The World Meteorological Organization reports that, for the first time in human history, atmospheric concentrations of carbon dioxide (CO2) eclipsed 400 parts-per-million (ppm) in the northern hemisphere for the entire month of April.CO2 concentration varies with the seasons, generally reaching their highest level early in the northern hemisphere's spring before seasonal intake of CO2 from vegetation.
The first recording of 400 ppm occurred briefly in the spring of 2012 at the National Oceanic and Atmospheric Administration's (NOAA) Arctic monitoring station in Barrow, Alaska. On May 9 2013, CO2 levels also topped 400 ppm at the CO2 observatory atop Mauna Loa on the Big Island of Hawaii.
The readings from April 2014 mark the first time CO2 concentrations of 400 ppm were observed for all monitoring stations throughout the northern hemisphere. The average concentration for April 2014 was 401.30 ppm. For April 2013 it was 398.ppm.
In 1958 Charles Keeling began the first continuous measurement of atmospheric carbon dioxide on the Mauna Loa Observatory in Hawaii, and is credited with the "Keeling Curve" graph showing both the seasonal variations in CO2 as well as it's steady and rapid rise since that time.
In 1959, after the first full year of data had been collected, atmospheric carbon dioxide was measured at 315.97. In 2013 average CO2 levels stood at 396.48, up from 393.82 in 2012. Average levels of CO2 are expected to reach 400 ppm or higher by 2015 or 2016.
Not only is CO2 on a steady rise, the rate of increase is accelerating as well, from about 0.7 ppm in the late 50's to 2.1 ppm during the last decade.
As of May 28, 2014, CO2 levels at the Mauna Loa station were 402.11 ppm
“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,” said WMO Secretary-General Michel Jarraud. “Time is running out.”
Graph courtesy of NOAA