NOAA Earth System Research Laboratory, Global Monitoring Division
http://www.esrl.noaa.gov/gmd/

Scripps Institution of Oceanography:The Keeling Curve
https://scripps.ucsd.edu/programs/keelingcurve/

Charles David Keeling biography
http://scrippsco2.ucsd.edu/sub_program_history/charles_david_keeling_biography.html

New York Times: A Scientist, His Work, and a Climate Reckoning
http://www.nytimes.com/2010/12/22/science/earth/22carbon.html?pagewanted=all&_r=0

NOAA Climate.gov
http://www.climate.gov/news-features/understanding-climate/climate-change-atmospheric-carbon-dioxide
http://www.climate.gov/news-features/decision-makers-toolbox/how-noaa-keeps-track-carbon-dioxide-and-other-greenhouse-gases
http://www.climate.gov/news-features/decision-makers-toolbox/tracking-carbon-dioxide-across-globe

Climate Central: 400 ppm—A Milestone That Means Everything, and Nothing
http://www.climatecentral.org/blogs/400-ppm-a-milestone-that-means-everything-and-nothing