Step 1: Understanding the Concept:
The greenhouse effect is the process by which radiation from a planet's atmosphere warms the planet's surface to a temperature above what it would be without its atmosphere.
Step 2: Detailed Explanation:
While Carbon dioxide ($CO_2$) is the most discussed gas regarding human-induced (anthropogenic) climate change, Water vapour ($H_2O$) is actually the most abundant and potent contributor to the natural greenhouse effect.
Water vapour accounts for about 60% to 70% of the natural greenhouse warming, whereas $CO_2$ contributes significantly less in a purely natural state.
The concentration of water vapour is highly variable and depends on temperature (warmer air holds more moisture), creating a powerful feedback loop.
Step 3: Final Answer:
Water vapour is the primary natural greenhouse gas.