Richard Black, BBC News 23 May 08;
Higher atmospheric levels of the greenhouse gas methane noted last year are probably related to emissions from wetlands, especially around the Arctic.
Scientists have found indications that extra amounts of the gas in the Arctic region are of biological origin.
Global levels of methane had been roughly stable for almost a decade.
Rising levels in the Arctic could mean that some of the methane stored away in permafrost is being released, which would have major climatic implications.
The gas is about 25 times more potent than carbon dioxide as a greenhouse gas, though it survives for a shorter time in the atmosphere before being broken down by natural chemical processes.
Northern lights
Indications that methane levels might be rising after almost a decade of stability came last month, when the US National Oceanic and Atmospheric Administration (Noaa) released a preliminary analysis of readings taken at monitoring stations worldwide.
Noaa suggested that 2007 had seen a global rise of about 0.5%.
Some stations around the Arctic showed rises of more than double that amount.
One is the station at Mount Zeppelin in Svalbard, north of Scandinavia.
In addition to the long-term monitoring carried out there by Norway and Sweden, a British team has recently started gathering samples and analysing them in a way that could reveal where the methane is coming from.
Methane produced by bacteria contains a high proportion of molecules with the lighter form (isotope) of carbon, carbon-12, rather than the heavier form, carbon-13.
"Anything where bacteria form methane, you get depletion in C-13 because methanogens (the bacteria) preferentially use C-12," said Rebecca Fisher from Royal Holloway, University of London, who has been running the Svalbard experiments.
"The results we have so far imply a predominantly biogenic source," she told BBC News.
The researchers also match methane levels with wind direction, so they can see where the gas is being produced. This analysis also implies a source in the Arctic regions, rather than one further afield such as the additional output from Asia's rapid industrialisation.
Warm and wet
Ed Dlugokencky, the scientist at Noaa's Earth System Research Laboratory (ESRL) who collates and analyses data from atmospheric monitoring stations, agrees that the 2007 rise has a biological cause.
"We're pretty sure it's not biomass burning; and I think 2007 is probably down to wetland emissions," he said.
"In boreal regions it was warmer and wetter than usual, and microbes there produce methane faster at higher temperatures."
Dr Dlugokencky also suggested that the drastic reduction in summer sea ice around the Arctic between 2006 and 2007 could have increased release of methane from seawater into the atmosphere.
A further possibility is that the gas is being released in increasing amounts from permafrost as temperatures rise.
Researchers will be keeping a close eye on this year's data which will indicate whether 2007 was just a blip or the beginning of a sustained rise.
Methane concentrations had been more or less stable since about 1999 following years of rapid increases, with industrial reform in the former Soviet bloc, changes to rice farming methods and the capture of methane from landfill sites all contributing to the levelling off.
In the recent past, concentrations have risen during El Nino events, whereas the world is currently amid the opposite climatic pattern, La Nina.
Solid evidence
An upturn in methane concentrations emissions could have significant implications for the Earth's climatic future.
A sustained release from Arctic regions or tropical wetlands could drive a feedback mechanism, whereby higher temperatures liberate more of the greenhouse gas which in turn forces temperatures still higher.
A particularly pertinent question is whether methane is being released from hydrates on the ocean floor.
These solids are formed from water and methane under high pressure, and may begin to give off methane as water temperatures rise.
The amount of the gas held in oceanic hydrates is thought to be larger than the Earth's remaining reserves of natural gas.
In collaboration with other British institutions, Dr Fisher's team will begin work this summer sampling water near hydrate deposits to look for indications of gas emerging.
