Peat (turf) is an accumulation of partially decayed vegetation or organic matter that is unique to natural areas called peatlands, bogs, or mires. The peatland ecosystem is the most efficient carbon sink on the planet because peatland plants capture the CO2 which is naturally released from the peat, thus 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 peatlands". One of the most common components is Sphagnum moss, although many other plants can contribute. Soils that contain mostly peat are known as histosols. Peat forms in wetland conditions, where flooding obstructs flows of oxygen from the atmosphere, slowing rates of decomposition. Peatlands, also known as mires, particularly bogs, are the most important source of peat, but other less common wetland types also deposit peat, including fens, pocosins, and peat swamp forests. Other words for lands dominated by peat include moors or muskegs. Landscapes covered in peat also have specific kinds of plants, particularly Sphagnum moss, ericaceous shrubs, and sedges (see bog for more information on this aspect of peat). Since organic matter accumulates over thousands of years, peat deposits also provide records of past vegetation and climates stored in plant remains, particularly pollen. Hence, they allow humans to reconstruct past environments and changes in human land use. Peat is harvested as an important source of fuel in certain parts of the world. By volume, about 4 trillion cubic metres (5.2 trillion cubic yards) of peat are in the world, covering a total of around 2% of global land area (about 3 million square kilometres or 1.2 million square miles), containing about 8 billion terajoules of energy. Over time, the formation of peat is often the first step in the geological formation of other fossil fuels such as coal, particularly low-grade coal such as lignite. Depending on the agency, peat is not generally regarded as a renewable source of energy, as its extraction rate in industrialized countries far exceeds its slow regrowth rate of 1 mm per year (0.039 in), and as peat regrowth is also reported to take place in only 30-40% of peatlands. Because of this, the United Nations Framework Convention on Climate Change (UNFCCC), and another organization affiliated with the United Nations classifies peat as a fossil fuel. However, the Intergovernmental Panel on Climate Change has begun to classify peat as a "slow-renewable" fuel. This is also the classification used by many in the peat industry. 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). Peat fires have been responsible for some large public health disasters, including the 1997 Southeast Asian haze.

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