The unthinkable has happened: atmospheric carbon dioxide (CO2) levels have permanently surpassed 400 parts per million (ppm).
That means we’ve crossed a line that scientists warned us about for decades—and we won’t be going back.
Why does 400 ppm matter? Because the ‘safe’ level of CO2 is considered to be 350 ppm, a threshold we blew past years ago.
To put this into perspective, the last time Earth experienced CO2 concentrations this high was between 2 and 4 million years ago—long before humans walked the planet.
Now, researchers at the Scripps Institution of Oceanography have confirmed that CO2 levels for September 2016 have stayed above 400 ppm, despite this typically being the time of year when levels reach their annual low. And the numbers will only go up from here.
“Is it possible that October 2016 will yield a lower monthly value than September and dip below 400 ppm?
Almost impossible,” says Ralph Keeling, director of the Scripps CO2 Program.
“At best, we might see stabilization, but CO2 levels probably won’t change much for decades—maybe centuries.”
The Carbon Clock Is Ticking
For years, scientists have warned about crossing this climate threshold.
Back in 2013, the Mauna Loa Observatory in Hawaii—widely regarded as the world’s most accurate CO2 measuring station—registered 400 ppm for the first time.
Other observatories followed, and by May 2016, the global average surpassed 400 ppm.
The final holdout, Antarctica’s South Pole Observatory, crossed the mark last, but it too succumbed to rising emissions.
Now, the situation is irreversible in the foreseeable future.
Even if we stopped emitting CO2 tomorrow, the gas would linger in the atmosphere for centuries.
According to NASA’s chief climate scientist Gavin Schmidt, “In my opinion, we won’t ever see a month below 400 ppm again.”
But Didn’t CO2 Fluctuate Naturally in the Past?
Skeptics argue that CO2 levels have changed throughout history, so why worry now?
The answer is simple: the speed and scale of current CO2 increases are unprecedented.
Scientists can analyze ancient CO2 levels through ice cores dating back 800,000 years, and they show that natural fluctuations rarely pushed CO2 above 300 ppm.
Further research into sea sediments suggests that CO2 levels haven’t been this high for at least 2 million years.
A 2009 study in Science estimated that sustained CO2 levels above 400 ppm haven’t been seen for 15–20 million years, while a 2011 study in Paleoceanography suggested the most recent comparison might be 2–4.6 million years ago.
During that time, global temperatures were much warmer, polar ice caps were smaller, and sea levels were significantly higher—conditions that would be catastrophic today.
A World We’ve Never Known
For modern humans, this is uncharted territory. We’ve never lived in a world with CO2 levels this high, and the effects are already being felt:
- Rising Temperatures: The planet has already warmed by about 1.1°C (2°F) since pre-industrial times, and the trend is accelerating.
- Melting Ice and Rising Seas: Greenland and Antarctic ice sheets are shrinking, leading to rising sea levels that threaten coastal cities worldwide.
- Extreme Weather: Hurricanes, wildfires, and droughts are becoming more frequent and intense, fueled by a hotter atmosphere.
What Happens Next?
Even if we slash emissions today, CO2 levels will remain elevated for generations. But that doesn’t mean we should do nothing.
Scientists stress that we can still limit the worst effects of climate change if we act fast.
That means transitioning to renewable energy, reducing fossil fuel consumption, and investing in carbon capture technologies.
If we don’t? We’re locking in a future with more extreme weather, rising sea levels, and an unstable climate that could push ecosystems—and societies—to their breaking point.
We may have passed the 400 ppm threshold, but the bigger question remains: will we do what’s necessary to prevent even worse milestones from being reached?
