Final answer:
Global warming is primarily caused by the emission of greenhouse gases like carbon dioxide and methane from burning fossil fuels, deforestation, and various industrial processes that change the heat-absorbing characteristics of the Earth's surface and atmosphere.
Step-by-step explanation:
The three main reasons that contribute to global warming are the emission of greenhouse gases, deforestation, and various industrial processes. The primary greenhouse gases include carbon dioxide (CO2) and methane (CH4), which are released in large amounts through the burning of fossil fuels such as coal, oil, and natural gas. Deforestation reduces the Earth's capacity to absorb CO2 through photosynthesis, thus accelerating warming. Additionally, industrial activities contribute to the release of other pollutants that increase global temperatures by changing the heat-absorbing characteristics of the Earth's surface and atmosphere.