May 11, 2016, 407.70 parts per million
May 11, 2015, 403.58 ppm
Average for month April 2016, 407.57 ppm April 2015, 403.45 ppm March 2016 was warmest since 1880, an increase of 2.20 degrees above the 20th century average. From CO2-earth, www.CO2.earth