Climate Central : “April will be the first time in human history where levels of atmospheric carbon dioxide were higher than 400 parts per million for an entire month, one scientist who monitors the levels said. And they could stay above that mark into July.”  

“Carbon dioxide concentrations, as measured at a site atop Hawaii’s Mauna Loa volcano since 1958, surpassed the 400 ppm mark for the first time in recorded history on May 9, 2013. While the particular mark is symbolic , it serves to show how far concentrations have risen from their pre-industrial levels of 280 ppm as fossil fuels such as coal and oil have continued to be burned.”  

Crossposted at Wonk Wire.