In 2013, atmospheric concentrations of CO2 surpassed 400 parts per million (ppm), a level not seen in 2 to 3 million years.7 This grim milestone exceeds by 50 ppm the 350 ppm level that scientists have identified as the safe upper limit of CO2 in the earth???s atmosphere.8 In practical terms, carbon dioxide and other greenhouse gases are causing global temperatures to rise.