Chapter 2 Introduction
Sea-level rise drives a whole-ecosystem dynamic response in tidal wetlands which involves flood-stimulation of root production and mineral sediment deposition, resulting in new soil formation (Morris et al. 2002). This new soil formation results in long term burial of carbon which is effectively sequestered (Chmura et al. 2003), and stabilized from decay by oxygen-poor water (Mueller et al. 2018). Many tidal wetlands are both a store of carbon as well as a comparatively minimal source of methane (Poffenbarger, Needelman, and Megonigal 2011). This makes them both effective long term carbon sinks, as well as landscapes that can be managed for climate mitigation (Howard et al. 2017).
Because of the high carbon sequestration capacity of tidal marshes, mangroves, swamps and seagrasses, Blue Carbon research has come into prominence in the last decade to quantify, map, and account for ecosystem services associated with tidal wetland preservation and restoration. In addition to preserving the ongoing carbon sequestration capacity of undisturbed tidal wetlands, the restoration of tidal wetlands has tremendous potential to offset greenhouse gas emissions (Kroeger et al. 2017).
The contiguous United States has been a focal point as the first country to add Blue Carbon ecosystems to the national greenhouse gas inventory report to the Intergovernmental Panel on Climate Change. The NASA Carbon Monitoring System and NASA Carbon Cycle and Ecosystems programs have funded multiple projects focused on building inventories at various scales, including those that intercompare soil carbon stock mapping strategies (James R. Holmquist, Windham-Myers, Bliss, et al. 2018), map biomass using remote sensing (Byrd et al. 2018), report on the magnitude and sources of uncertainty in the inventory arising from literature review datasets and mapping sources (James R. Holmquist, Windham-Myers, Bernal, et al. 2018), and develop a full terrestrial carbon export budget to coastal aquatic systems for the U.S. northeastern Atlantic coast (Najjar et al. 2018). Coastal wetlands were included in the Second State of the Carbon Cycle Report (Cavallaro et al. 2018).
In recent years multiple state governments have implemented policies on greenhouse gas emission reductions through the U.S. Climate Alliance. These are part of a wider portfolio of emission reduction and mitigation strategies under the theme of Natural Climate Solutions (NCS) (Fargione et al. 2018), which are land management practices that reduce emissions or draw down carbon. Blue carbon strategies are one of the NCS strategies available in a subset of coastal states including: California, Connecticut, Delaware, Hawaii, Maine, Maryland, Massachusetts, New Jersey, New York, North Carolina, Oregon, Pennsylvania, Puerto Rico, Rhode Island, Virginia, and Washington. In 2018 the Nature Conservancy hosted a participatory workshop with multiple state-level stakeholder groups in order to educate them on the potential of NCS, and discussed scenarios where NCS could be implemented in state-level policy and planning. James Holmquist, lead author of this report, was among those providing data and consulting on behalf of Blue Carbon strategies. Informational one-sheets were developed educating state-level stakeholders on Blue carbon potential, and provided cursory numbers on the potential for drained wetland restoration and impoundment reconnection to offset greenhouse gas emissions.
The state-level maps and assessments developed for this workshop were preliminary. However, they highlighted the need for baseline knowledge of the status of available data for Blue Carbon ecosystems. Since 2017 the Coastal Carbon Research Coordination Network has been building open-source datasets for community use. This has resulted in two products. The first is the Coastal Carbon Library, a public GitHub repository where synthetic datasets and workflows for downloading, standardizing, and stitching together data are stored. The second is the Coastal Carbon Atlas, a graphical interface for mapping, subsetting and downloading these data. As far as we know, the Coastal Carbon Library and Atlas serve as the most comprehensive publicly available database of soil cores that exists for blue carbon ecosystems.
Furthermore, at the time of the participatory workshop, no metrics on the quantity, quality and representativeness of soils available for a given state were provided. The goal of this report is to integrate analyses of the Coastal Carbon Library and national coastal wetland mapping products to generate quantitative metrics of soil carbon data availability, quality, and representativeness so that stakeholders can scope new Blue Carbon proposals and policies. We provide in-depth analysis for a subset of coastal U.S. states, as well as addenda describing local mapping efforts that improve upon the national scale products used here, and updated estimates for the potential of coastal wetland restoration activities to reduce or offset greenhouse gas emissions. We hope the results of this report will serve to establish whether coastal U.S. states with policies and programs favorable to Blue Carbon activities have enough existing soil carbon data to support those activities.
There are a few overarching considerations and caveats to this report. The authors are not responsible for how the data in this report is used. We recommend that any new carbon projects be planned with additional site-specific data and monitored before and after the activity in order to quantify whether or not greenhouse gas targets have been met. The Verified Carbon Standard Manual (Emmer et al. 2015) for Blue Carbon is a good resource for anyone interested in starting new carbon projects.