According to scientists, at every location on Earth where scientists routinely monitor carbon dioxide levels, the CO2 level is at highest level in 4 million years.
For reference, carbon dioxide levels were at about 280 ppm at the start of the industrial revolution. The levels have kept on increasing since then.
Pieter Tans, the lead scientist for the Global Greenhouse Gas Reference Network, 400 ppm is the highest level that carbon dioxide levels have reached in at least 4 million years.
"This is the first time a sustained reading of 400 ppm, over the period of a day, has been recorded at a research station on the ice," according to a press release from the British Antarctic Survey.
"The remoteness of the Antarctic continent means it is one of the last places on Earth to see the effects of human activities, but the news that even here the milestone of carbon dioxide levels reaching 400 parts per million has been reached shows that no part of the planet is spared from the impacts of human activity," said David Vaughan, director of science at the Antarctic Survey, in a press release.
"... Today at Halley Station, CO2 is rising faster than it was when we began measurements in the 1980s. We have changed our planet to the very poles."
A separate press release from the U.S. National Oceanic and Atmospheric Administration said the South Pole observation occurred on May 23, but was announced on June 15.
In 2015, the global average carbon dioxide level was 399 ppm. It is expected that each month this year will likely see carbon dioxide levels remain above 400 ppm for the first time.
"We know from abundant and solid evidence that the CO2 increase is caused entirely by human activities," Tans said. "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."