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Credit: Mac Stone

Land Use

Coastal Wetlands

Sawgrasses and lilies in the Everglades, a complex region of tropical wetlands that has been drained to make way for sugarcane and development for more than 125 years. Ranking with the Camargue in France and the Pantanal in South America as one of the greatest wetlands in the world, it is a rain-fed grassland on a limestone shelf. Slow-moving water moves through grasses to remove pollutants, making it a highly effective water treatment system. In 2000, following an act of Congress, the most expensive and complex environmental restoration in human history began and continues to this day despite constant political resistance.

Along the fringes of coasts, where land and ocean meet, lie the world’s salt marshes, mangroves, and sea grasses. These coastal wetland ecosystems are found on every continent except Antarctica.

They provide nurseries for fish, feeding grounds for migratory birds, a first line of defense against storm surges and floodwaters, and natural filtration systems that boost water quality and recharge aquifers. Relative to their land area, they also sequester huge amounts of carbon in plants aboveground and in roots and soils below.

Coastal wetlands can store five times as much carbon as tropical forests over the long term, mostly in deep wetland soils. The soil of mangrove forests alone may hold the equivalent of more than two years of global emissions—22 billion tons of carbon, much of which would escape if these ecosystems were lost.

Wetlands face a myriad of threats, but thanks to research and advocacy efforts, awareness is growing about the role they play in curbing climate change and coping with its impacts. It is vital to preserve healthy coastal wetlands—keeping a lid on the carbon they contain—while also rehabilitating and restoring those that already have been degraded. 

References

coastal wetlands [vs.] tropical forests: Boyd, Robynne. “Blue Carbon: An Oceanic Opportunity to Fight Climate Change.” Scientific American. March 10, 2011.

mangrove forests…two years of global emissions: “Editorial: Blue Future.” Nature 529 (2016): 255–256.

one-third of…mangroves…lost: Boyd, “Blue Carbon.”

“sequestration…in ‘blue-carbon’ wetlands”: “Blue Future,” Nature; Ramsar Convention on Wetlands: Ramsar Convention Bureau. The Ramsar Convention Manual and Guide to the Convention on Wetlands, Ramsar, Iran, 1971. Gland: Ramsar Convention Bureau, 2013.

“living shorelines”: Stutz, Bruce. “Why Restoring Wetlands Is More Critical Than Ever,” Yale Environment 360. July 28, 2014; Bilkovic, Donna Marie. Living Shorelines: The Science and Management of Nature-based Coastal Protection. Boca Raton: CRC Press, 2017.

European companies…in Senegal: Bird, Winifred. “African Wetlands Project: A Win for the Climate and the People?” Yale Environment 360. November 3, 2016.

view all book references

Technical Summary

Coastal Wetlands

Project Drawdown defines coastal wetlands as: the legal protection of carbon-rich mangroves, seagrasses, and saltmarshes, leading to reduced degradation rates and the safeguarding of carbon sinks. This solution replaces non-protected coastal wetlands.

Coastal wetlands significantly impact global carbon cycles. 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 wetlands solution proposes increased protection 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 and the Coming Attractions marine permaculture and ocean farming.

Methodology

Total Land Area [1]

The total global area of coastal wetlands is 49 million hectares, of which 7.4 million hectares are already protected. [2] Thus, the total available land for the coastal wetlands solution is 41.6 million hectares. The modeling of the coastal wetlands was based on the individual modeling of mangroves, seagrasses, and salt marshes. The total land area available for this solution was modeled using the annual rate of degradation for mangroves, seagrasses, and salt marshes, i.e. 1.24 percent, 3.41 percent, and 1.50 percent, respectively. Thus, an estimation was made for future degraded and non-degraded mangroves, seagrasses, and salt marshes, and the non-degraded area (which is not yet protected) was considered the available area for protection as part of this solution.

Adoption Scenarios [3]

Five custom adoption scenarios were developed for each of these three coastal wetlands using a linear growth curve. Given the small area of coastal wetlands, the high urgency because of the annual degradation of unprotected coastal wetlands, and the high mitigation efficiency of protection, several scenarios emphasized early peak adoption by 2030.

Impacts of increased adoption of coastal wetlands from 2020-2050 were generated based on three growth scenarios, which were assessed in comparison to a Reference Scenario where the solution’s market share was fixed at the current levels.

  • Plausible Scenario: This scenario results in the protection of 30.5 million hectares of unprotected coastal wetlands.
  • Drawdown Scenario: Adoption is intensified under this scenario based on the most aggressive adoption scenarios, and results in the protection of 33.4 million hectares of unprotected coastal wetlands.
  • Optimum Scenario: Adoption is intensified under this scenario based on the most aggressive adoption scenarios, and results in the protection of 35.3 million hectares of unprotected coastal wetlands.

Achieving 100 percent protection of unprotected coastal wetlands, even under the most aggressive adoption scenarios, was assumed to be unachievable due to the continuous annual degradation of coastal wetlands.

Emissions, Sequestration, and Yield Model

Sequestration rates for mangroves are set at 1.7 tons of carbon per hectare per year, based on meta-analysis of 11 data points from 10 sources. Emissions from degraded or deforested mangroves are set at 31.3 tons of carbon dioxide-equivalent per hectare per year, based on meta-analysis of 15 data points from 8 sources. Salt marsh sequestration rates are 1.8 tons of carbon per hectare per year, based on 20 data points from 10 sources. Emissions from degraded salt marshes are 9.8 tons of carbon dioxide-equivalent per hectare per year, based on 2 data points from 2 sources. Seagrass sequestration is set at 1.1 tons of carbon per hectare per year, based on 8 data points from 5 sources. Emissions from degraded seagrass beds are set at 5.9 tons of carbon dioxide-equivalent per hectare per year, based on 3 data points from 3 sources.

Financial Model

Financials are not modeled, as costs are not necessarily carried out by the landowner or land manager.

Integration [4]

The coastal wetlands 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.

Results

Total adoption in the Plausible Scenario is 30.5 million hectares in 2050, representing 62.2 percent of the total available land. Of this, 23.1 million hectares are adopted from 2020-2050. The emissions impact of this scenario is 3.2 gigatons of carbon dioxide-equivalent reduced by 2050. Total carbon protected is 14.6 gigatons of carbon dioxide-equivalent. Financial impacts are not modeled.

Total adoption in the Drawdown scenario is 33.4 million hectares in 2050, representing 90.2 percent of the total available land. Of this, 26.0 million hectares are adopted from 2020-2050. The impact of this scenario is 4.1 gigatons of carbon dioxide-equivalent by 2050. Total carbon protected is 15.9 gigatons of carbon dioxide-equivalent.

Total adoption in the Optimum scenario is 35.3 million hectares in 2050, representing 72.0 percent of the total available land. Of this, 28.1 million hectares are adopted from 2020-2050. The impact of this scenario is 4.5 gigatons of carbon dioxide-equivalent by 2050. Total carbon protected is 16.8 gigatons of carbon dioxide-equivalent.

Discussion

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). The Drawdown model shows a maximum annual reduction of 0.14-0.18, relatively close to this benchmark.

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.


[1] To learn more about the Total Land Area for the Land Use Sector, click the Sector Summary: Land Use link below.

[2] Based on meta-analysis of protection rates from 6 sources.

[3] To learn more about Project Drawdown’s three growth scenarios, click the Scenarios link below. For information on Land Use Sector-specific scenarios, click the Sector Summary: Food link.

[4] For more on Project Drawdown’s Land Use integration model, click the Sector Summary: Food link below.

Full models and technical reports coming in late 2017.

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