"What I see now with existing investments for plants under construction...we are seeing the door for a 2 degree Celsius target about to be closed and closed forever," Fatih Birol, the IEA's chief economist, told a Reuters' Global Energy & Environment Summit.

"This door is getting slimmer and slimmer in terms of physical and economic possibility," he warned.

The IEA said last November that around 80 percent of total energy-related carbon emissions permissible by 2035 to limit warming were already accounted for by existing power plants, buildings and factories, leaving little room for more.