Fossil fuels are hydrocarbon fuels formed over many millions of years from the decomposition of dead organisms. Because this process takes so long, they are not considered a form of renewable energy. Their amount on this earth is limited, and we will be nearing their limit in the near future. This is one of the two main drawbacks to using fossil fuels as the primary source of energy. The other main drawback to fossil fuels is the by-products released during the combustion. Carbon dioxide is a greenhouse gas released from the burning of fossil fuels and it can be very harmful to the environment. After the sun's radiation reflects off of the earth's crust, carbon dioxide and other greenhouse gases absorb the radiation keeping it trapped in the atmosphere, as opposed to it reflecting out back into space. This process slightly warms the earth and atmosphere. Over time, this warming could melt the polar ice caps, which would throw off the oceanic oscillations, which could destroy many organisms' habitats, which would throw off the whole food chain. This whole chain of events would be very bad for the entire earth and all its inhabitants including YOU.