The fact that we humans are pumping more and more carbon dioxide into the air is well-known, but the extent to which the gas is building up in the atmosphere continues to surprise scientists, and not in a good way. The latest readings of atmospheric C02 concentration reveal that it is now at levels never reached before in the entirety of human existence. As one meteorologist puts it, "We don't know a planet like this."

Back in 2016, scientists warned that we had moved into "uncharted territory" as atmospheric C02 levels tipped to more than 400 parts per million over the South Pole, which was the last region on Earth to pass this threshold. As a point of reference, 350 ppm is a concentration that experts consider safe – the international environmental organization 350.org even dedicates its name to such effect.

For years, the UN has been warning that the world has to go beyond the requirements of the Paris Agreement and do more to rein in global carbon emissions, even calling for immediate action after late last year stating that "we are running out of time."

And new measurements from the Mauna Loa Observatory in Hawaii, gathered by scientists from the Scripps Institution of Oceanography at UC San Diego and the National Oceanic and Atmospheric Administration, are a rather clear indicator of what that inaction looks like. Known as the Keeling Curve since its inception in 1958, the graph tracks the buildup of carbon dioxide in the atmosphere and the latest reading, taken on May 11, shows that concentration to be 415.26 ppm.

"This is the first time in human history our planet's atmosphere has had more than 415 ppm CO2," tweeted meteorologist Eric Holthaus, after promptly picking up on the news. "Not just in recorded history, not just since the invention of agriculture 10,000 years ago. Since before modern humans existed millions of years ago. We don't know a planet like this."

If carbon emissions continue unabated, they are expected to drive up global temperatures at the same time. But something of an unknown is what effect this will have on the planet. Scientists warn that an increase of 2° C (3.6° F) above pre-industrial levels would limit the severity of more intense storms, flooding, sea-level rise and the loss of agriculture and ecosystems. The bad news is we are currently on track to go far beyond that.