257chemical carbon cycle but also in the cycles of many inor-
ganic elements. CH 4 is therefore at the center of scientific
and environmental concerns but also of societal ones. In the
context of climate change, CH 4 is one of the several green-
house gases responsible for increased radiative forcing on
the climate system. Atmospheric CH 4 concentration has
increased by about 1000 p.p.m. (parts per million) since the
beginning of the industrial era representing the fastest
changes in CH 4 atmospheric concentrations over the last
80,000 years (IPCC 2007 ). Despite a short lifetime in the
atmosphere of approximately 8 years, CH 4 is 25 more potent
than carbon dioxide (CO 2 ) as a greenhouse gas over a 100-
year horizon due to its higher efficiency in trapping radiation
(Shindell et al. 2009 ; Nazaries et al. 2013 ). Whereas atmo-
spheric CH 4 has stabilized at a value of 1.77–1.78 p.p.m
since 2005, an increase in the atmospheric concentration of
CH 4 to 2.55 p.p.m. is predicted by 2050 (Lelieved et al. 1998 ;
Nazaries et al. 2013 ).
16.2.1.2 Budget and Sources of Methane^1
The global budget of atmospheric CH 4 is on the order of
500–600 Tg.yr−1 (Lelieveld et al. 1998 ; Wang et al. 2004 ;
Conrad 2009 ). CH 4 is emitted from a range of natural and
anthropogenic (relating to human activity) sources as a result
of the anaerobic decomposition of organic matter, land use
changes and fossil fuel emissions. Whereas much attention is
currently focused on the anthropogenic sources of the green-
house gases, there is ample evidence that emissions of these
gases from natural sources have also changed over time
(EPA 2010 ). Natural sources of CH4 are estimated to pro-
duce 37 % of the total CH4 flux into the atmosphere every
year. The largest source of natural CH4 emissions is natural
wetlands (170 Tg CH4.yr−1). Several other sources contrib-
ute substantially as well, including geologic emissions (42 to
64 Tg CH4.yr−1), lakes (10–50 Tg CH4.yr−1), and vegetation
(20 to 60 Tg CH4.yr−1) (EPA 2010 ). About 70 % of all CH 4
formation is the result of microbial processes showing that
most of the atmospheric CH 4 originates from microbial
metabolism.
16.2.1.3 Methane Sinks
The global CH 4 sources are balanced by sinks of similar
magnitude. The largest sink (>80 % of the total) is the photo-
chemical oxidation of CH 4 initiated by the reaction with •OH
radicals (Cicerone and Oremland 1988 ). The loss of CH 4 in
the stratosphere accounts for ~ 7 % of the sink and finally
CH 4 is also eliminated from the atmosphere uptake in upland
soils due to microbial oxidation (~5 % of the sink, Conrad
2009 ). Noteworthy that rates of CH 4 production are much
larger than rates of CH 4 emission since a large fraction (50–
90 %) of the initially produced CH 4 is oxidized by microor-
ganisms and does not reach the atmosphere (Frenzel 2000 ;
Reeburgh 2003 ; Kvenvolden and Rogers 2005 ).16.2.2 Freshwater Lakes as Methane SourcesLakes and ponds (excluding impoundments and reservoirs)
are natural sources of CH 4. They contribute substantially to
CH 4 emissions, although analysis of this source has been
limited to date. Current emissions to the atmosphere are esti-
mated to 10–50 Tg CH 4 .yr−1 but one key uncertainty involves
the total surface area of lakes and ponds (for comparison the
oceans emit 9 Tg CH 4 .yr−1, EPA 2010 ). The total lake surface
area is estimated to 0.9 % of the Earth surface; although the
number and total area of large lakes is well known, the num-
ber and total area of small lakes and ponds is not. It is esti-
mated that there are about 300 × 10^6 natural lakes and ponds
worldwide, 90 % of which are smaller than 1 ha. Because
small lakes and ponds generally emit more CH 4 per unit area
than large lakes, a major uncertainty in the global estimation
of CH 4 emission relies on the precise estimation of the total
lake surface area (EPA 2010 ). Contribution of freshwater
lakes to CH 4 emissions is thus probably underestimated.
Climate warming impacts on permafrost and the develop-
ment of thermokarst lakes could substantially affect future
CH 4 emissions from lakes. A long-term decline of emissions
from lakes north of 45°N should be observed due to lake area
loss and permafrost thaw. But, a period of increase of CH 4
emissions associated with thermokarst lake development in
the zone of continuous permafrost will precede this decline.
CH 4 emission rates from northern lakes should rise to 50 to
100 Tg CH 4 .yr−1 during this transitional period, which would
last hundreds of years (EPA 2010 ).16.2.3 Methane Cycle in Freshwater Lakes^2CH 4 cycling in lakes contributes to 6–16 % of the non-
anthropogenic emissions of this gas. It is therefore poten-
tially important for the present global CH 4 budget (Bastviken
et al. 2004 ). CH 4 emissions are dependent on the relative
rates of CH 4 production and consumption, but also on the
dominant pathways for its transport to the water-atmosphere
interface.16.2.3.1 Biological Production of Methane
(Methanogenesis)
CH 4 is a major product of carbon metabolism in lakes and is
produced by the activity of CH 4 -generating microbes (i.e.,(^1) For detailed informations see Conrad 2009 ; EPA 2010.^2 For more details, see Bastviken et al. 2004 , 2008 , 2009.
16 Methanogens and Methanotrophs in Lake Pavin
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