Today's atmosphere is about four-fifths (about 78%) nitrogen and about one-fifth (about 21%) oxygen. The remaining 1% or so is made of small amounts of other gases: argon, carbon dioxide (about 0.04%), water vapour and traces of others.
Carbon dioxide and methane are greenhouse gases. They absorb heat radiated from the Earth and release energy that keeps the Earth warm. Human activity has increased their levels. Burning fossil fuels releases carbon dioxide, and livestock farming releases methane, for example from cattle digestion. More greenhouse gases absorb more heat, so the average temperature of the Earth may rise. This is global warming, and it may cause climate change: rising sea levels, more extreme weather and changes to rainfall.
These effects may be reduced. Options include using renewable energy instead of fossil fuels, planting trees, capturing carbon dioxide, and reducing livestock numbers. Each option must be judged on its scale (how much it could change), its risk (how likely it is to work) and its environmental implications (for example, wind farms use land, and carbon capture is costly and not yet used widely).