evidence: Measurements of the forms (isotopes) of carbon in the modern atmosphere show a clear fingerprint of the addition of "old" carbon (depleted in natural radioactive 14C) coming from the combustion of fossil fuels (as opposed to "newer" carbon coming from living systems). In addition, it is known that human activities (excluding land use changes) currently emit an estimated 10 billion tonnes of carbon each year, mostly by burning fossil fuels, which is more than enough to explain the observed increase in concentration. These and other lines of evidence point conclusively to the fact that the elevated CO2 concentration in our atmosphere is the result of human activities.