Levels of carbon dioxide in the atmosphere stayed above 400 parts per million for the entire month of April, in what researchers say is a significant and worrying climate-change milestone.

The concentrations of the potent greenhouse gas over the Northern Hemisphere exceeded 400 ppm for the full term of the month, the first instance of that happening, the United Nations' World Meteorological Organization reported.

That level is a 40 percent increase in CO2 since the start of the Industrial Revolution and widespread fossil fuel use, the WMO said.

While that level has been recorded before in some parts of the world, it has never remained that high for that long, experts said.

"Four hundred ppm is not in and of itself particularly important physically," says climate scientist Gavin Schmit at NASA's Goddard Institute for Space Studies in New York, "but it is emblematic of the fact that we are pushing the climate system into territory that is uncharted."

Environmental organizations have considered 400 ppm an important threshold, although Schmidt reiterates it has no particular scientific significance.

"Humans like big round numbers," says Schmidt, who was not part of the WMO measurement efforts.

The Northern Hemisphere experiences CO2 levels higher than those normally recorded in the Southern Hemisphere, a result of the higher numbers of industrial emission sources in the north, the researchers pointed out.

WMO officials said the annual global average CO2 concentration is predicted to cross the 400 ppm threshold by 2015 or 2016.

 "Time is running out," WMO Secretary-General Michel Jarraud said in a statement.

"This should serve as yet another wake-up 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."

Carbon dioxide emitted from coal-fired power plants and factories and fossil-fueled vehicles can persist in the atmosphere for hundreds of years.

Carbon dioxide levels in the atmosphere have probably cycled between 180 and 280 ppm for the last 800,000 years, scientists say, and it has probably been millennia since they were last above 400 ppm.

The U.S. National Oceanographic and Atmospheric Administration has posted an online animation showing the history of atmospheric carbon dioxide.