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The Earth is surrounded by the atmosphere.
This is made up of 78% nitrogen (N2), 21% oxygen (O2) and 1% other, including argon, water vapour and carbon dioxide (CO2).
O2 is required for humans and animals to respire, and CO2 is required for photosynthesis.
CO2 is also the byproduct of respiration.
Because of human activity, the atmosphere is changing. This is causing global warming.
Carbon dioxide is a greenhouse gas, and more of this is being added to the atmosphere, causing the greenhouse effect. Water vapour and methane are also greenhouse gases.
Greenhouse gases are mostly released from burning fossil fuels which include coal, oil and gas, or from farming. This is because farm animals release a lot of methane too.
As plants and trees are carbon stores, deforestation also has an impact since trees take in carbon dioxide for photosynthesis. Therefore, when trees are cut down and burned, this releases the greenhouse gases back into the atmosphere.