The Earth passed another unfortunate milestone May 23 when carbon dioxide surpassed 400 parts per million (ppm) at the South Pole for the first time in 4 million years.
![The National Science Foundation’s Atmospheric Research Observatory illuminated by the sun approaching South Pole sunset in 2014. (Image credit: NOAA-LT Joseph Phillips) The National Science Foundation’s Atmospheric Research Observatory illuminated by the sun approaching South Pole sunset in 2014.](/sites/default/files/styles/landscape_width_1275/public/legacy/image/2019/Jun/PHOTO-NSF%20observatory.jpg?itok=7yP7W9Qp)
The National Science Foundation’s Atmospheric Research Observatory illuminated by the sun approaching South Pole sunset in 2014. (Image credit: NOAA-LT Joseph Phillips)