The NOAA recorded this reading on May 9, receiving a reading of 400.03 ppm from their station in Mauna Loa, Hawaii. UCSD Scripps Institution of Oceanography recorded a reading of 400.08 ppm during that same 24-hour period.

“That increase is not a surprise to scientists,” NOAA senior scientist Pieter Tans said in an NOAA release. “The evidence is conclusive that the strong growth of global carbon dioxide emissions from the burning of coal, oil and natural gas is driving the acceleration.”

Scripps has been providing updates on the Keeling Curve, a record of atmospheric carbon dioxide levels. American scientist Charles Keeling produced the Keeling Curve in 1956 to measure carbon dioxide. In the last reading, from May 12, the carbon dioxide concentration was 399.41 ppm. It is estimated that the carbon dioxide level has not reached such levels since the Pliocene epoch, around 3.2 to 5 million years ago.

“The 400-ppm threshold is a sobering milestone and should serve as a wakeup call for all of us to support clean energy technology and reduce emissions of greenhouse gases, before it’s too late for our children and grandchildren,” Scripps oceanographer Timothy Lueker said to UCSD News.

Scripps geochemist and Charles Keeling’s son, Ralph Keeling, predicted that if the carbon dioxide levels continue to increase at this pace, the levels would hit 450 ppm in a few decades.

“There’s no stopping CO2 from reaching 400 ppm,” Keeling said in an NOAA release. “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.”