The human being per se Every human being excretes CO 2 by nature. With each breath, CO 2 is released from the lungs when we exhale. In the lungs, gas exchange takes place between the blood and the air. As the blood is supersaturated in CO 2 , this is released into the air in the lungs. The CO 2 is produced in the body cells during the energy production of the cells, whereby organic substances (food) are oxidized ("burned"). Biochemically, the CO 2 is produced in the so-called respiratory chain. Man in the Stone Age, who had no heating and fed on the substances of his immediate environment, lived on renewable raw materials. His life was CO 2 neutral and climate neutral. The way of life of today's man What makes today's humans different from those in the Stone Age? We have means of transportation that need to be powered. Predominantly, these are internal combustion engines that burn organic fuel and convert it into CO 2 . Our fuels are predominantly fossil fuels (coal, petroleum, natural gas) that were removed from the earth's carbon cycle over millions of years and safely deposited. Now we are releasing the CO 2 bound in them again in a short time. Even electrically powered vehicles indirectly release CO 2 from fossil fuels as long as the electricity is not generated from renewable energy sources. If we look at by category in Germany, the energy sector accounts for the largest quantities of carbon dioxide equivalents (CO 2 eq), at 84.2% .

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We have heaters in our apartments. Again, we use mostly fossil fuels for this. We have houses, cars and a wide variety of everyday objects, all of which have to be produced. This requires a lot of energy, which in turn is mainly generated using fossil fuels. And in transporting the items to the consumer, we are back to fuels. We afford ourselves the luxury of importing some of our food from distant countries, so again consumption of fuels. Even the fertilizer we use for intensive agriculture is produced with high energy input. Due to the high proportion of meat in our food, many cows are kept, which particularly contribute to the emission of the greenhouse gas methane. With the desire to get to know distant countries on our travels, we are back to fuels....... Modern communication via data exchange also consumes a great deal of energy. But we cannot return to the Stone Age. Therefore, our lives and actions will always result in CO 2 being released. This measure of CO 2 release due to human activity is also figuratively referred to as our CO 2 footprint.

Table: Shares of CO 2 eq emissions by category in Germany (2021) (German federal environmental agency)