The term blue carbon was coined in 2009. At the time, the term was coined to highlight that coastal vegetated ecosystems have a disproportionately large contribution to global carbon sequestration. Others use the term to describe the carbon captured by the entire ocean, not just the coastal ecosystems. The role of blue carbon in climate change mitigation and adaptation has now reached international prominence.
The vegetated coastal ecosystems of tidal marshes, mangroves and seagrasses (which are grouped as "blue carbon") have high carbon burial rates. This is because they accumulate carbon in their soils and sediments.
Such ecosystems can contribute to climate change mitigation and also to ecosystem-based adaptation. However, when coastal blue carbon ecosystems are degraded or lost they release carbon back to the atmosphere.
Mangroves, salt marshes and seagrasses can store carbon and are highly efficient carbon sinks. They capture  from the atmosphere by sequestering the carbon in their underlying sediments, in underground and below-ground biomass, and in dead biomass.
Although vegetated coastal ecosystems cover less area and have less aboveground biomass than terrestrial plants they have the potential to impact long term C sequestration, particularly in sediment sinks.
One of the main concerns with blue carbon is that the rate of loss of these important marine ecosystems is much higher than any other ecosystem on the planet, even compared to rainforests. Current estimates suggest a loss of 2-7% per year, which is not only lost carbon sequestration, but also lost habitat that is important for managing climate, coastal protection, and health.
As habitats that sequester carbon are altered and decreased, that stored amount of C is being released into the atmosphere, continuing the current accelerated rate of climate change. Impacts on these habitats globally will directly and indirectly release the previously stored carbon, which had been sequestered in the sediments of these habitats. Declines in vegetated coastal habitats are seen worldwide.
Quantifying rates of decrease are difficult to calculate, however measurements have been estimated by researchers indicating that if blue carbon ecosystems continue to decline, for any number of reasons, 30-40% of tidal marshes and seagrasses and approximately 100% of mangroves could be gone in the next century.
Reasons for the decline of mangroves, seagrass, and marshes include land use changes, climate and drought related effects, dams built in the watershed, convergence to aquaculture and agriculture, land development and sea-level rise due to climate change. Increases in these activities can lead to significant decreases in habitat availability and thus increases in released C from sediments.
As anthropogenic effects and climate change are heightened, the effectiveness of blue carbon sinks will diminish and CO emissions will be further increased. Data on the rates at which CO is being released into the atmosphere is not robust currently; however, research is being conducted to gather better information to analyze trends. Loss of underground biomass (roots and rhizomes) will allow for CO to be emitted changing these habitats into sources rather than carbon sinks.