According to NOVA and PBS, of all the planets in our solar system, the Earth is the only one that—as far as we know—supports life. So why is our planet alone so hospitable? In part, we owe our existence to a process called the greenhouse effect.

Inside commercial greenhouses filled with plants, the surrounding glass traps the sun’s energy, therefore making it warm inside even while the outdoor temperature may be much colder. This same effect happens every day on Earth. Gases within the atmosphere act like glass and trap the sun’s heat. These gases include water vapor, carbon dioxide, methane, nitrous oxide, ozone, and chlorofluorocarbons (CFCs).

Just like any other planet, the Earth absorbs the sun’s heat and radiates it back towards space. Then the greenhouse gases counteract that heat loss, trapping heat. It then reflects 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. On the other hand, without any greenhouse gases, much of the sun’s heat would be lost. So the Earth would become a frozen wasteland with an average temperature of 0 degrees Fahrenheit (-18 degrees Celsius).

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 is carbon dioxide which contributes an additional 25 percent. Some gases trap solar radiation from the sun better than others. For example, while man-made CFCs are one of the least plentiful gases, they actually have a greater relative impact than many others.

Do we live in a greenhouse on this planet? schematic of CO2's actions in atmosphereThe changes in the balance and concentration of all these gases can affect the Earth’s temperature. Therefore these temperature changes are often referred to as “global warming” or “global cooling.”

Greenhouse gas concentrations in the atmosphere have been naturally rising and falling for billions of years, creating cold and warm periods in the Earth’s history. For example, as the Ice Age progressed, scientists believe the amount of natural carbon dioxide in the atmosphere dropped over thousands of years, reducing the greenhouse effect, and making the Earth cooler. But many disagree on how that change in carbon dioxide occurred (see “Big Chill”
by Kirk Maasch).

Today, scientists are looking at effects of global warming as they debate the long-term impact of man-made carbon dioxide and CFCs entering the atmosphere. Many climatologists argue that we are artificially increasing the greenhouse effect, warming the Earth faster than would occur naturally, which could cause problems for the Earth in the future.

In conclusion and even as scientist debate the impact of changes to the greenhouse gases, there is still one fact with which they all can agree.That’s without the greenhouse effect, life on this planet would not be the same. In fact, we would not be here at all.

Finally and to learn more about the atmosphere and its many layers, take a look at The Atmosphere, part of the “Balloon Race Around the World” Online Adventure.