CARBON dioxide levels throughout the northern hemisphere hit 400 parts per million for the first time in human history in April, an ominous threshold for climate change.
The 400 ppm level is rapidly spreading southwards, the World Meteorological Organization said.
The WMO expects the global annual average carbon dioxide concentration to be above 400 ppm in 2015 or 2016.
Rising concentrations of the heat-trapping gas raise risks of more heatwaves, droughts and rising sea levels.
“Time is running out,” WMO Secretary-General Michel Jarraud said.
“This should serve as yet another wake-up call about the constantly rising levels of greenhouse gases which are drivingclimate change.
“If we are to preserve our planet for future generations, we need urgent action to curb new emissions of these heat-trapping gases.”
Almost 200 governments have agreed to work out a deal by the end of 2015 to slow climate change as part of efforts to limit the average temperature increase to two degrees Celsius (3.6degrees Fahrenheit) above pre-industrial times.
Temperatures have already risen about 0.8C (1.4F).
In April the UN’s panel of climate experts said that greenhouse gas concentrations, led by carbon dioxide, would have to be kept below 450 ppm to give a good chance of achieving the 2C goal.