The world has crossed a major greenhouse-gas milestone, and it may never turn back.
The Manua Loa Observatory in Hawaii has maintained a continuous record of atmospheric carbon-dioxide levels since 1958. Here's the complete record of readings from the observatory over that time period:
As you can see, atmospheric CO2 cycles every year, but there's a significant upward trend in the measurements.
Right now we're at the low point in that cycle, just at the end of September. And, according to a post from the Scripps Institute of Oceanography (which we first saw covered over at Motherboard), atmospheric CO2 is holding at 401 parts per million. That's the first time in recorded history that the annual carbon cycle has bottomed out at over 400 ppm. And it means the 2016 carbon trough is about 25% higher than the 1958 peak — just under 320 ppm.
As Scripps points out, there have been a few years when the readings in October have been a bit lower than September. But they've never seen one full ppm drop, so it's unlikely we'll see a reading under 400 ppm this year.
And given the steady upward trend in atmospheric CO2, it also means we'll likely never see a reading under 400 ppm in the foreseeable climate future.
The last time atmospheric carbon held above 400 ppm was 15 million years ago. The world was three to six degrees warmer, and sea levels were between 75 and 120 feet higher, according to research published in the journal Science.
Which is to say: This is perhaps a milestone to note.
SEE ALSO: TRUMP: Claims of global warming still 'need to be investigated'
Join the conversation about this story »
NOW WATCH: Here's what could happen to Earth over the next 500 years