The last time the Earth had this much carbon dioxide in its atmosphere, human beings were nowhere to be found.

Scientists say the level of CO₂ has averaged 400 parts per million (ppm) for the past three months at the Mauna Loa observatory in Hawaii. That last happened, climatologists say, between 800,000 and 15 million years ago, long before modern humans roamed the earth.

The 400 ppm level contrasts with a level of about 280 ppm before the beginning of the Industrial Revolution less than two centuries ago.

With the rise in CO₂ has come a corresponding bump in the planet’s overall temperature, about 1.6°F since the dawn of the 20th century.

“It is urgent that we find a way to transition to non-carbon fuels as our source of primary energy,” Pieter Tans, a climate scientist with the National Oceanic and Atmospheric Administration, said according to Climate Central. That’s because temperatures will only go up further, creating more climate unpredictability.

The 400 ppm milestone was first reached last year, on May 9, 2013, but the concentration was not sustained then. Beginning in April and continuing through June of this year, the CO₂ measurement held steady at the 400 mark.

Levels of CO₂ are expected to fall beginning this month as Northern Hemisphere plant life takes in more of the gas. However, they’re not able to deal with all the excess CO₂, so the base level is expected to increase each year.

-Noel Brinkerhoff

To Learn More:

New CO2 Milestone: 3 Months Above 400 PPM (by Andrea Thompson, Climate Central)

The Last Time CO2 Was This High, Humans Didn’t Exist (by Andrew Freedman, Climate Central)

Researchers Find U.S. Responsible for 22% of World’s Global Warming (by Noel Brinkerhoff, AllGov)

Scientists Claim 95% Certainty that Humans are “Dominant Cause” of Global Warming (by Noel Brinkerhoff and Danny Biederman, AllGov)