United Press International: The World Meteorological Association (WMO) announced Monday that the average monthly concentration of carbon dioxide (CO2) in the northern hemisphere exceeded 400 parts per million (ppm) in April, the highest monthly average on record.
A press release from the United Nations' weather agency says crossing the "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...