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 an artificial greenhouse filled with plants, the surrounding glass traps
the sun's energy, making it warm inside, even while outside the temperature may
be much colder. This same effect happens every day on the Earth. Gases within
the atmosphere act like glass, trapping the sun's heat. These gases include
water vapor, carbon dioxide, methane, nitrous oxide, ozone, and
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. On the other hand, without any greenhouse gases, much of the sun's
heat would be lost, and 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 in line, carbon
dioxide 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.
The changes in the balance and concentration of all these gases can affect the
Earth's temperature, and 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.
But even as scientist debate the impact of changes to the greenhouse gases,
there is still one fact with which they all can agree - without the greenhouse
effect, life on this planet would not be the same. In fact, we would not be
here at all.