A press release from the United Nations' weather agency says crossing the "threshold is of symbolic and scientific significance and reinforces evidence that the burning of fossil fuels and other human activities are responsible for the continuing increase in heat-trapping greenhouse gases warming our planet."
According to the WMO's release, CO2 levels have risen more than 40 percent, up from pre-industrial levels of around 278 ppm, since human's began burning fossil fuels. CO2 can remain in the atmosphere for hundreds of years, longer still in the oceans. Because plants absorb more carbon dioxide during summer months when their foliage is more dense and plentiful, CO2 levels fluctuate from season to season and tend to peak in the spring. The northern hemisphere, due to higher levels of human industrial activity than the southern hemisphere, tends to have a more pronounced seasonal cycle.
But even with April's reading representing a seasonal peak, Earth's atmosphere hasn't seen levels as high as 400 ppm for millions of years.
"This should serve as yet another wakeup call about the constantly rising levels of greenhouse gases which are driving climate change. If we are to preserve our planet for future generations, we need urgent action to curb new emissions of these heat trapping gases," WMO Secretary-General Michel Jarraud said in a statement. "Time is running out."
Notable deaths of 2014 [PHOTOS]