The greenhouse effect is a phenomenon caused by industrial pollution. According to the data taken by scientists, our earth has been warming up since the Industrial Revolution. Why does it happen? Emissions from factories have created smog in the atmosphere. The earth's atmosphere becomes like a greenhouse, and the earth and everything inside of it have become like the plants in a greenhouse. The sunlight can pass through the ozone layer but the heat transferred by the sunlight can not escape easily from the layer of smog and is deflected to the earth's surface again.