NOTICE (March 2018): This website provides access to the CDIAC archive data temporarily. It will be gradually transitioned into data packages in the new ESS-DIVE archive. This site will continue to operate in parallel during and after the transition, and will be retired at a future date. If you have any questions regarding the data or the transition, please contact

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Carbon-13 in CO2

Carbon-13 in CO2 is decreasing, as the fraction of atmospheric CO2 that is realized from combustion of fossil carbon is increasing. Ratios of 13C/12C in CO2 tend to be lower in the Northern Hemisphere, suggesting a fossil-fuel source that resides mainly in the Northern Hemisphere.