The concentration of carbon dioxide in the Earth’s atmosphere has reached levels not seen for 3 million years, scientists at Mauna Loa Observatory in Hawaii said Monday – offering a dire warning about the impact of human activity on the planet.

The observatory’s sensors registered carbon dioxide levels of 415 parts per million (ppm) on Saturday, meaning CO2 made up 415 of ever 1 million molecules of gas in the atmosphere.

CO2 – which is emitted when we burn fossil fuels like coal, oil and natural gas – is a greenhouse gas which traps heat in the earth’s atmosphere, contributing to the global temperature increases which drives climate change.

The concentration of CO2 has been rising by an average of 2.5 ppm over the last decade. But the increase from 2018 to 2019 will likely be around 3ppm, Ralph Keeling, director of the Scripps C02 program which runs the Manua Loa Observatory, said in a statement.

“Every year it goes up like this we should be saying ‘No, this shouldn’t be happening. It’s not normal,” Keeling said. “This increase is just not sustainable in terms of energy use and in terms of what we are doing to the planet.”

This is the first time in human history that the concentration of the gas has topped 415 ppm, meteorologist Eric Holthaus warned on Twitter.

The last time the proportion of CO2 was this this high was during the Pliociene period between 5.3 and 2.6 million years ago. Back then, the Earth was a very different place, with a much warmer climate. Average sea levels are thought to have been around 50 ft higher than they are today and forests grew as far north as the Arctic, a professor of earth system science at Stanford University told NBC.

Write to Ciara Nugent at ciara.nugent@time.com.