 Coal, oil, natural gas, shale gas and gas from fracking are fossil fuels formed hundreds of millions of years ago from living things that got trapped by layers of sediment before they had time to decompose. Let's see why people are worried about burning these fossil fuels. If you burn 12 tons of coal, how much carbon dioxide do you think is formed? Pause and think. Well, coal is made mainly of carbon, and when it burns it reacts with oxygen like this. Carbon atoms have an atomic mass of 12 and oxygen, 16. So 32 tons of oxygen react with 12 tons of carbon to produce 44 tons of carbon dioxide. If you said there would be much more than 12 tons of carbon dioxide, you were right. Humans have been burning fossil fuels for about 300 years. As a result, the amount of carbon dioxide in the atmosphere has kept on rising, so we need to replace them with an energy supply that does not put extra carbon dioxide into the atmosphere. Perhaps firewood is the oldest biofuel mankind has used. We now have biogas, gas-a-hole and many more. Carbon dioxide is produced when these burn, but as long as the trees and the plants are allowed to regrow, burning them simply helps carbon on its way round its natural cycle. Consider biogas. When living things die and there's a lack of oxygen, bacteria can use anaerobic respiration. Although it only releases a very small amount of energy, some of the carbon atoms stay bonded to hydrogen to form methane. This is the reaction, highly simplified, that happened millions of years ago to form deposits of natural gas, but it's also happening all the time today where ever organic matter is allowed to rot in the absence of oxygen, for example in the gut of cows, who produce 200 litres of methane from their rear ends every day. If we collect the feces from humans and farm animals and place them in a digester, we can mimic what happens in a cows intestine, and we get biogas. There are many such digesters in use all round the world, where gas is used for cooking and lighting, and the remains are a rich fertilizer. Most digesters are used in most sewage works, where the methane is burnt to generate electricity. Another example of biofuel is agricultural waste, for example wheat straw, which is burnt to generate electricity. But nowadays we are growing crops especially to use as a fuel. In Brazil, sugar from sugarcane is fermented to make alcohol, or gasahol, to fuel their cars. In Europe and USA, vegetable oil is made into fuel for diesel engines. When these fuels are burnt, the carbon atoms return to the atmosphere ready to be used again. The next year, when the crops grow again, which explained why these fuels are called carbon neutral, but this is not quite the full story. Growing biofuel crops uses large amounts of fossil fuel for fertilizer and manufacturing, and it uses large areas of agricultural land needed for food production. Action aid suggests that half the world's hungry, 400 million people, could eat properly using the food that rich countries burn in their petrol tanks as biofuels. However, there is hope scientists are trying to grow oil-rich algae using seawater and sunshine. This does not use valuable agricultural land and needs no fertilizers.