Wetlands are an important natural source of methane to the atmosphere.
The amounts of methane produced vary greatly from area to area,
with temperature, water level and organic carbon content all being
important controlling factors.
Current estimates of global methane emission from wetlands range
between 100-250 million tonnes of methane per year, equivalent
to around a quarter of total global methane emissions.
The process of methane production (methanogenesis) involves the
microbial mineralization of organic carbon under anaerobic conditions
in the waterlogged soil by microorganisms called Archaea. When
water levels fall, as they often do in the Summer months, methane
emission from wetlands can be greatly reduced or even cease completely
as oxygen concentrations rise in the soil.
Through widespread land drainage and changes in land-use, man
has had a great impact on wetland methane emissions in many areas
of the world. Additionally, increased nitrogen deposition from
the atmosphere, again resulting from human activity, may also
greatly increase net methane emissions from wetland areas by inhibiting
any methane consumption (methane oxidation).
Potential for control
The potential for control of methane emission from wetlands lies
largely in land-use policy. Under certain circumstances draining
of wetlands could greatly reduce methane emissions. However, certain
wetland environments, such as peat bogs, have been shown to vastly
increase their emissions of Carbon dioxide in response to such
draining - so offsetting any net cut in GHG emissions. On top
of this there is the problem of habitat destruction, with many
wetland animal, plant and insect species already being endangered.
Overall, decisions as to whether to lower wetland water tables,
and so methane emission, by increased drainage or tree planting
must be made on an individual wetland basis.