The rise in CO2 is unambiguously caused by human activity, principally fossil-fuel burning. This is clear from the numbers: We know how much fossil fuel is converted into CO2 each year and emitted into the atmosphere. -- Scripps Institution of Oceanography geochemist Ralph Keeling
Sources:
The Basics: Isotopic Fingerprints, National Oceanic and Atmospheric Administration Earth System Research Laboratory
Is the Current Rise in CO2 Definitely Caused by Human Activities?, on the Keeling Curve website