We may have just passed a point of no return when it comes to Earth's climate.

The low point of carbon dioxide in the atmosphere typically occurs around the last week of September. But this year, levels of late have failed to drop below 400 parts per million (ppm) – and it looks like they're not going to.

"Brief excursions towards lower values are still possible but it 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," Ralph Keeling, director of the Scripps CO2 Program at the University of California-San Diego's Scripps Institution of Oceanography, wrote Friday.

Carbon dioxide just hit its annual minimum at Mauna Loa Observatory and failed to dip below 400 ppm https://t.co/m0ZoyzgcEf — Climate Central (@ClimateCentral) September 28, 2016

Why's that a big deal? The 400 ppm mark is considered by many to be a "red line," the crossing of which poses dangerous climate ramifications.

Climate activist Bill McKibben's climate change initiative 350.org was so named because 350 ppm is considered by many scientists to be the upper limit of safe CO 2 levels in the atmosphere.

"Right now we’re at 400 ppm, and we’re adding 2 ppm of carbon dioxide to the atmosphere every year," the initiative's website reads. "Unless we are able to rapidly turn that around and return to below 350 ppm this century, we risk triggering tipping points and irreversible impacts that could send climate change spinning truly beyond our control."

But that doesn't mean there's no hope. While some of the world's greatest climate change offenders have appeared a bit apathetic on the issue, many countries are investing in clean energy.