Concentrations of carbon dioxide (CO2) in the Earth's atmosphere have crossed a new threshold, the U.N. weather agency confirmed Monday, warning that "time is running out" to reduce rising greenhouse-gas emissions. In April, monthly concentrations of CO2 in the atmosphere topped 400 parts per million (ppm) throughout the northern hemisphere, the World Meteorological Organization (WMO) reported. "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 wrote. Levels of CO2 first reached 400 ppm in April 2012, but this is the first time the monthly average passed the level. The WMO predicts the global annual average CO2 concentration will cross the threshold in 2015 or 2016. "Time is running out," WMO Secretary-General Michel Jarraud said in a statement. "This should serve as yet another wake-up call about the constantly rising levels of greenhouse gases which are driving climate change. If we are to preserve our planet for future generations, we need urgent action to curb new emissions of these heat-trapping gases." The U.N. Intergovernmental Panel on Climate Change (ICCC) reported in April that to keep the global average temperature increase to 2 degrees Celsius means lowering global greenhouse-gas emissions by 40 to 70 percent compared with 2010 by mid-century, and to near-zero by the end of this century.