Mongabay: For the first time since homo sapiens evolved, concentrations of carbon dioxide in the atmosphere have struck 400 parts per million (ppm) due to burning fossil fuels. The National Oceanic and Atmospheric Administration (NOAA) reports that readings of carbon dioxide at Mauna Loa Observatory in Hawaii hit the symbolic number on Thursday and are expected to continue rising in coming years. The last time concentrations were this high for a sustained period was 4-5 million years ago when sea levels were...