Earth has warmed by about 1º F over the past 100 years. But why? and how? well, scientists are not exactly sure. The Earth could be getting warmer on its own, but many of the world's leading climate scientists think that things people do are helping to make the Earth warmer... .
One of the main greenhouse gases is carbon dioxide (CO2). As trees grow they take in CO2 from the air. When the wood dies the CO2 is returned to the air. Forest clearance and wood burning (such as happens in tropical rain forests) is increasing the latter half of the process, adding to the CO2 in the atmosphere. Deforestation is now out of control. For example in 1987 an area of the Amazon rain forest the size of Britain was burned, adding 500 million tonnes of CO2 to the atmosphere. The loss of the forests also means that there are fewer trees to absorb CO2.