Atmospheric carbon dioxide hit a new record in April, averaging about 431 parts per million at NOAA's Mauna Loa Observatory. That's up from under 320 ppm when the site began measurements in 1958. Scientific American reports: Greenhouse gases, such as carbon dioxide, are measured as a proportion of ... Atmospheric carbon dioxide hit a new record in April, averaging about 431 parts per million at NOAA's Mauna Loa Observatory. That's up from under 320 ppm when the site began measurements in 1958. Scientific American reports: Greenhouse gases, such as... [1293 chars]