Global warming is the rise in temperature of the air and oceans globally. It is happening mainly because humans burn coal, oil, and natural gas; and cut down forests. Average temperatures today are about 1 °C (1.8 °F) higher than before people started burning a lot of coal around 1750. In some parts of the world it is less, and in some, more. Most climate scientists say that by the year 2100, temperatures will be 2 °C (3.6 °F) to 4 °C (7.2 °F) higher than they were before 1750.