Greenhouse gases are made out of:
water vapourcarbon dioxidemethanenitrous oxideozonechlorofluorocarbons (CFCs)
They are all natural gases, but extra greenhouses gases can be made by humans.
Environment experts are especially concerned about carbon dioxide.
How are extra greenhouse gases made?
Although plants absorb carbon as they grow, extra carbon is released into the atmosphere when humans cut down trees.
Extra greenhouse gases are produced through activities which release carbon dioxide, methane, nitrous oxide and ozone CFCs (chlorofluorocarbons).
These activities include:
Burning coal and petrol, known as 'fossil fuels'
Cutting down of rainforests and other forests
Animal waste which lets off methane