many different things. first, there might be natural phenomena that lead to global warming, but i guess we are talking here about the non-natural reasons which mainly reduce to: - many different types of pollution.
global warming is caused by air pollution, deforestation, etc.
uncertain origin, but co2 emission is suspected.
cars
mainly humans. mainly vehicles around the world are contributing to global warming, as well as the burning of fuels in power plants.
humans
pollution, deforestation, modernization of the world, etc.
sunlight came to earth but did not reflect back, which causes global warming.