Question

Which one is a more effective greenhouse gas, $C{O}_2$ or $H_{2}O$?

Answer 1

The average amount of water vapor in the atmosphere averaged for all locations is between 2 and 3%. Carbon dioxide levels are near 0.04%. That means there is more than 60 times as much water vapor in the atmosphere than carbon dioxide in average conditions. Both water vapor and Carbon dioxide are greenhouse gases. They both trap outgoing longwave radiation between the earth and the atmosphere. This has the effect of keeping temperatures warmer. Carbon dioxide is a more efficient greenhouse gas than water vapor when both are in equal quantities. However, they are not in equal quantities. There is much more water vapor than carbon dioxide in the atmosphere. In day-to-day weather forecasting, the greenhouse effect from water vapor is important while carbon dioxide is not.
So, $H_{2}O$ is a more effective greenhouse gas.