Final answer:
Carbon dioxide (CO2) and methane (CH4) have both natural and human-caused sources. Human activities such as burning fossil fuels and agricultural production have significantly increased levels of these greenhouse gases. Oxygen (O2) is not a greenhouse gas and does not contribute to global warming.
Step-by-step explanation:
Greenhouse gases with both natural and human-caused sources include carbon dioxide (CO2) and methane (CH4). CO2 is naturally present in the Earth's atmosphere as part of processes like photosynthesis and volcanic eruptions, but its levels have risen dramatically due to human activities such as the burning of fossil fuels, cement production, deforestation, and agricultural production, which includes livestock farming and rice cultivation.
Methane is a more potent greenhouse gas than CO2, and its levels have also been greatly increased by human activities. These activities include agricultural processes such as enteric fermentation in ruminant animals like cows, management of manure, rice cultivation, as well as the decay of organic matter in landfills and the extraction of fossil fuels.
The question also inquires about a greenhouse gas that is not considered a significant contributor to global warming: oxygen (O2) is not a greenhouse gas, and therefore it does not contribute to global warming.