RE: The food carbon calculator: How does the food you eat affect global warming?
The CO2 that you exhale and the CO2 that comes from burning biofuel is from carbon that is already in the carbon cycle. It was absorbed from the atmosphere in the process of the plant growing and gets released again when you or another animal eats it so the net increase of CO2 from your breathing is zero. Carbon from fossil fuels is carbon that has been sequestered from the carbon cycle for (in most cases) 100's of millions of years so burning it increases the total carbon in the carbon cycle.
Oxygen is necessary for life but breathing a 100% oxygen atmosphere at normal pressures for any length of time is toxic. Same thing for CO2, not enough is not good but too much is also not good.
