Methane changes the equation IN last week's report "Spread ash to turn landfills into power stations", the effectiveness of this method for capturing methane was underestimated by a factor of 2.5, as methane's global warming potential, relative to 1kg carbon dioxide, is 24.5 over 100 years and 62 over 20 years. If global warming is to be taken seriously, the 20-year figure should be used and hence only 112 landfill sites (not 280) would need to be capped to neutralise coal-fired power's CO2 output.  Perhaps coal has a well-deserved place in Britain's energy portfolio after all. This technological development should receive similar funding to that of highly variable wind power.  andrew Porteous emeritus professor of environmental engineering, The Open University