“This threshold is of symbolic and scientific significance and reinforces evidence that the burning of fossil fuels and other human activities are responsible for the continuing increase in heat-trapping greenhouse gases warming our planet,” the WMO continued.

CO2 levels first reached 400 ppm in April 2012, but this is the first time the monthly average passed the threshold. The agency predicts the global annual average CO2 concentration will cross this threshold in 2015 or 2016.

Available at: http://www.un.org/apps/news/story.asp?NewsID=47892#.U4cQQ3KSzSk