Some human activities increase the amount of greenhouse gases in the atmosphere. Two important gases are carbon dioxide and methane.
Carbon dioxide is released when fossil fuels such as coal, oil and natural gas are burned, for example in power stations and in vehicles. It is also released when forests are cleared and burned, and deforestation means fewer trees are left to take in carbon dioxide by photosynthesis.
Methane is released by farming and waste. Cattle and other livestock produce methane in their digestion, and rice paddy fields release it too. Methane also comes from rubbish decomposing in landfill sites.
Many scientists believe, based on peer-reviewed evidence, that these activities will raise the temperature of the Earth's atmosphere at the surface and cause global climate change. Peer review means other scientists check the work before it is published. However, climate is a very complex system and is hard to model, so models are simplified. Media reports may use only part of the evidence and may be biased, so the quality of evidence in a report should be judged carefully.