Atmospheric carbon dioxide concentrations have been closely monitored for some time now. Just last month it was predicted that if we did not break through the 400 parts per million (ppm) barrier this May, it would happen next year, according to the longest continuous record of carbon dioxide levels in the atmosphere.
On May 9, true to form, the daily mean concentration of atmospheric carbon dioxide levels passed 400 ppm at Mauna Loa according to independent measurements taken by both the National Oceanic and Atmospheric Administration (NOAA) and the Scripps Institution of Oceanography.
“I wish it weren’t true, but it looks like the world is going to blow through the 400-ppm level without losing a beat,” said geophysicist Ralph Keeling of the Scripps Institution of Oceanography at UC San Diego last month, who took over the Keeling Curve measurement begun by his father, Charles David (Dave) Keeling, after he developed the ultraprecise measurement device known as a manometer.
“At this pace we’ll hit 450 ppm within a few decades.” 
The jagged saw-tooth graph was primed and ready to hit 400 ppm this May, and not wanting to ignore expectations, atmospheric carbon dioxide levels did just that.
The rate of increase has accelerated since Dave Keeling first started measuring carbon dioxide levels in the atmosphere, from approximately 0.7 ppm per year in the late 1950s to 2.1 ppm per year during the last 10 years.
“That increase is not a surprise to scientists,” said NOAA senior scientist Pieter Tans, with the Global Monitoring Division of NOAA’s Earth System Research Laboratory in Boulder, Colorado. “The evidence is conclusive that the strong growth of global CO2 emissions from the burning of coal, oil, and natural gas is driving the acceleration.”
“There’s no stopping CO2 from reaching 400 ppm,” said Ralph Keeling. “That’s now a done deal. But what happens from here on still matters to climate, and it’s still under our control. It mainly comes down to how much we continue to rely on fossil fuels for energy.”
Image Credit: NOAA
NOAA announced last year that its global cooperative air sampling network had detected 400 ppm for the first time over all its Arctic sites, just a prelude to what is now being detected over Mauna Loa. According to NOAA, locations throughout the Southern Hemisphere will follow over the next few years, as the increase in Northern Hemisphere levels is always a little ahead of the Southern Hemisphere, due to the fact that the majority of carbon dioxide producing behemoths are in the Northern Hemisphere.
