Atmospheric levels of methane, the greenhouse gas which is much more powerful than carbon dioxide, have risen significantly for the last three years running, scientists have warned � leading to fears that a major global-warming �feedback' is beginning to kick in. For some time there has been concern that the vast amounts of methane, or 'natural gas', locked up in the frozen tundra of the Arctic could be released as the permafrost is melted by global warming. This would give a huge further impetus to climate change, an effect sometimes referred to as 'the methane time bomb'. This is because methane is even more effective at retaining the Sun's heat in the atmosphere than carbon dioxide, the main focus of international climate concern for the last two decades. Over a relatively short period, such as 20 years, methane has a global warming potential more than 60 times as powerful as carbon dioxide, although it decays more quickly. Now comes the first news that levels of methane in the atmosphere, which began rising in 2007 when an unprecedented heatwave in the Arctic caused a record shrinking of the sea ice, have continued to rise significantly through 2008 and 2009. Although researchers cannot yet be certain, and there may be non-threatening explanations, there is a fear that rising temperatures may have started to activate the positive feedback mechanism. This would see higher atmospheric levels of the gas producing more warming, which in turn would release more methane, which would produce even further warming, and so on into an uncontrollable 'runaway' warming effect. This is believed to have happened at the end of the last Ice Age, causing a very rapid temperature rise in a matter of decades.