For the first time in human history, the amount of carbon dioxide in the air has passed 400 parts per million, a symbolic mark that has not been reached for 3 to 5 million years.
Atmospheric CO2 is measured by the U.S. government at a lab on Mauna Loa, a volcano in Hawaii. CO2 is generally recognized as the greenhouse gas most responsible for raising the global temperature.
While 400 ppm is a high mark, the number is expected to fall, as it does seasonally at the measuring station. The station sees high numbers in winter and lower numbers in summer, but the overall trend is upward.
See the story at BBC News.