Carbon dioxide is a colourless and odourless soluble gas that can readily dissolve in water. It is commonly used to create fizzy drinks.
In the early Earth’s atmosphere, water vapour condensed to form oceans. As oceans formed, large amounts of carbon dioxide dissolved in them, forming soluble carbonate compounds. This process reduced the amount of carbon dioxide in the atmosphere.
Over time, the carbonates in the ocean were precipitated and formed sedimentary rocks, including limestone which is mainly composed of calcium carbonate.
As green plants and photosynthetic algae evolved, they absorbed significant amounts of carbon dioxide through photosynthesis. These organisms, and the organisms from the food chains they supported, eventually turned into fossil fuels.