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Peat (), sometimes known as turf (), is an accumulation of partially decayed vegetation or organic matter. It is unique to natural areas called peatlands, bogs, mires, moors, or muskegs. The peatland ecosystem covers 3.7 million square kilometres and is the most efficient carbon sink on the planet, because peatland plants capture CO2 naturally released from the peat, maintaining an equilibrium. In natural peatlands, the "annual rate of biomass production is greater than the rate of decomposition", but it takes "thousands of years for peatlands to develop the deposits of 1.5 to 2.3 m [4.9 to 7.5 ft], which is the average depth of the boreal [northern] peatlands", which store around 415 gigatonnes of carbon (about 46 times 2019 global CO2 emissions). Sphagnum moss, also called peat moss, is one of the most common components in peat, although many other plants can contribute. The biological features of Sphagnum mosses act to create a habitat aiding peat formation, a phenomenon termed 'habitat manipulation'. Soils consisting primarily of peat are known as histosols. Peat forms in wetland conditions, where flooding or stagnant water obstructs the flow of oxygen from the atmosphere, slowing the rate of decomposition.
Peatlands, particularly bogs, are the primary source of peat; although less-common wetlands including fens, pocosins, and peat swamp forests also deposit peat. Landscapes covered in peat are home to specific kinds of plants including Sphagnum moss, ericaceous shrubs, and sedges (see bog for more information on this aspect of peat). Because organic matter accumulates over thousands of years, peat deposits provide records of past vegetation and climate by preserving plant remains, such as pollen. This allows the reconstruction of past environments and study changes in land use.
Peat is harvested as a source of fuel in certain parts of the world. By volume, there are about 4 trillion cubic metres (5.2 trillion cubic yards) of peat in the world, covering a total of around 2% of the global land area. Over time, the formation of peat is often the first step in the geological formation of fossil fuels such as coal, particularly low-grade coal such as lignite.
The Intergovernmental Panel on Climate Change (IPCC) classifies peat as neither a fossil fuel nor a renewable fuel, and notes that its emission characteristics are similar to fossil fuels. At 106 g CO2/MJ, the carbon dioxide emission intensity of peat is higher than that of coal (at 94.6 g CO2/MJ) and natural gas (at 56.1) (IPCC). Peat is not a renewable source of energy, due to its extraction rate in industrialized countries far exceeding its slow regrowth rate of 1 mm per year, and as it is also reported that peat regrowth takes place only in 30–40% of peatlands.
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