Global wetlands appear to contribute about 75% of natural methane emissions, relatively larger in peat-rich wetlands. Since most of the methane produced in organic soils of wetlands is oxidized within 20 cm of the water table, changing water tables can dramatically alter the role of wetlands as large source or sink. Past drainage of temperate swamps may already have reduced global emissions from these sources, and additional drainage of swamps or development of drier conditions due to global warming could cause further reductions and even add new sinks.
About 70% of the estimated 510 Mt of methane released into the atmosphere each year comes from natural and human-induced surface biological processes, about 20% from the escape of natural gas from fossil fuel sources, and 10% from biomass burning.