Climate scientists have long said that the earth cannot withstand more than 400 parts per million of carbon dioxide in the atmosphere for very long without tipping the scales against human life once and for all.

And according to Ralph Keeling, who runs the carbon dioxide monitoring program at the Scripps Institute of Oceanography, that atmospheric concentration was recorded last week for the first time this year, two months earlier than last. Now, warns Keeling, it is only a matter of time before the atmosphere permanently maintains those levels.

“We’re already seeing values over 400. Probably we’ll see values dwelling over 400 in April and May. It’s just a matter of time before it stays over 400 forever,” said Keeling whose team at Scripps have taken daily measurements from the Mauna Loa Observatory in Hawaii since March 1958 when carbon was at 313 ppm.

The 400 parts per million threshold was first recorded during a brief moment in the month of May last year, notes Brian Kahn at Climate Central. It was the first time carbon levels had reached that high in human history.

But as Robert Monroe writes on Keeling’s blog, “Fossil fuel burning continues to increase concentrations of the greenhouse gas to levels not seen in human history and not in perhaps as many as 3 to 5 million years.”