Atmospheric carbon dioxide concentrations have crossed a new threshold, the UN's weather agency saidon on Monday, highlighting the urgency of curbing manmade, climate-altering greenhouse gases.
In April, for the first time, the mean monthly CO2 concentration in the atmosphere topped 400 parts per million (ppm) throughout the northern hemisphere, which pollutes more than the south, the World Meteorological Organisation (WMO) said.
"This should serve as yet another wake-up call about the constantly rising levels of greenhouse gases which are driving climate change," WMO chief Michel Jarraud said in a statement.
"If we are to preserve our planet for future generations, we need urgent action to curb new emissions of these heat-trapping gases," he warned. "Time is running out."
Spring values in the northern hemisphere had previously spiked over the 400 ppm level, but this was the first time the monthly mean concentration of CO2 in the atmosphere exceeded the threshold.
The global annual average is set to exceed the 400 ppm level in 2015 or 2016, the agency added.