Final answer:
Carbon monoxide is not considered a greenhouse gas because it does not effectively trap heat in the Earth's atmosphere like other greenhouse gases such as carbon dioxide, methane, and nitrous oxide.
Step-by-step explanation:
Greenhouse gases include all of the following EXCEPT carbon monoxide. The greenhouse gases that are most commonly associated with global warming and the enhanced greenhouse effect are chlorofluorocarbons (CFCs), ozone, and carbon dioxide. While carbon monoxide does have an effect on the atmosphere, it is not classified as a greenhouse gas because it does not effectively trap heat in the Earth's atmosphere. Major greenhouse gases include substances like carbon dioxide which is a byproduct of burning fossil fuels and deforestation, and it is the most abundant long-lived greenhouse gas contributing to climate change. Other significant greenhouse gases include methane, nitrous oxide, and synthetic gases like hydrofluorocarbons (HFCs).