Trends in Concentration of Atmospheric Carbon Dioxide (CO2 ppm). Data is sourced from US Government's Earth System Research Laboratory, Global Monitoring Division. Two main series are provided: the Mauna Loa series (which has the longest continuous series since 1958) and a Global Average series (a global average over marine surface sites).

Data

From the site:

Data are reported as a dry air mole fraction defined as the number of molecules of carbon dioxide divided by the number of all molecules in air, including CO2 itself, after water vapor has been removed. The mole fraction is expressed as parts per million (ppm). Example: 0.000400 is expressed as 400 ppm.

Citation:

Mauna Loa data: Dr. Pieter Tans, NOAA/ESRL (www.esrl.noaa.gov/gmd/ccgg/trends/) and Dr. Ralph Keeling, Scripps Institution of Oceanography (scrippsco2.ucsd.edu/). Global data: Ed Dlugokencky and Pieter Tans, NOAA/ESRL (www.esrl.noaa.gov/gmd/ccgg/trends/)

Mauna Loa:

• Monthly: ftp://aftp.cmdl.noaa.gov/products/trends/co2/co2_mm_mlo.txt
• Annual: ftp://aftp.cmdl.noaa.gov/products/trends/co2/co2_annmean_mlo.txt
• Growth rates: ftp://aftp.cmdl.noaa.gov/products/trends/co2/co2_gr_mlo.txt

Global

• Monthly data: ftp://aftp.cmdl.noaa.gov/products/trends/co2/co2_mm_gl.txt
• Annual data: ftp://aftp.cmdl.noaa.gov/products/trends/co2/co2_annmean_gl.txt
• Growth rates: ftp://aftp.cmdl.noaa.gov/products/trends/co2/co2_gr_gl.txt

License

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The information on government servers are in the public domain, unless specifically annotated otherwise, and may be used freely by the public so long as you do not 1) claim it is your own (e.g. by claiming copyright for NOAA information – see next paragraph), 2) use it in a manner that implies an endorsement or affiliation with NOAA, or 3) modify it in content and then present it as official government material. You also cannot present information of your own in a way that makes it appear to be official government information.

Given this data is not "specifically annotated otherwise," the Data Package is licensed under the Public Domain Dedication and License (PDDL).