By Richard Black
Environment correspondent, BBC News website
Methane emissions from landfill sites can be captured and used as fuel
Levels of the greenhouse gas methane in the atmosphere seem to be rising having remained stable for nearly 10 years.
Data from the National Oceanographic and Atmospheric Administration (Noaa) in the US suggest concentrations rose by about 0.5% between 2006 and 2007.
The rise could reflect melting of permafrost, increased industrialisation in Asia or drying of tropical wetlands.
The rise in carbon dioxide levels was significantly higher than the average annual increase for the last 30 years.
Noaa figures show CO2 concentrations rising by 2.4 parts per million (ppm) from 2006 to 2007. By comparison, the average annual increase between 1979 and 2007 was 1.65ppm.
Concentrations now stand at 384 ppm, compared to about 280 ppm before the era of human industrialisation began.
The rise in CO2 is not exceptional compared with the previous few years, but does add more evidence that concentrations are rising faster than they were a decade or so ago.
The methane figure is more interesting, and potentially of more concern.
Concentrations have been more or less stable since about 1999 following years of rapid increases. Industrial reform in the former Soviet bloc, changes to rice farming methods and the capture of methane from landfill sites all contributed to the levelling off.
But the 2007 figure indicates that levels may be on the rise again.
"Looking at the curve, there is a sign that methane is showing some increase," commented Geir Braathen, senior scientific officer with the World Meteorological Organization, who was not involved in the Noaa publication.
"But the mechanism behind that would be uncertain; and it's too early to say if this is the start of a new increase or not.
"We will need several years of increase before we can state that there is a rising trend."
Methane concentrations have shown small rises and falls during the years of stability, but rises have been associated with El Nino conditions which are known to induce more wildfires.
Currently, the world is experiencing La Nina conditions, the opposite of El Nino.
A sustained rise could be due to several reasons. Asia's spectacular industrialisation, reversion to older rice farming techniques, and a drying out of tropical wetlands would all be candidates if the rising trend is confirmed.
Equally possible would be the release of methane from frozen zones of the world, notably the Arctic permafrost, as they warm.
Methane is the second most important gas causing man-made climate change. Each molecule causes about 25 times more warming than a molecule of CO2, but it survives for shorter times in the atmosphere before being broken down.