The amount of carbon dioxide in Earth’s atmosphere has officially surpassed levels seen in the entirety of human history, topping the highest point previously recorded in 800,000 years of data by more than 100 parts per million, or ppm.

Related Content CO2 Levels Are as High as They Were Three Million Years Ago

Researchers at Hawaii’s Mauna Loa Observatory measured the chart-topping figure of 415 ppm—meaning carbon dioxide made up 415 of every one million gas molecules in the atmosphere—last Saturday. Although it’s difficult to place this number in perspective, climate expert Peter Gleick offers an apt frame of reference, writing on Twitter that “the last time humans experienced levels this high was … never. Humans didn’t exist.” (Our species, Homo sapiens, evolved around 300,000 years ago.)

Mankind may not have been on the scene at this point, but ocean sediment and mummified plants dating to the period provide a sense of just how different the world was. As Jonathan Amos writes for BBC News, the last time Earth’s atmosphere contained the amount of carbon dioxide present today—during the Pliocene Epoch of 2.6 to 5.3 million years ago—Antarctica was a plant-covered oasis, sea levels were an estimated 10 to 20 meters higher, and global temperatures were an average of 2 to 3 degrees Celsius warmer. In the Arctic, summer temperatures were a full 14 degrees higher than they are now.

According to the National Oceanic and Atmospheric Administration’s climate portal, scientists started tracking carbon dioxide concentrations at Mauna Loa in 1958. At the time, the site’s carbon levels hovered around 315 ppm, but by 2013, they had surpassed 400 ppm. Global concentrations reached this same milestone in 2015.

Thanks to ice cores and paleoclimate evidence detailing 800,000 years of data, scientists know these numbers are far higher than any experienced in recorded history. As Alex Schwartz writes for Popular Science, the world’s average carbon dioxide levels stood at 280 ppm for the majority of the last one million years, never topping 300 ppm or falling below 160 ppm despite multiple periods of planetary warming and cooling.

Then, the Industrial Revolution upended this relative stability, introducing high levels of greenhouse gases into the atmosphere as humans burned fossil fuels to support an increasingly technology-driven lifestyle. Today, global temperatures stand about 1.8 degrees Fahrenheit, or 1 degree Celsius, higher than during the pre-industrial period. And while this uptick may seem numerically insignificant, the many extreme weather events—including more frequent droughts, stronger ocean wind and waves, and unprecedented warm seasons—associated with slightly elevated temperatures suggest otherwise.

“We keep breaking records, but what makes the current levels of CO2 in the atmosphere most troubling is that we are now well into the 'danger zone' where large tipping points in the Earth’s climate could be crossed,” Jonathan Overpeck, dean of the University of Michigan’s School for Environment and Sustainability, tells Live Science’s Yasemin Saplakoglu.

According to the Washington Post’s Jason Samenow, last week’s carbon dioxide high was just one of multiple simultaneous indications of “human-induced climate change.” As scientists in Hawaii measured carbon levels of 415 ppm, temperatures in northwest Russia surged to 84 degrees Fahrenheit—30 degrees higher than the region’s average high of 54 degrees—and Greenland’s ice sheets continued their relentless melt season, which began more than a month ahead of schedule.

As carbon levels continue to rise at a rate of around three ppm, such occurrences could become dangerously commonplace. And while the verdant Antarctic landscape of the Pliocene Epoch is still a far cry from our current climate, the fact that Earth has reached carbon levels seen during this period is a foreboding sign.

“We could soon be at the point where comparable reductions in ice sheet size, and corresponding increases in sea level, are both inevitable and irreversible over the next few centuries,” Overpeck concludes. “It's like we're playing with a loaded gun and don't know how it works.”