Today's atmosphere is about 78% nitrogen and 21% oxygen. The remaining 1% is mostly argon, with very small amounts of carbon dioxide (about 0.04%), water vapour and other gases. The amount of water vapour varies from place to place.
Carbon dioxide and methane are greenhouse gases. Human activity adds to both. Burning fossil fuels releases carbon dioxide. Livestock farming, especially cattle, releases methane, and so do rice fields and rubbish tips. More of these gases means more heat is trapped, so the average temperature of the Earth rises. This is global warming. It may cause climate change, such as melting ice caps, rising sea levels, more extreme weather and changes to where crops can grow and where animals can live.
These effects can be reduced, or mitigated. Examples are using renewable energy instead of fossil fuels, planting trees, and reducing livestock farming. Each option should be judged by its scale, its risk and its environmental implications. For example, a solution that works on a small scale may do little to change global levels, and some solutions carry risks or cost, or damage habitats in other ways.