Man-made or from natural sources?

A paper published July 17 in Atmospheric Chemistry and Physics finds that

only about 3.75% [15 ppm] of the CO2 in the lower atmosphere is man-made from the burning of fossil fuels, and thus, the vast remainder of the 400 ppm atmospheric CO2 is from land-use changes and natural sources such as ocean outgassing and plant respiration.

if use of fossil-fuels has contributed such a small part of total atmospheric CO2 levels, restricting use of fossil fuels will have little effect upon CO2 levels.

Read more here.