A U.N. agency says the world has reached a new milestone on carbon dioxide (CO2), which is driving climate change. The World Meteorological Organization warned that in April, for the first time in history, monthly concentrations of CO2 in the atmosphere topped 400 parts per million throughout the Northern Hemisphere. WMO Secretary-General Michel Jarraud called for urgent action to cut emissions, saying, “Time is running out.” Next week, President Obama is expected to announce a new regulation to cut carbon pollution from coal-fired power plants, the leading source of carbon pollution in the United States. The warning from the WMO came just days after House lawmakers passed an amendment to a major military spending bill, which bars the Pentagon from using funds to address climate change and its potential impact on national security.