Gases within the atmosphere act like glass, trapping the sun's heat. Just like any other planet, the Earth absorbs the sun's heat and radiates it back towards space. But greenhouse gases counteract that heat loss, trapping heat, and reflecting it back towards the Earth. The more greenhouse gases in the atmosphere, the more heat that is trapped. The less the amount of greenhouse gases, the less heat that is trapped.
Earth has just the right amount to help life flourish.
Too many of these gases, as is the case on Venus, would create a runaway greenhouse and a sizzling hot surface. Each greenhouse gas has its own important role in trapping the sun's heat, the most significant of which is water vapor. On a clear day, water vapor can comprise 60 to 70 percent of the greenhouse effect. Next in line, carbon dioxide contributes an additional 25 percent. Some gases trap solar radiation from the sun better than others.