Final answer:
Carbon dioxide (CO2) accounts for the majority of global greenhouse gas emissions due to human activities like the burning of fossil fuels and deforestation. Other greenhouse gases contribute to varying degrees, but CO2 is the most prevalent. Oxygen is not considered a significant contributor to global warming.
Step-by-step explanation:
Most of global greenhouse gas emissions are made up of carbon dioxide (CO2). Carbon dioxide has the highest concentration in the atmosphere of any of the greenhouse gases, and it is a direct product of combustion processes such as the burning of fossil fuels including coal, oil, and natural gas, as well as deforestation and various industrial activities. While other gases like methane (CH4) and nitrous oxide (N2O) also contribute significantly to the greenhouse effect, they are present in lower concentrations compared to carbon dioxide. It's also important to note that greenhouse gases like CFC's have been mostly phased out due to international environmental legislation, so their current contribution is less significant.
The gas that is not considered a significant contributor to global warming is oxygen (O2), as it does not trap heat in the atmosphere like the other gases mentioned.