Coal, oil and natural gas are fossil fuels . When they are burned, they change the Earth's atmosphere. How is that possible? C oal is a good example. Coal was formed hundreds of millions of years ago . Geologists say that a three-metre (10-foot) coal seam took between 12,000 and 60,000 years to form . Ancient trees and other plants lived, died and were fossilised. All those plants took carbon dioxide out of the atmosphere. Some larger coal seams are, for example, 10 metres thick. They took around 40,000 years to form, but have been mined and burned in a little over 100 years. The fastest rise of CO 2 in the air seen in the ice core record (800,000 years) is 20 ppm in 1000 years. The CO 2 level in the atmosphere is now rising at around 20 ppm per decade . The carbon joins up with oxygen when it burns. Each carbon atom joins with two oxygen atoms to make a carbon dioxide molecule . As a result, oxygen concentration i