Reuters: Levels of carbon dioxide, the main greenhouse gas emitted by burning fossil fuels, hit a record high in the atmosphere in 2006, accelerating global warming, the World Meteorological Organization (WMO) said on Friday. But concentrations of methane, the number two heat-trapping gas, flattened out in a hint that Siberian permafrost is staying frozen despite some scientists’ fears that rising temperatures might trigger a runaway thaw. "In 2006, globally averaged concentrations …