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2 Coastal Blue Carbon
Pages 45-86

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From page 45...
... , may be conducted in the H 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 NETs.
From page 46...
... . 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.
From page 47...
... . Reversing historic loss and degradation through restoration, incorporating wetland creation into coastal adaptation projects, and managing wetland area and carbon 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.
From page 48...
... COASTAL BLUE CARBON PROCESSES Coastal carbon sequestration is calculated for each ecosystem as the product of its areal extent (horizontal dimension) and its vertical OC accumulation rate (vertical dimension)
From page 49...
... SOURCE: Redfield, 1972. FIGURE 2.3  Carbon sequestration is a function of the rate of relative sea level rise (RSLR)
From page 50...
... , 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.
From page 51...
... . Current models suggest the existence of a threshold level of sea level where the vertical elevation and lateral migration of tidal wetlands may not keep pace with the water level, resulting in wetland drowning and a sudden decrease in OC burial (Figure 2.4)
From page 52...
... , ideal carbon burial rates (green) , and baseline carbon burial rates without human intervention (red)
From page 53...
... Vegetation shifts associated with wetland 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.
From page 54...
... 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 OC 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.
From page 55...
... The field has reasonable estimates of OC burial in coastal macrophyte systems in the United States, 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 practices to reduce the impacts of human drivers that cause coastal change.
From page 56...
... . 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., 2017a)
From page 57...
... However, significant uncertainty remains as to whether application of these approaches 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.
From page 58...
... However, although implementation of nature-based approaches to tidal wetland creation has reached the deployment level, the estimates for CO2 removal are based primarily on assumptions that these approaches attain OC accumulation rates similar to those for natural or restored wetlands.
From page 59...
... In other words, similar socioeconomic issues may exist when converting other coastal land uses to coastal wetlands (i.e., restoration)
From page 60...
... and biochar addition as a means to reduce nitrogen mineralization of peat and coastal wetlands (Luo et al., 2016; Zheng et al., 2018)
From page 61...
... , but the interest for coastal blue carbon is to increase lignin content of roots and rhizomes and reduce OC decomposition. Biofuels researchers have identified genes that encode the enzymes leading to the building blocks of lignin (Hoffmann et al., 2003)
From page 62...
... This section describes the source of these estimates. The total carbon flux per year, and potential carbon 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 to augment projects and strategies to manage wetland transgression.
From page 63...
... bTwenty-five percent of potential area restored by 2030, full area restored by 2060; c25 percent of potential area adapted by 2030, full potential area adapted by 2060; dProjected 0-2 ft SLR by 2060 and 2-4 ft SLR by 2100, land area estimate reflects assumption of 1 ft of accretion through 2100; eNo additional areas for any CO2 removal approach except managed wetland transgression were included in the 2100 scenario. fAugmentation of projects with carbon-rich materials implemented at 25 percent potential area for resto ration and adaptation projects by 2030, full potential area by 2060 (annual rate based on area of projects implemented by year indicated)
From page 64...
... 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.
From page 65...
... adopts a risk-based model, existing areas of developed land with high frequency of losses due to flooding or storm surge may become more readily available for other uses that can sequester OC. These considerations introduce significant knowledge gaps because of the potential variability in the annual carbon flux of restored and created coastal wetlands.
From page 66...
... . Summary of Coastal Blue Carbon Estimates Combining the annual fluxes for each approach based on the potential rate at which they could be implemented, the committee estimated an annual carbon flux rate for 66
From page 67...
... However, proactive management is needed to maintain their rates of flux and area. b25% of potential area restored by 2030, full area restored by 2060; c25% of potential area adapted by 2030, full potential area adapted by 2060; dProjected 0-2ft SLR by 2060 and 2-4ft SLR by 2100, land area estimated includes assumption of 1ft of accretion through 2100; eAugmentation 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; fNo additional areas for any coastal blue carbon approach EXCEPT managed wetland transgression were included in the 2100 scenario.
From page 68...
... managed wetland transgression for 0-2 ft and 2-4 ft of SLR, and (d) total cumulative annual flux of all coastal blue carbon approaches combined.
From page 69...
... , slightly higher than ocean water. Considering the small total area of coastal wetlands not only for the United States, but also for the world, coastal carbon approaches would have a trivial effect on the overall global radiation balance, considering albedo alone.
From page 70...
... . These risks include: • potential for sediment contaminants, toxicity, bioaccumulation and biomagni fication in organisms, • issues related to altering degradability of coastal plants, • use of subtidal areas for tidal wetland carbon removal, • effect of shoreline modifications on sediment redeposition and natural marsh accretion, and • abusive use of coastal blue carbon as means to reclaim land for purposes that degrade capacity for carbon removal.
From page 71...
... living space Organic matter Generates biogeochemical $31/ha/y N/A $30.5/ha/y (2011 accumulation activity, sedimentation, (2011 USD)
From page 72...
... To overcome these barriers, managers must plan and design the coast in a way that allows for continued development by humans while enhancing carbon removal capacity of larger areas of wetlands (Stark et al., 2016)
From page 73...
... Incremental Costs for Monitoring Coastal Blue Carbon If projects are implemented for purposes other than or in addition to carbon removal, then costs are reduced to the incremental cost of monitoring coastal carbon removal. Such costs approximate $0.75/t CO2 for tidal wetlands and $4/t CO2 for seagrass meadows)
From page 74...
... RESEARCH AGENDA The committee developed a research agenda with the overarching goal to preserve and enhance the high rates of OC sequestration in existing tidal wetlands and seagrasses in the coastal zone and to expand the area covered by these ecosystems. As discussed earlier, carbon sequestration rates 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 ensure wetland elevation r ­ emains in balance with increasing rates of SLR, • hybrid "engineering" (restoration, creation, coastal adaptation)
From page 75...
... 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 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 is an important challenge.
From page 76...
... Justification Basic research in 6 5-10 5 projects at $2M/y for 10 understanding years to address fate of OC and using coastal produced and buried in ecosystems as a soils/sediments of coastal NET 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 SLR and management drivers; 5 projects at $2M/y for 5 years to address selection of materials and coastal plants/ phenotypes producing high OC density materials with slow decay rates buried in coastal sediments carbon. Mapping current 2 20 Former NASA CMS projects and future (i.e., (wetland: $1.5M/y; seagrass after SLR)
From page 77...
... at a cost of $200k/y per activity. Coastal blue 5 10 Policies, incentives, and carbon project barriers will change as coastal Deployment deployment risk increases.
From page 78...
... 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, SLR, 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 areal expanse in conjunction with transgression into upland areas as they flood with rising seas is poorly understood.
From page 79...
... Similarly, for seagrass meadows, OC accumulation rates are roughly understood, but the 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.
From page 80...
... 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, so that adaptive management can be facilitated.
From page 81...
... 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, is needed. What remains uncertain is the ability of large-scale shoreline modifications, in areas reliant on sediment resuspension and redeposition for marsh accretion, to maintain the marsh platform and wetland function in the long-term.
From page 82...
... Using adaptive management, corrective action can be applied if management activities do not maintain OC accumulation rates. Uncertainties about predictions of engineered wetland performance and wetland survival may create a barrier to any coastal blue carbon approach without sustained and new research, utilizing an adaptive management approach that enables planning, trial and error, and corrective actions if performance criteria are not met (Zedler, 2017)
From page 83...
... Research budget: $30M/y for 20 years. Deployment Socioecological and economic research to quantify the costs and benefits of coastal blue carbon Numerous social and policy aspects emerge from coastal blue carbon approaches, particularly related to land-cover and land-use change.
From page 84...
... 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 the cost-benefits of coastal blue carbon may incentivize governments to convert lands exposed to coastal flooding to wetlands or private property owners to abandon vulnerable properties (e.g., NFIP repetitive loss properties)
From page 85...
... Research budget: $5M per year for 10 years. Monitoring and Verification and Research Management The cost of coastal blue carbon ($/tCO2)
From page 86...
... Although wetland restoration is under way is some coastal zones, uncertainty regarding its projected capacity for coastal carbon removal renders it immature as a longterm NET. Biological and geomorphic controls on the rate and permanence of carbon accumulation and sequestration are not well enough understood to predict its future impact alongside high rates of sea level rise and future coastal management practices.


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