During respiration, plants and animals consume oxygen and emit carbon dioxide. Such is life! Just for breathing, humans emit per person each day some 1,140 grams of CO2, assuming they eat normally and follow the mean diet of 2,800 kcal (more or less, since the caloric efficiency of burnt carbon also depends on the type of food: fat, protein or carbohydrates).

The amount of CO2 emitted per person per day is not negligible. It is equivalent to the emission of a car in a 5 km stretch.

If we multiply the 1,140 grams/day by 6 billion persons then, just for breathing, humanity emits per year some 2.5 billion tons of CO2 . . . a considerable amount, more than the reduction required by the Kyoto Protocol (that reduction is a bit less than 1 billion tons, 5% of the 1990 emission).

But the net balance between the carbon absorbed by feeding and the carbon emitted by breathing is almost zero, so is the same how many people we are. Nevertheless a little bit of the carbon we eat is transformed in methane, wich molecule has a greater potential of warming than the CO2 ...