Final answer:
Carbon dioxide (CO2) makes the greatest relative contribution to increasing the greenhouse effect due to its high concentrations and increased emissions from human activities like the burning of fossil fuels and deforestation.
Step-by-step explanation:
The human-generated gas that makes the greatest relative contribution to increasing the greenhouse effect is carbon dioxide (CO2). Despite methane (CH4) being more effective per molecule, CO2 is released in significantly larger quantities due to human activities such as the burning of fossil fuels and large-scale deforestation. Thus, CO2 has a more pronounced impact on the greenhouse effect and is the primary greenhouse gas contributing to recent global climate change. Additionally, methane results from sources like livestock and decaying organic matter in landfills, but CO2 has a greater overall volume in the atmosphere, leading to more heat being trapped.
It should be noted that nitrous oxide (N2O) and ozone (O3) are also greenhouse gases affected by human activities. However, CO2 is more prevalent in the atmosphere and thus contributes more to the enhanced greenhouse effect. Oxygen (O2), on the other hand, is not considered a significant contributor to global warming.