The only gas in these selections that is made only by man is _, which is a CFC or Chlorofluorocarbon. , methane, can be produced by many other organisms; a key example of this are cows. , or water vapor, has many natural origins such as bodies of water, and finally  also has natural sources like volcanoes.

The variance of annual  levels can be accounted for by a combination of both the  tilt of the Earth on its axis by as well as the revolution of the Earth around the sun. These two factors in conjunction with one another create the seasons. Since the Northern Hemisphere has vastly more plant coverage compared to the Southern Hemisphere, the Earth can uptake more carbon dioxide during the summer and spring seasons due to high levels of photosynthesis; however, when the Northern Hemisphere is in fall and winter, many of its plants cannot photosynthesize, leading to a spike in atmospheric carbon dioxide.

The Northern hemisphere has a much larger basal area comparatively to that of the Southern hemisphere. When the Northern hemisphere is in winter, many of its plants are not able to photosynthesize, therefore the global  levels rise due to much fewer plants actively uptaking that  for their photosynthetic processes. By the same token, when the Northern hemisphere is experiencing spring and summer, the carbon dioxide levels fall due to higher photosynthetic activity.

All of these greenhouses gasses, except , occur naturally in the environment.  was produced by humans and was commonly known as Freon-12. It was marketed as a manmade refrigerant and aerosol propellant. 

The greenhouse effect occurs as the temperature within the Earth's atmosphere rises. Greenhouse gases trap the heat within the Earth's atmosphere. As sunlight enters the Earth's atmosphere, it passes through the gaseous layer and the Earth's surface absorbs part of the energy and reflects some energy. These reflected rays of energy get trapped between the Earth's surface and the atmosphere. Some common greenhouse gasses include: carbon dioxide, methane, nitrous oxide, and tropospheric ozone. Helium and nitrogen are two gases in the atmosphere, but they do not contribute to the greenhouse effect.

Methane gas produced by livestock is a very potent greenhouse gas, with molecules that trap far more heat in our atmosphere than carbon dioxide. When methane is burned, carbon dioxide and water vapor are released into the atmosphere, but these vapors have a relatively benign impact on global warming when compared to the release of methane, had it not been used as fuel. Therefore, harvesting methane gas from agricultural waste and livestock prevents potent greenhouse gases from escaping into the atmosphere and also reduces dependence on energy derived from fossil fuels. 

Chlorofluorocarbons, aka CFCs, are nontoxic, nonflammable chemicals containing atoms of carbon, chlorine, and fluorine. They are used in the production of aerosol sprays, foams and packing materials, as solvents, and as refrigerants. Though mostly inert in the lower atmosphere, that low reactivity affords CFCs a lifespan that can exceed 100 years, giving them time to diffuse into the upper atmosphere and stratosphere. Once in the stratosphere, the sun's ultraviolet (or thermal) radiation is strong enough to cause the cleavage of the  bond, making it an active ozone reducer. Furthermore, CFCs absorb and emit absorb and emit infrared radiation in the wavelength range emitted by Earth, make them potent greenhouse gases.

The greenhouse effect plays a role in all of the options because of the interaction between the layer of gasses surrounding the earth and the solar radiation that approaches the earth. An easy way to remember this is to follow the path of solar radiation traveling from the sun to the earth and think of all the possible ways in which this radiation can travel. The radiation can bounce off and not enter, it can enter and be absorbed in the surface, or it can enter and both bounce off the earth's surface and then be absorbed.

Every scientist listed but Norman Burlaug are responsible for the early studies of the greenhouse effect. Joseph Fourier spoke about the effect in 1824. Claude Pouillet studied its existence further in 1827 and 1838. John Tyndall conducted experimental observations in 1859. 

This term was first used by Nils Gustaf Ekholm in 1901. Ekholm was an early adopter of climate control. His early studies included how the burning of coal could add unnecessary levels of carbon dioxide to the atmosphere.