So you probably know that we breathe out CO₂ and plants consume CO₂ in the process of photosynthesis. In addition there are many other sources of CO₂ for instance, when most things are burned, they produce CO₂. Volcanoes, hot springs, and geysers also release CO₂ into the atmosphere. Deep under lakes and the world’s oceans there is an abundance of CO₂ dissolved in the water. In fact, there is 50 times more carbon dioxide in the sea water than in the atmosphere. All of these different sources of CO₂ are connected through the “Carbon Cycle” which is important in making Earth sustainable for life. One answer to your question is that carbon dioxide comes from other sources of carbon in the world.

A more cosmic question is where did the carbon dioxide come in the first place? The answer to this question is that Carbon and Oxygen are produced deep within a massive star. These elements are made from lighter elements in a process called “nuclear fusion.” When a star dies, its insides are thrown through space and some of them will end up on a planet. This is the original source of the carbon dioxide on Earth.

Carbon dioxide is generated from the oxidation of carbon. So you have to have both oxygen and carbon atoms and some energy to get the reaction started. For example, you could have a spark that begins the burning of coal (which contains mostly carbon) in air which contains oxygen. This produces carbon dioxide. Other sources of carbon dioxide include animal respiration (we exhale carbon dioxide), which is a result of the oxidation of carbon-containing food that we eat for energy. Burning wood or gas or other carbon-based fuels also produces carbon dioxide.

Carbon dioxide comes from many, many sources in the world. Most commonly, though, it is the end result of breaking big molecules into smaller ones. Carbon dioxide is very energetically stable (think: it does not burn) and it is so stable your body cannot do anything with it. Plants can use it because they use energy from the sun to build other molecules with it, but this takes a lot of energy. So, think of some processes that release energy by breaking down bigger molecules into smaller ones and energy. Combustion (burning fuel) in a car engine breaks large hydrocarbons (carbon-based molecules) into much smaller, more stable carbon dioxide. This releases a lot of energy, and also carbon dioxide. In the same way, your body breaks down sugars and combines them with oxygen you breathe to make carbon dioxide. These are two main sources, but carbon dioxide also occurs naturally.

Carbon dioxide results when you combine carbon or any carbon-containing compound with oxygen from the atmosphere. You can also get it from dissolving carbonate rocks in acid (and yes, water is a very weak acid). You breathe in oxygen and breathe out carbon dioxide, because the fuel that your body runs on is carbon-containing molecules. Burning almost anything will release carbon dioxide (the exceptions are burning metals or sulfur), since most things that burn contain carbon.