Well there’s many reason but the most common one is because of the green house effect. The stuff that causes greenhouse gases are the smoke from cars, factories and even those old hair spray, the bad thing about those is that they take YEARS to fully dissolve is the air. Also there’s a video that explains it more, really interesting also HELLLOOOO!!!!
Answer: Global warming is happening because the greenhouse gases are becoming more thicker which makes more of the suns radiation (heat) stay in the earth; as a result, this is increasing the earth's temperature.
