Peatlands (also known as peat bogs or simply bogs) are a type of wetland. Peat bogs generally lack tree cover, and the soil is saturated with acidic water. The acidic and anaerobic conditions slow the decay of organic matter that dies within the bog.
Peat is partially-decomposed biomass, primarily vegetation such as mosses and shrubs. Peat is rich in carbon, making it effective for CO2 sequestration, and also making it an effective fuel when combusted for residential or industrial energy. Peat is formed very slowly in peatlands, requiring thousands of years to reach a depth of two to three meters.
As long as peatlands remain saturated with water, peat formation is faster than peat decomposition, and the lands are a net carbon sink. However, when peatlands dry out, the formation of peat stops and peat decomposition accelerates, releasing CO2 into the air. Humans drain peatlands (by digging drainage canals) to make the land more suitable for agriculture. Plantations on drained peatlands emit roughly 55 metric tons of CO2 (equivalent to the annual emissions of 12 cars) per hectare per year from decomposing peat.
Drained peatlands are also susceptible to peat fires. A peat fire occurs when the dried-out peat and other plant matter ignites due to natural or human causes. Peat fires are very difficult to extinguish. They may smoulder in layers of the ground up to a meter deep, making them resistant to weather that would quench surface fires and even allowing the fires started in the summer to survive throughout a wet winter and expand again the following spring. Some peat fires burn for years. Peat fires release large quantities of CO2, as well as particulate pollution that is harmful to human health.
To stop CO2 emissions from peatlands, it is necessary to "rewet" them: to restore them to their natural, waterlogged state. This can be difficult to accomplish, as blocking the drainage canals only tends to rewet peat areas near to those canals. It is more cost-effective to prevent the draining of peatlands in the first place.