Global levels of atmospheric carbon dioxide hit a worrying 400 parts per million in March for the first time on record, U.S. government scientists are reporting.
Researchers at the National Oceanic and Atmospheric Administration said that although that figure has been reached before, the March readings were the first showing levels averaging 400 ppm for an entire month.
The level of carbon dioxide present in the atmosphere has been tracked since the late 1950s.
The burning of fossil fuels since the beginning of the Industrial Revolution has released "greenhouse gases" including CO2 into the atmosphere at a steadily increasing rate, researchers say.
"It was only a matter of time [before] we would average 400 parts per million globally," says Pieter Tans, head scientist of NOAA's Global Greenhouse Gas Reference Network.
"This marks the fact that humans burning fossil fuels have caused global carbon dioxide concentrations to rise more than 120 parts per million since pre-industrial times," he added. "Half of that rise has occurred since 1980."
Greenhouse gases have increased global warming over the last century at a rate that cannot be tied to natural variability, scientists say.
The last time global carbon dioxide levels reached 400 ppm was 3 to 5 million years ago during the Pliocene Epoch, the result of massive and long-lasting volcanic activity on Earth.
Invisible, odorless and colorless, CO2 is the cause of 63 percent of all global warming linked to greenhouse gases, say researchers at NOAA's Earth System Research Laboratory in Boulder, Colorado.
The average rate of increase of atmospheric carbon dioxide concentrations was 2.25 ppm per year from 2012 to 2014, the highest ever recorded over three consecutive years, NOAA says.
The agency determined worldwide CO2 concentration by analyzing air samples gathered from 40 sites around the globe then shipped to the Boulder lab for testing.
Global efforts to reverse the increase in greenhouse gases causing global warming face difficult challenges, notes James Butler, director of NOAA's Global Monitoring Division.
Even if international agreements to reduce emissions can be arrived at, the trend will be hard to reverse, he says.
"Elimination of about 80 percent of fossil fuel emissions would essentially stop the rise in carbon dioxide in the atmosphere, but concentrations of carbon dioxide would not start decreasing until even further reductions are made and then it would only do so slowly."
