The petroleum industry extracts from the Earth and refines fossil fuels like oil, gasoline, and natural gas. Fossil fuels are based on hydrocarbons, or chemicals that are composed of carbon and hydrogen. When people use these fuels in their cars, homes, and businesses, they combust (burn) them for heat or other energy. This produces carbon dioxide as a byproduct. Carbon dioxide is not toxic to living organisms at all. In fact, plants require it to produce food. But carbon dioxide is a greenhouse gas, meaning that CO2 in the atmosphere will trap in heat from the Earth and keep it close to the surface instead of letting it escape into space. Over time, this can cause the general temperature of the planet to increase, a phenomenon called global warming. Global warming can affect weather, sea levels, and food production.