Climate questions: Have methane levels stabilised?
Methane is a powerful greenhouse gas with a warming potential approximately 25 times greater than carbon dioxide, integrated over a 100 year time frame - this means that a tonne of methane released now will have the same warming impact over the next 100 years as 25 tonnes of carbon dioxide released now.
Methane has increased from a pre-industrial level of about 715 parts per billion molar (ppb) to approximately 1 775 ppb in 2005, up to 1790 ppb by the end of 2008.
Ice cores reveal this concentration far exceeds the natural range (320-790 ppb) for the last 800 000 years.
Why are methane levels changing?
This significant increase is attributed to emissions from human activities including agriculture, burning fossil fuels and landfills.
Increases in methane can also be associated with positive and negative feedback processes.
As the planet warms, changes such as warmer wetlands, melting permafrost soils and increased fires can all release stores of methane.
The drying of wetlands increases the soil methane sink.
Steep increase in methane levels
Between 1999 and 2007 there was essentially no growth in the mean annual atmospheric concentrations of methane, compared to an eight to nine per cent rise over the preceding 20 years (Figure 2).
However, in 2007 a steep rise in methane levels was detected at all monitoring stations in both the northern and southern hemispheres.
This is interesting, because the majority of methane emissions come from the northern hemisphere and it takes more than a year for the gases to mix between the two hemispheres (Rigby et al. 2008).
North and south
While it is likely that very warm summer conditions over Siberia in 2007, and perhaps again in 2008, could have caused an increase in bacterial methane emission from boreal wetlands, the timing of the southern hemisphere increase suggests tropical sources and/or sinks must also be involved.
Rigby et al. (2008) examined the possibility that there was a decrease in hydroxyl free radicals, which destroy methane, largely in the tropical atmosphere.
This theoretical study showed that if there had been a drop in the level of hydroxyl free radicals, the required global methane emissions to reproduce the atmospheric observations would have been smaller and less strongly biased to the northern hemisphere.
At present, it is still uncertain whether such a drop in hydroxyl free radical concentrations occurred.
Dlugokencky et al. (2009) suggest that increased tropical rainfall (La Nina driven) may be stimulating tropical wetland emissions, and this could be the source of enhanced methane emissions.
Methane growth anomalies
Because the atmospheric methane growth rate is now showing signs of dropping in 2009 (Rigby et al., 2009; Dlugokency et al., 2009), it is now likely that this steep increase in methane growth in 2007 and 2008 does not represent a return to long-term sustained methane growth.
This current event appears to be yet another methane growth rate anomaly, somewhat smaller than the last major anomaly in 1997-1998, which was driven by El Nino-related tropical and boreal biomass burning.
Thus the renewed concern expressed by Mascarelli (2009) that the Earth’s extensive methane stores are possibly destabilising is not supported by the atmospheric evidence to date.
Unstable frozen methane
Nevertheless there are two areas of potential destabilisation that are of particular concern, including methane frozen beneath the sea floor and trapped in frozen soils.
Below the sea floor, methane gas is frozen in the pores of sediment known as hydrates or clathrates.
The Siberian Shelf alone is thought to contain 1 400 billion tonnes of methane gas, equivalent to about twice as much carbon as contained in all the trees, grasses and flowers on the planet.
This methane is stored at high pressure and if it becomes destabilised massive quantities of methane could be released very quickly.
While observed plumes could indicate methane is already escaping from below the sea floor, it is not known whether this process has been initiated recently or has been occurring for a long time.
If just one per cent of the Siberian Shelf methane escaped within a few decades, it would be enough to cause abrupt climate change.
Methane in permafrost
Extensive deposits of methane are also found in the Arctic's frozen soils, known as permafrost.
Permafrost in the Northern Hemisphere is thought to contain some 950 billion tonnes of carbon in the form of methane.
Much of this methane is released through bubbling in thaw lakes. Research conducted in the northern part of Siberia found that not only was the methane emissions from thaw lakes in the area five times higher than previously estimated, but Pleistocene age (35 260 to 42 900 year old) methane was being released (Walter et al., 2006).
This finding indicates a positive feedback, where warming of the climate is releasing old carbon stocks that were previously stored in permafrost.
However the atmospheric data do not support the hypothesis that this is now a recent, significant methane source.
The use of radiocarbon data to identify the source of additional methane during past shifts in climate suggests that much of the new methane that sustained warming came initially from wetlands (Petrenko et al., 2009; Nisbet and Chappellaz 2009).
With very rapid wetland responses playing a major role in driving methane increases in the past, it is likely that a similar response will occur with anthropogenic warming, especially in the Arctic.
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