According to the latest figures, levels of atmospheric carbon have officially surpassed 400 parts per million (ppm), and there's little hope of returning them to safe levels - the situation is now permanent.
What's the big deal about 400 ppm? Well, the 'safe' level of CO2 in the atmosphere is considered to be 350 ppm, and the last time Earth experienced levels that were consistently this high was roughly 4 million years ago. That means humans have literally never experienced CO2 like this before.
According to researchers at the Scripps Institution of Oceanography, the CO2 value for September 2016 will definitely be above 400 ppm, and will likely be around 401 ppm. And here's the thing - September typically has the lowest atmospheric CO2 levels of the year.
"Is it possible that October 2016 will yield a lower monthly value than September and dip below 400 ppm? Almost impossible," Ralph Keeling, director of the Scripps CO2 Program, writes in a blog post.
Keeling says that by November this year, we could be pushing towards new highs, and perhaps even breaking the 410 ppm barrier.
"[I]t already seems safe to conclude that we won't be seeing a monthly value below 400 ppm this year - or ever again for the indefinite future," he adds.
Even if, by some miracle, we all stopped emitting carbon dioxide tomorrow, it would take decades to get us back below the 400 ppm threshold - and we all know that's never going to happen.
"At best (in that scenario), one might expect a balance in the near term, and so CO2 levels probably wouldn't change much - but would start to fall off in a decade or so," Gavin Schmidt, NASA's chief climate scientist, told Brian Kahn at Climate Central.
"In my opinion, we won't ever see a month below 400 ppm."
For years now, scientists have been predicting that we'd hit the 400 ppm threshold and eventually tip over. Back in 2013, the Mauna Loa Observatory in Hawaii - described as the world's "gold standard" carbon dioxide observatory - hit the 400 ppm mark, and gradually, all other observing stations followed suit.
In May 2016, the world collectively passed the 400 ppm threshold, with the South Pole Observatory in Antarctica being the last to clear 400 ppm.
And this September, which should have been our low point for the year, failed to deliver, and now we're stuck with this mess until we can figure out how to significantly decrease our emissions.
So when was the last time the planet had CO2 levels like this? Analysis of carbon levels in ice cores can give us indications of atmospheric CO2 as far back as 800,000 years ago, and scientists have estimated that it's "inconceivable" that they would have been much above 300 ppm at that time.
According to David Etheridge, principal research scientist at Australia's CSIRO, analysis of sea sediments can push our estimates of historic CO2 levels back to about 2 million years ago. Based on these values, scientists have created climate models that give us an idea of what conditions on Earth were like tens of millions of years ago.
As Graham Readfearn reported for The Guardian earlier this year, a 2009 study in Science found the last time in Earth's history that atmospheric CO2 levels were this high for a sustained period was between 15 and 20 million years ago.
More recently, a 2011 study in Paleoceanography found that atmospheric CO2 levels could have been comparable to today's levels much later than that - between 2 and 4.6 million years ago.
Regardless of whether Earth has experienced these levels 15 or 4 million years ago, humans have never been around to experience them until now. And that means there's really no telling what's going to happen next.