Global warming
When people burn fossil fuels like coal, oil and natural gas this adds carbon dioxide into the air. This is because fossil fuels contain lots of carbon and burning means joining most of the atoms in the fuel with oxygen. When people cut down many trees (deforestation), this means less carbon dioxide is taken out of the atmosphere by those plants. Animals which have four places in their stomachs, like cows and sheep, also cause global warming, because their burps contain a greenhouse gas called methane.