Climate Central: A new carbon dioxide milestone has been reached according to the World Meteorological Agency. Average carbon dioxide measurements at all monitoring stations in the northern hemisphere were above 400 parts per million for the month of April, the first time that's been recorded in human history.
The location and April average CO2 levels measured at 12 World Meteorological Organization monitoring stations around the globe.
Previous reports from the Mauna Loa Observatory, the longest continually...