The U.N. weather agency says levels of carbon dioxide and methane, the two most important greenhouse gases, reached record highs last year. The World Meteorological Organization says CO2 levels rose to nearly 398 parts per million, from 396 ppm in 2013. The CO2 level fluctuates throughout the year and the monthly average crossed the symbolic 400 ppm threshold in March 2015. The WMO says the annual average "is likely to pass 400 ppm in 2016." Pushed by the burning of coal, oil and gas for energy, global CO2 levels are now 143 percent higher than before the industrial revolution. Scientists say that's the main driver of global warming. WMO says methane levels reached a new high of about 1,833 parts per billion in 2014.