding and modeling how alternative management scenarios will result in changes to ecosystem structure and function, what service(s) will be provided as a result, and finally the value of the ecosystem service(s). As an example, InVEST (Integrated Valuation of Ecosystem Services and Tradeoffs) models value ecosystem functions using a suite of biophysical, economic, and human well-being metrics based on how the resulting services are prioritized by the local population, particularly at the community level. These values will vary on a spatial scale and thus can help managers prioritize and site conservation, restoration, and development projects.
Edward Barbier from the University of Wyoming14 described approaches to valuing carbon storage in coastal wetlands. The quantity and longevity of carbon storage are important considerations when compensating projects for carbon removal. Barbier identified two methods for valuing this carbon storage. The first is to use a social cost of carbon, which considers the damages associated with greenhouse emissions, and therefore requires extensive information about physical, ecological, and economic impacts. A National Academies of Sciences, Engineering, and Medicine committee15 recommended improvements to the method for developing a social cost of carbon. An alternative method is to use the price established in existing carbon markets. This price will vary over time, and because there are no global markets, this price varies across countries. Barbier said that the value of the carbon storage should be considered in combination with the other wetland ecosystem services in determining the overall value of wetland conservation or restoration.
POLICY, LEGAL, SOCIAL, AND INSTITUTIONAL CONTEXT FOR INCLUDING CARBON REMOVAL IN COASTAL MANAGEMENT
Government policies may also incentivize wetland carbon projects, some of which were described by Ariana Sutton-Grier from the University of Maryland and the Nature Conservancy during the final session of the workshop. She noted that wetland restoration is an activity that is already being implemented, allowing CDR benefits to be achieved until larger technological advances are ready. Sutton-Grier described her analysis of federal policies that could support the implementation of coastal wetland management specifically for CDR. The Clean Water Act, the Coastal Zone Management Act, the Natural Resources Damage Assessment process, and the National Environmental Policy Act (NEPA) could serve as potential policy tools. Though these laws do not currently incorporate CDR activities, the National Oceanic and Atmospheric Administration introduced the consideration of wetland carbon into two NEPA Environmental Impact Statements. Wetland carbon storage has also been incorporated into national climate efforts, notably in the U.S. Inventory of Greenhouse Gas Emissions and Sinks. A methodology has been included under the Verified Carbon Standard for crediting coastal wetland restoration with carbon removal in the voluntary carbon market, and a standard for also crediting coastal wetland protection activities is under review, according to Sutton-Grier.
A law or a policy that protects and restores coastal wetlands will have some degree of carbon benefits. Sutton-Grier identified coastal wetland uses for fishery habitats, nutrient reduction, or in coastal stabilization projects (i.e., hybrid or living shorelines). Despite this broad opportunity, Sutton-Grier identified several barriers to implementing policies that target CDR, such as lack of guidance, protocols, and expertise for implementing coastal carbon as a CDR approach; lack of a standard carbon cost for valuing CDR; challenges in obtaining permits for restoration projects; complications from external processes such as sediment delivery, nutrient management, groundwater extraction, and local zoning; and public acceptance of nature-based land management strategies. As noted by
14 As of Fall 2017, Edward Barbier is at Colorado State University.
15 National Academies of Sciences, Engineering, and Medicine. 2017. Valuing climate damages: Updating estimation of the social cost of carbon dioxide. Washington, DC: The National Academies Press.
previous speakers, the future survival of many wetlands may depend on their ability to migrate inland, which Sutton-Grier noted is not well incorporated into land-use planning and pointed to a study that spatially modeled where coastal wetlands may be able to migrate now and in the future16 (see Figure 3).
Sam Brody from Texas A&M University at Galveston spoke about local planning opportunities and barriers to coastal CDR activities. Generally, climate mitigation is a relatively low priority for people in the United States. However, high-risk communities, such as those prone to sea-level rise and flooding, are more engaged. He noted that highlighting coastal wetlands' role in infrastructure planning, such as in flood risk reduction, is an opportunity for increasing the acceptance of wetland conservation and restoration. His work is focused on the Houston area, where urban development is rapid and will continue to be a cause of wetland loss. While these losses are often small scale and thus hard to detect, the number of individual impacts, which he identified from Clean Water Act permits, is large. At the local level, the value of coastal property is significant and it is difficult to convince people of the value of ecosystem services in comparison. However, he described local-level policy frameworks that can be used to encourage wetland protection, including land acquisition, open space protection, setbacks, and comprehensive coastal and hazard planning.
Scott Pippin from the University of Georgia also emphasized the importance of local decision making. Pippin described relevant federal laws and policies (Federal Emergency Management Agency Disaster Response and Mitigation, the National Flood Insurance Program, Clean Water Act Section 404 permitting, and federal taking jurisprudence) and state laws and policies (state-level environmental statutes related to wetland permitting or sediment control, property laws, public trust doctrines, and state taking jurisprudence) for coastal wetland management. However, Pippin noted that laws with direct influence on land use will require local cooperation to be implemented.
Figure 3. Maps of county-level barriers and opportunities for landward migration of coastal wetlands under alternative sea-level rise scenarios. (a) Relative ratio of areas available for migration. (b) Relative ratio of areas where migration is prevented by future urban land. Counties with crosshatching are expected to have less than 1 km2 of area available for migration. SOURCES: Enwright et al., 2016 (Sutton-Grier, slide 11).16
16 Enwright, N., K. Griffith., and M. Osland. 2016. Barriers to and opportunities for landward migration of coastal wetlands with sea-level rise. Frontiers in Ecology and the Environment 14(6):307–316.
Local jurisdictions also influence land use through infrastructure management, land-use planning and zoning, building standards, floodplain and stormwater management, and law enforcement. Local governments can decide how and whether to provide public services, such as roads and utilities, in areas into which marshes could migrate. However, he said that there are limitations to the degree which local governments will be able or willing to interfere with private property rights, economic development, resource availability, and other local political issues. In order to predict the influence local policies will have on national coastal CDR potential, Pippin recommended research to develop typologies, or classifications, of local entities and their priorities (e.g., revenue streams) that will influence their likelihood to implement climate change mitigation practices. Additionally, better evaluation of the co-benefits of coastal wetland conservation can tie it more closely to government responsibilities and economic opportunities.
In the last presentation of the workshop, Clark Miller from Arizona State University described the governance needs for supporting the implementation of CDR approaches. He described three areas that need focus so policy makers can incorporate coastal CDR into their decisions. The first is that knowledge systems—the institutional processes for applying knowledge to policy—will need to be developed to understand carbon and wetland science at many scales. Second, coastal management decisions affect human lives and livelihoods, and policies supporting coastal carbon projects should have social values that are distributed equitably. Lastly, issues of greenhouse gas mitigation and ocean management are global issues that require negotiation of international policies, which in turn require new and innovative international governance arrangements. Miller noted that governance frameworks for forest carbon can be used as models for coastal carbon.
DISCLAIMER: This Proceedings of a Workshop—in Brief was prepared by Emily Twigg as a factual summary of what occurred at the meeting. The statements made are those of the rapporteur or individual meeting participants and do not necessarily represent the views of all meeting participants; the planning committee; or the National Academies of Sciences, Engineering, and Medicine.
REVIEWERS: To ensure that it meets institutional standards for quality and objectivity, this Proceedings of a Workshop—in Brief was reviewed by John Callaway, University of San Francisco; Charles Hopkinson, University of Georgia; Denise Reed, University of New Orleans; and Michael Wara, Stanford University.
Committee for Developing a Research Agenda for Carbon Dioxide Removal and Reliable Sequestration: Stephen Pacala (NAS) (Chair), Princeton University; Mahdi Al-Kaisi, Iowa State University; Mark Barteau (NAE), University of Michigan; Erica Belmont, University of Wyoming; Sally Benson, Stanford University; Richard Birdsey, Woods Hole Research Center; Dane Boysen, Cyclotron Road; Riley Duren, Jet Propulsion Laboratory; Charles Hopkinson, University of Georgia; Christopher Jones, Georgia Institute of Technology; Peter Kelemen (NAS), Columbia University; Annie Levasseur, International Reference Centre for the Life Cycle of Products, Processes and Services (CIRAIG); Keith Paustian, Colorado State University; Jianwu (Jim) Tang, Marine Biological Laboratory; Tiffany Troxler, Florida International University; Michael Wara, Stanford Law School; Jennifer Wilcox, Colorado School of Mines
SPONSORS: This activity was supported by Incite Labs, the National Oceanic and Atmospheric Administration, the U.S. Department of Energy, the U.S. Environmental Protection Agency, the U.S. Geological Survey, and the V. Kann Rasmussen Foundation, with support from the National Academy of Sciences' Arthur L. Day Fund.
For a record of presentations and additional information regarding the workshop, including the statement of task for this study, visit http://nas-sites.org/dels/studies/cdr.
Suggested citation: National Academies of Sciences, Engineering, and Medicine. 2017. Coastal Blue Carbon Approaches for Carbon Dioxide Removal and Reliable Sequestration: Proceedings of a Workshop—in Brief. Washington, DC: The National Academies Press. doi: https://doi.org/10.17226/24965.
Division on Earth and Life Studies
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