Carbon dioxide levels passed 400 parts per million at the South Pole last month, according to a new report from the National Oceanic and Atmospheric Administration.

It's the last place on Earth to hit the global warming milestone. Researchers say CO2 levels haven't been this high at the South Pole in 400 million years.

"The far southern hemisphere was the last place on earth where CO2 had not yet reached this mark," Pieter Tans, the scientists in charge of NOAA's Global Greenhouse Gas Reference Network, said in a news release. "Global CO2 levels will not return to values below 400 ppm in our lifetimes, and almost certainly for much longer."

Carbon dioxide levels wax and wane of the course of the year, but average CO2 levels have gone up every year since 1958. It's unlikely 2016 will be any different. Last year's average was 399 ppm. This year's is sure to be above 400 ppm.

"We know from abundant and solid evidence that the CO2 increase is caused entirely by human activities," Tans added.

While climate change mitigation may be able to contain temperature rise, it's unlikely CO2 levels will drop below 400 ppm.

It's what many scientists are calling the "new normal."

"Since emissions from fossil fuel burning have been at a record high during the last several years, the rate of CO2 increase has also been at a record high," Tans said. "And we know some of it will remain in the atmosphere for thousands of years."