The airborne fraction is a scaling factor defined as the ratio of the annual increase in atmospheric  to the emissions from anthropogenic sources. It represents the proportion of human emitted that remains in the atmosphere. The fraction averages about 45%, meaning that approximately half the human-emitted is absorbed by ocean and land surfaces. There is some evidence for a recent increase in airborne fraction, which would imply a faster increase in atmospheric for a given rate of human fossil-fuel burning. However, other sources suggest that the "fraction of carbon dioxide has not increased either during the past 150 years or during the most recent five decades".

Changes in carbon sinks can affect the airborne fraction.