In the face of global warming, coastal wetlands—including submerged seagrasses, mangrove forests, and salt marshes, and pelagic ecosystems—provide a vital service to the global community by storing large amounts of carbon.
Coastal carbon, or blue carbon, is carbon captured by living organisms in oceans and coastal habitats that is stored in the form of biomass and sediments. The secret of blue carbon lies in the soil. Of course, they also have an important living biomass, especially mangroves, but coastal wetlands, unlike almost any other ecosystem, have thick, waterlogged organic muds. As it falls to the ground, dead plant matter is trapped by the wetlands’ complex root system. There is little or no oxygen available to break it down and so it accumulates in a rich peat. Unlike freshwater peats, however, the saline waters in coastal wetlands also prevent bacterial breakdown which would otherwise lead to the release of methane—itself a powerful greenhouse gas.
The carbon rich soil remains tightly packed, layer upon layer, out of circulation in the soil, sometimes for thousands of years. These systems also store external carbon as coastal ecosystems act as sediment traps for runoff from terrestrial systems. Once trapped in the wetland, they too are buried in the soil. Intact coastal habitats have soils that range in depth from less than one meter to over ten meters, storing hundreds to thousands of metric tons of carbon per hectare.
The high rate of loss creates twin problems—first, they expose us to large amounts of a carbon being released from the biomass and the soil, accelerating global warming. Secondly we are losing some of the only effective carbon scrubbers on the planet. Coastal habitats loss is estimated to be between 0.5–3 percent of their global area per year resulting in 0.15–1.02 billion metric tons of CO2 released annually (equivalent to burning 112 billion gallons of gasoline). This loss is largely due to human causes of conversion related to coastal development, aquaculture and agriculture.
While only occupying 13.8 million hectares of tropical coastlines, per hectare mangroves are among the most carbon-rich forests in the world. Similar to terrestrial forests, mangroves capture carbon from the atmosphere via photosynthesis, and return some to the atmosphere through respiration and oxidation. The remaining carbon is stored in living biomass such as leaves, branches and roots. Through this process, carbon is stored in biomass for relatively shorter time scales of years to decades.
The true potential for mangrove climate mitigation lays in the soil, where, undisturbed, the carbon can remain stable for centuries or more. It is estimated the average age of 1.5 m of sediment in mangrove forest in Brazil to be between 400 and 770 years old.
Mapping Ocean Wealth: Mangrove Biomass
We know, mangroves are an efficient carbon store, but we also know that not all mangroves are equal in this respect. Desert mangroves can be little more than shrubs, while in the wet tropics mangroves form vast forests with canopies 30-40 meters high. From a carbon-storage perspective, the latter are of particular importance, and knowing how much biomass is stored and where can be critical for optimizing efforts to achieve maximum returns on investment in conservation.
Working with partners in the University of Cambridge, we conducted a review of all the available science and developed a model describing the relationship between mangrove biomass and climate (temperature, rainfall and seasonality).
The results of the study confirmed that mangroves are a biomass powerhouse, but it also called out the most valuable areas of all. This knowledge can be translated into action. For the countries with high biomass value such as Indonesia, Brazil and Nigeria, including mangroves in national policies could yield beneficial results in offsetting a certain amount of their greenhouse gas emissions. For smaller island countries, like Cuba and Solomon Islands, mangroves may represent a significant portion of their total forest. When thinking about ways to finance conservation, market-based mechanisms may provide a significant opportunity as they represent one of the islands’ large assets.
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Mangrove Soil Carbon
More than 90% of the total carbon stock in mangrove forests can be found in the soil; however, to date, direct measurements of carbon storage have not been conducted in the majority of nations where mangroves occur. The Woods Hole Research Center collaborated with The Nature Conservancy to develop a global model of carbon stored in mangrove soil. The researchers developed a machine-learning based model to predict soil carbon storage based on climatic, vegetation, topographic and hydrologic properties that can be inferred from satellite data. Applying these data to maps of known mangrove distribution, they were able to estimate the carbon storage value of any mangrove in the world. Using this method, the study revealed that mangrove soils hold more than 6.4 billion tons of carbon globally. The paper can be accessed here, and preliminary study data can be viewed here, on the Mapping Ocean Wealth Explorer.
Seagrass and Saltmarshes
Lacking the stature of forests, intertidal saltmarshes and sub-tidal seagrasses were left out of carbon models for years, but ecologists have long known that they were among the most productive ecosystems on earth. Increasingly, studies are also finding deep layers of centuries-old, carbon-rich soil beneath these habitats. Despite their importance, however, we still lack reliable global maps of both saltmarsh and seagrass worldwide. Accordingly, estimates of their extent range widely from 200,000 to 1,000,000 square kilometers. With this uncertainty, we are unable to accurately predict their global role in carbon cycles and climate change, but research at local scales are impressive and should spark further study. For example, the average hectare of seagrass stores 139.7 metric tons of carbon in its soil.
Top image: ©Ami Vitale. Photo Credits: © Tim Calver, Mark Godfrey