Reducing greenhouse gas (GHG) emissions globally will require a multifaceted approach. Conserving, restoring, and managing natural habitats such as forests, grasslands, and wetlands is one strategy that can help moderate emissions and slow the rate of climate change. Coastal wetlands—including salt marshes, scrub-shrub, tidal forested wetlands, and submerged aquatic vegetation (SAV) such as seagrasses—are especially efficient at sequestering (capturing and storing) atmospheric carbon dioxide in their roots and soils. This stored carbon is known as “blue carbon.”
North Carolina officials sought to better understand and harness the carbon sequestration benefits of their state’s natural habitats, including coastal wetlands. So, beginning in 2021, the state embarked on a process to develop a greenhouse gas inventory (GGI) for its coastal wetlands. The resulting blue carbon GGI, finished in 2023, is among the first in the world to include not only above-water habitats such as salt marshes—often called “emergent wetlands”—but also SAV habitats, particularly seagrass beds.
Seagrasses are found along most coastlines in the United States and occur naturally in a range of salinities, from ocean water to estuaries. And because they grow in water-saturated, low-oxygen soil, seagrasses form dense root structures that capture two to five times as much carbon as their aboveground vegetation.1 These entangled networks create ideal conditions for carbon from other marine sediments to settle in the meadows: As much as 50% of carbon stored in seagrass soils comes from sources other than the seagrasses themselves.2
Worldwide, however, seagrasses are under threat from development and degraded water quality, and their degradation or loss can lead to stored carbon being rapidly released back into the environment. In some parts of North Carolina, seagrass beds are declining faster than the global average of 1.5% a year.3
Nevertheless, the state still has the most seagrass on the Eastern Seaboard of the U.S., and with the GGI data, it will be better able to conserve those seagrass meadows, restore others, and sequester even more carbon. The state’s innovative GGI process and approach for coastal wetlands—particularly the inclusion of seagrass—offer a model for other regions and states interested in incorporating coastal blue carbon into their climate mitigation efforts.
In 2018, North Carolina Governor Roy Cooper issued an executive order that led to the creation of the Natural and Working Lands (NWL) Action Plan two years later.4 The plan, which represents the state’s most comprehensive efforts to date to address its vulnerability to climate change, identifies opportunities to help conserve and manage the state’s coastal wetlands and maximize their benefits for carbon sequestration, climate resilience, and the state’s economy. Although the plan recognizes coastal wetlands for their significant per-unit-area carbon storage, a lack of available data precluded their inclusion in the state’s 2018 GGI update
Although the United Nation’s Intergovernmental Panel on Climate Change (IPCC) has issued guidance for developing GGIs of vegetated coastal wetlands such as seagrasses, most jurisdictions have not included these habitats in their GGIs, largely because of challenges in mapping underwater habitats and a lack of studies of U.S. seagrass carbon.5 (Even the U.S. Environmental Protection Agency’s [EPA’s] Inventory of U.S. Greenhouse Gas Emissions and Sinks, commonly known as the national greenhouse gas inventory [NGGI], the country’s primary source of greenhouse gas data, does not include seagrasses.6 As a result, regions and states aiming to capture the blue carbon in these habitats, including North Carolina, have had to develop their inventories from the ground up.
In 2021, North Carolina officials involved in the NWL plan appointed a blue carbon work group to enact the recommendations. The work group modeled the state’s seagrass inventory on the IPCC’s “2013 Supplement to the 2006 IPCC Guidelines for National Greenhouse Gas Inventories: Wetlands” and the NGGI to ensure consistency with national reporting and to make inclusion of the states’ data easier for EPA officials when they update the NGGI to add seagrasses.7
Although the state’s coastal habitat mapping provided the most up-to-date information on the extent of seagrass beds, it still lacked data on low-salinity SAV and carbon storage rates. To compensate, the work group included only high-salinity SAV in the inventory and used the IPCC’s “default values” for carbon storage, which are globally accepted aggregated data that can be substituted when regional or state-specific information is not yet available.8 Additionally, the work group convened two workshops with seagrass mapping and blue carbon experts from federal and state agencies, academic institutions, and nonprofits to develop methods for dealing with other missing or inexact data.9
Those efforts yielded information from various other sources, including:
After identifying the necessary data sources, the work group created North Carolina’s first GGI for seagrasses by:
The work group finalized North Carolina’s seagrass GGI in September 2023. It showed that seagrasses along the state’s coast stored more carbon than they emitted between 2007 and 2021 but also that annual removals of carbon had slightly decreased over that period because of the loss of seagrass acreage. In 2021, the state’s seagrasses sequestered approximately 55.14 kilotons of carbon dioxide equivalent in the soils alone, comparable to removing roughly 12,270 automobiles from the state’s roads for one year.16
The seagrass inventory will be incorporated into the next update of North Carolina’s statewide GGI, which is scheduled to be released in January 2024.
The seagrass GGI process and results also uncovered research gaps and needs that the state can address in future updates, including:
Despite these questions, North Carolina’s work highlights the importance of protecting seagrasses and other coastal wetlands, as well as the role conservation and restoration can play in advancing state climate goals. North Carolina’s efforts give other coastal states a model for developing estimates of the carbon stored in their seagrass habitats that maximize the values of wetlands GGIs for land-use managers and scientists, even in the absence of perfect data.