Coastal ecosystems play a significant role in global carbon cycles, capturing and storing substantial amounts of carbon, often referred to as “blue carbon.” This carbon is sequestered within the living organisms and sediments of these unique environments, highlighting their importance in maintaining atmospheric balance.
Coastal Ecosystems and Their Carbon Storage
Coastal ecosystems, such as mangroves, salt marshes, and seagrass beds, are highly effective at storing carbon. Mangroves are trees and shrubs found in tropical and subtropical coastal areas, while salt marshes consist of salt-tolerant grasses and herbs thriving between land and saltwater. Seagrass beds are flowering plants that grow in salty marine environments, forming extensive underwater meadows. These ecosystems collectively cover a relatively small global area, yet they disproportionately contribute to carbon sequestration.
These environments store carbon primarily through the accumulation of organic matter in their waterlogged soils and sediments. Plants in these ecosystems absorb carbon dioxide through photosynthesis, and when they die, their organic material gets buried in oxygen-poor conditions. This anoxic environment slows decomposition, allowing carbon to accumulate over centuries or even millennia. The density of carbon stored in these deep, water-logged soils can be many times greater per acre than in terrestrial forests, making them significant carbon sinks.
Quantifying Carbon Emissions from Degradation
Degraded coastal ecosystems release substantial amounts of stored carbon into the atmosphere. Experts estimate that the conversion and degradation of these ecosystems annually release between 0.15 and 1.02 billion tons of carbon dioxide, with a central estimate around 0.45 billion tons. These emissions are equivalent to 3% to 19% of those from global deforestation.
Mangrove deforestation, for instance, contributes approximately 0.02 gigatons (20 million tons) of carbon annually, accounting for up to 10% of global deforestation emissions despite mangroves covering less than 1% of tropical forest area. Seagrass degradation globally could lead to emissions of up to 0.65 gigatons (650 million tons) of carbon dioxide each year, comparable to the annual emissions of the entire global shipping industry. Salt marsh losses from 2000 to 2019 are estimated to have resulted in net global emissions of 16.3 teragrams of carbon dioxide, equivalent to the annual emissions from about 3.5 million motor vehicles. Major causes of this degradation include conversion for agriculture and aquaculture, coastal development, and pollution.
How Degradation Releases Stored Carbon
The degradation of coastal ecosystems leads to carbon release through specific physical and biological processes. When these waterlogged environments are drained, cleared, or otherwise disturbed, the previously anoxic (oxygen-depleted) soils become exposed to oxygen. This exposure alters the chemical environment within the sediment, allowing aerobic microbes to become active.
These microorganisms rapidly decompose the accumulated organic matter, which had been preserved for extended periods due to the lack of oxygen. The rapid microbial decomposition converts the stored organic carbon into carbon dioxide and, in some cases, methane, which are released into the atmosphere. Furthermore, the loss of vegetation cover eliminates the ongoing natural process of carbon sequestration that these ecosystems provide. The removal of plants also exposes the carbon-rich sediments to erosion and further disturbance, accelerating the release of stored carbon that would otherwise remain sequestered for centuries.
Global Implications and Restoration Efforts
The carbon emissions from degraded coastal ecosystems significantly contribute to atmospheric carbon dioxide concentration and global warming. This undermines climate change mitigation efforts and highlights the link between coastal health and global climate stability. The impacts extend beyond climate, affecting biodiversity, water quality, and coastal protection.
Recognizing these implications, significant efforts are underway to conserve and restore “blue carbon” ecosystems. Initiatives focus on protecting existing healthy mangroves, salt marshes, and seagrass beds to prevent further carbon release and maintain their sequestration capacity. Active restoration strategies include replanting mangroves, re-establishing salt marshes, and transplanting seagrass shoots into degraded areas. These actions not only reduce emissions by stabilizing stored carbon but also enhance the natural ability of these ecosystems to absorb more carbon from the atmosphere, providing a dual benefit for climate change mitigation.