Greenhouse gases rose to a symbolic milestone in 2015, taking climate change into a new phase which could last generations even if governments act to curb man-made global warming, the U.N. World Meteorological Organization (WMO) said on Monday. Globally averaged concentrations of carbon dioxide, the main man-made greenhouse gas, reached 400.0 parts per million (ppm) in the atmosphere for the first time on record and were 44 percent above levels before the Industrial Revolution, it said. The rise, continuing in 2016, contrasts with accords by almost 200 governments to start reining in emissions, led by the Paris Agreement last year to phase out fossil fuels in favour of renewable energy in the second half of the century. "The year 2015 ushered in a new era of optimism and climate action with the Paris climate change agreement. But it will also make history as marking a new era of climate change reality with record high greenhouse gas concentrations," WMO Secretary-General Petteri Taalas said in a statement. Carbon dioxide levels had reached 400 ppm in some places and some months but "never before on a global average basis for the entire year," the WMO said in an annual Greenhouse Gas Bulletin. And the gas is set to keep building up in the atmosphere. "The real elephant in the room is carbon dioxide, which remains in the atmosphere for thousands of years," Talaas said.