Coastal Wetland Restoration
Project Drawdown defines coastal wetland restoration as “any process that aims to return a system to a preexisting condition (whether or not this was pristine) (sensu Lewis, 1990c)” including both “natural restoration” or anthropogenic- led recovery (Lewis, 2005) of carbon-rich mangroves, seagrasses, and salt marshes. This solution recovers coastal wetlands ecosystems capacity as carbon sinks. Coastal wetlands significantly impact global carbon cycles and their disturbance contributes to an estimated 1–10 percent of anthropogenic carbon emissions. Unlike most terrestrial ecosystems, coastal wetlands can continue sequestering carbon for centuries without becoming saturated. As a result, they have accumulated vast stores of carbon, making their global significance high despite their small area. Yet, these ecosystems are being degraded rapidly due to human activity, and relatively few are protected.
Coastal wetlands also provide important ecosystem services. These ecosystems are being replaced for other uses, including coastal development and agriculture, releasing their stored carbon and preventing future sequestration. The coastal wetland restoration solution proposes increased restoration of these important carbon reservoirs, with mitigation impact via emissions reduction and biosequestration.
The oceans are the world's largest carbon sink, yet their climate mitigation potential has received little attention. Coastal wetland protection is one component of a “blue carbon” strategy, which also includes restoration of coastal wetlands, marine permaculture, and ocean farming.
Total Land Area
The total global area of coastal wetlands is 7.32 million hectares, while current adoption was set to 0.
Six custom adoption scenarios were developed for each of these three coastal wetlands using a linear growth curve based on the Guatemala’s National Restoration Strategy, which aims to restore 10,000 hectares on mangroves between 2015 and 2045, and Green India Mission, which included a commitment to restore 100,000 hectares of mangroves between 2010 and 2020. In the absence of similar information for the seagrasses and salt marshes, the adoption of these coastal wetlands is projected based on mangroves' growth rate, which is applied to their respective total land area and degradation rate. Given the small area of coastal wetlands and the high mitigation efficiency of restoration, some scenarios emphasized early peak adoption by 2030.
Impacts of increased adoption of coastal wetland restoration from 2020 to 2050 were generated based on two growth scenarios. These were assessed in comparison with a Reference Scenario in which the solution’s market share was fixed at the current levels.
- Scenario 1: This scenario results in the protection of 6.07 million hectares of unprotected coastal wetlands.
- Scenario 2: Adoption is intensified under this scenario based on the most aggressive adoption scenarios, and results in the protection of 7.24 million hectares of unprotected coastal wetlands.
Emissions, Sequestration, and Yield Model
Sequestration rates for mangroves are set at 6.58 tons of carbon per hectare per year, based on meta-analysis of 27 data points from 13 sources. Salt marsh sequestration rates are 0.93 tons of carbon per hectare per year, based on 15 data points from six sources. Seagrass sequestration is set at 0.99 tons of carbon per hectare per year, based on 16 data points from four sources.
Financials are not modeled, as costs are not necessarily carried out by the landowner or land manager.
The coastal wetland restoration solution was not affected by integration with other solutions, as coastal wetlands are largely distinct from terrestrial land uses. An exception is mangroves, which were included in the “forest” land use model but were given highest priority and were therefore not limited by any other forest land use.
Total adoption in Scenario 1 is 6.07 million hectares in 2050, representing 83 percent of the total available land. Of this, 6.07 million hectares are adopted from 2020 to 2050. The emissions impact of this scenario is 0.99 gigatons of carbon dioxide-equivalent reduced by 2050. Financial impacts are not modeled.
Total adoption in Scenario 2 is 7.24 million hectares in 2050, representing 99 percent of the total available land. Of this, 7.24 million hectares are adopted from 2020 to 2050. The impact of this scenario is 1.01 gigatons of carbon dioxide-equivalent by 2050.
The International Union for the Conservation of Nature and The Nature Conservancy report that if costal wetland destruction was halved, it would result in an annual emissions reduction of 0.23 gigatons of carbon dioxide per hectare per year (Herr and Landis, 2016). Griscom et al. (2017)’s “Natural climate solutions” calculate 0.18–0.30 gigatons of carbon dioxide equivalent per year in 2030. The Project Drawdown coastal wetland protection and coastal wetland restoration model shows a maximum annual reduction of 0.03–0.06 gigatons in 2030. The lower impact could be attributed to the lower degradation rate in the Project Drawdown models, where multiple sources citing the same degradation rate are excluded from the analysis in order to avoid double counting.
Net costs and savings should be calculated for future upgrades of this solution. More data on salt marsh and seagrass sequestration and emissions rates are also needed. It would be also useful to model restoration in addition to protection.
Like peatlands, coastal wetlands harbor a disproportionate share of the world's stored carbon. They also provide critical ecosystem services. Thus, aggressive efforts to protect these ecosystems are an important component of climate change mitigation efforts.
Note: August 2021 corrections appear in boldface.