The trapping of heat in the lower atmosphere (the troposphere), referred to as the greenhouse effect, is a natural process that helps regulate the temperature of our planet, allowing for life on Earth to exist and flourish. Nitrogen and oxygen molecules make up 99% of Earth's atmosphere, with water and carbon dioxide accounting for less than 0.5%. This natural greenhouse effect is the result of heat absorption by H2O and CO2 molecules in the lower atmosphere, and re-radiation of this trapped heat back to Earth's surface. Without the natural greenhouse effect, Earth's temperature would be 0°F instead of its present 57°F. The question for some is whether human activities are causing an increase of the greenhouse effect, and thus global climate change. Scientific data collected in the past fifty years indicates that the concentration of greenhouse gases in the atmosphere has been increasing and the greenhouse gas that has stabilized in the lower atmosphere is
CFC.

Which greenhouse gas is most abundant naturally?

a. Trace gases

b. Water vapor

c. Carbon dioxide

d. Methane

e. Nitrous oxide