This weekend, Earth's atmosphere had a concentration of carbon dioxide last seen more than 3 million years ago, before humans had evolved, NBC News reported.
The greenhouse gas was detected at 415 parts per million by sensors at the Mauna Loa Observatory in Hawaii on Saturday. The level has been steadily rising over the past decade and doesn't appear set to stop.
"We're racing toward a state very different from the kind humans evolved in and that civilization developed in," said geochemist Ralph Keeling, of the Scripps Institution of Oceanography in La Jolla, California.
When Earth last saw atmospheric carbon dioxide at these levels, in the Pliocene Epoch, average sea levels were about 50 feet higher than they are today, according to Stanford earth system science professor Rob Jackson.