also account for more than 10% of the ocean's total carbon storage. Per hectare, they hold twice as much carbon dioxide as rain forests and can sequester about 27 million tons of CO2 annually. This ability to store carbon is important as atmospheric carbon levels continue to rise. Blue carbon refers to carbon dioxide removed from the atmosphere by the world's coastal marine ecosystems, mostly mangroves, salt marshes, seagrasses and potentially macroalgae, through plant growth and the accumulation and burial of organic matter in the sediment. Although seagrass meadows occupy only 0.1% of the area of the ocean floor, they account for 10–18% of the total oceanic carbon burial. Currently global seagrass meadows are estimated to store as much as 19.9 Pg (petagrams or gigatons, equals a billion tons) of organic carbon. Carbon primarily accumulates in marine sediments, which are anoxic and thus continually preserve organic carbon from decadal-millennial time scales. High accumulation rates, low oxygen, low sediment conductivity and slower microbial decomposition rates all encourage carbon burial and carbon accumulation in these coastal sediments. Compared to terrestrial habitats that lose carbon stocks as CO2 during decomposition or by disturbances like fires or deforestation, marine carbon sinks can retain C for much longer time periods. Carbon sequestration rates in seagrass meadows vary depending on the species, characteristics of the sediment, and depth of the habitats, but on average the carbon burial rate is about 140 g C m−2 yr−1. Coastal protection Seagrasses are also ecosystem engineers, which means they alter the ecosystem around them, adjusting their surroundings in both physical and chemical ways. The long blades of seagrasses slow the movement of water which reduces wave energy and offers further protection against coastal erosion and storm surge. Many seagrass species produce an extensive underground network of roots and rhizome which stabilizes sediment and reduces coastal erosion. Seagrasses are not only affected by water in motion; they also affect the currents, waves and turbulence environment. Seagrasses prevent erosion of the seafloor to the point that their presence can raise the seafloor. They contribute to coast protection by trapping rock debris transported by the sea. Seagrasses reduce erosion of the coast and protect houses and cities from both the force of the sea and from sea-level rise caused by global warming. They do this by softening the force of the waves with their leaves, and helping sediment transported in the seawater to