What are greenhouse gasses? how does carbon dioxide enter the atmosphere? click through all four pages of the article.

Greenhouses basically function thanks to the greenhouse effect.
The greenhouse effect is a natural occurrence, which, helps maintain the  stable, livable, range of temperatures that we find on earth. To better understand how this phenomenon works, scientists have used the analogy of the greenhouse. In a greenhouse, the glass (the atmosphere) allows the radiant energy (light) of the sun to enter. As the radiant energy strikes various plants, gases, and objects in the greenhouse, they heat up. Their heat is then released back into the air of the greenhouse as infrared radiation (heat). This infrared radiation cannot travel through the glass and remains inside the greenhouse, increasing the temperature inside the greenhouse. Temperatures can be controlled, or kept in balance, by allowing some of the heat to remain inside and some to be vented to the outside. Under normal conditions, the earth's atmosphere works in a similar manner. Solar radiation enters the atmosphere and heats up the air molecules, the surface of the earth, buildings, plants, and other objects. These objects in turn, radiate infrared radiation back out into the atmosphere, where some of this radiation is absorbed by the greenhouse gases, and some is radiated out into space. As some of the heat stays in the atmosphere and some escapes into spaces, a balance is maintained allowing our planet to have a relatively stable range of temperatures. Scientists believe that as the greenhouse gases become more abundant in the atmosphere, due mainly to the burning of fossil fuels, they retain more and more infrared radiation, allowing less and less to escape into space, tipping the balance toward heating up the atmosphere, resulting in a changing and warmer climate, planet-wide. 

It is c. naturally in the environment

Migration people get up and go cause they feel it is best for them