Carbon dioxide, methane and nitrous oxide all hit annual record highs in the atmosphere in 2015. It's the first year carbon dioxide reached an annual peak of 400 ppm. At a Glance 2015 was the first time annual average carbon dioxide levels topped 400 ppm.It’s the first year in human history where carbon dioxide levels have reached the symbolic milestone for an entire year. On Monday, the World Meteorological Organization released another reminder of the planetary predicament we’re in: The...