Peatland distribution is closely associated with climatic conditions. Peat accumulation is a consequence of low decomposition rates rather than high biologic productivity, which are caused by water-logged conditions found in peatlands and poorly drained environments. Few organisms responsible for plant decomposition can live in water saturated soils because of lack of air and oxygen or anaerobic conditions. These environments are found under climatic regimes characterized by an annual water surplus that depends on the balance between precipitation and evaporation. Peatland distribution is limited northward by low precipitation and southward by a high evaporation rate. Most of temperate and boreal regions of the northern hemisphere offer favourable conditions for peatland development. They have cold and wet winters that compensate for summer evaporation.