Global warming
There are several  greenhouse gases that cause the Earth to warm. The most important one is carbon dioxide (CO2). CO2 comes from power plants which burn coal and natural gas to make electricity. Cars also emit CO2 when they burn petrol. About 35 billion tons of carbon dioxide are released into the Earth's atmosphere each year. The amount of CO2 in the air is about 50% more than it was around 1750. About three-quarters of the CO2 that people have put in the air during the past 20 years are due to burning fossil fuel like coal or oil. The rest mostly comes from changes in how land is used, like cutting down trees.