Concentrations of carbon dioxide in the global atmosphere could rise above 400 parts per million (ppm) for sustained lengths of time throughout much of the Northern Hemisphere as soon as mid-May.
Today, measurements at the Hawaii station show concentrations of 399.50 ppm ... pretty darn close to 400, according to scientists at Scripps Institution of Oceanography at University of California/ San Diego.
If you'll remember, 350 ppm is the level the entire world has agreed not to exceed because that's when feedback loops and uncontrollable climate change is likely to kick in (as we've begun to see with vastly more wildfires, droughts, floods and storms).
"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," says Ralph Keeling, whose has taken over for his father who created the "Keeling Curve" to track daily carbon levels.
It's hard to comprehend, but last time there was 400 ppm of carbon in the atmosphere was 3.2 million to 5 million years ago, during the Pliocene period.
Carbon levels reached about 415 ppm during the Pliocene, with global average temperatures 3-4 degrees C (5.4-7.2 degrees F) higher than today and as much as 10 degrees C (18 degrees F) warmer at the poles. Sea level ranged between 16-131 feet higher than today, according to Scripps.
"At this pace we'll hit 450 ppm within a few decades," warns Keeling.
Prior to the Industrial Revolution carbon levels were 280 ppm and rose gradually to 316 ppm by 1958 when Keeling's father began daily measurements. The rate of rise of CO2 over the past century is unprecedented; there is no known period in geologic history when such high rates have been found. The continuous rise is a direct consequence of society's heavy reliance on fossil fuels for energy, Keeling says.
You can track daily levels of carbon here or on Twitter.