Final answer:
Carbon dioxide (CO₂) is the primary gas that contributes most significantly to the greenhouse effect and global warming, due to its abundance and persistence in the atmosphere coupled with human activities like burning fossil fuels.
Step-by-step explanation:
The gas known to contribute the most toward the greenhouse effect and global warming is Carbon dioxide (CO₂). While other gases like methane (CH₄) and nitrous oxide (N₂O) have a higher global warming potential per molecule, CO₂ is much more abundant and lasts longer in the atmosphere, making it a significant contributor to global warming.
Human activities, such as the burning of fossil fuels and changes in land use, release large amounts of CO₂, further exacerbating the problem.