The Greenhouse Effect
Energy from the Sun that makes its way to Earth can have trouble finding its way back out to space. The greenhouse effect causes some of this energy to be waylaid in the atmosphere, absorbed and released by greenhouse gases.
Without the greenhouse effect, Earth’s temperature would be below freezing. It is, in part, a natural process. However, Earth’s greenhouse effect is getting stronger as we add greenhouse gases to the atmosphere. That’s is warming the climate of our planet.
How Does It Work?
Solar energy absorbed at Earth’s surface is radiated back into the atmosphere as heat. As the heat makes its way through the atmosphere and back out to space, greenhouse gases absorb much of it. Why do greenhouse gases absorb heat? Greenhouse gases are more complex than other gas molecules in the atmosphere, with a structure that can absorb heat. They radiate the heat back to the Earth's surface, to another greenhouse gas molecule, or out to space.
There are several different types of greenhouse gases. The major ones are carbon dioxide, water vapor, methane, and nitrous oxide. These gas molecules all are made of three or more atoms. The atoms are held together loosely enough that they vibrate when they absorb heat. Eventually, the vibrating molecules release the radiation, which will likely be absorbed by another greenhouse gas molecule. This process keeps heat near the Earth’s surface.
Most of the gas in the atmosphere is nitrogen and oxygen – both of which are molecules made of two atoms. The atoms in these molecules are bound together tightly and unable to vibrate, so they cannot absorb heat and contribute to the greenhouse effect.
A Couple of Common Greenhouse Gases
- Carbon dioxide: Made of one carbon atom and two oxygen atoms, carbon dioxide molecules make up a small fraction of the atmosphere, but have a large effect on climate. There was about 270 parts per million volume (ppmv) of carbon dioxide in the atmosphere in the mid-19th Century at the start of the Industrial Revolution. The amount is growing as burning fossil fuels releases carbon dioxide into the atmosphere. There is about 380 parts per million volume (ppmv) now.
- Methane: A powerful greenhouse gas, able to absorb far more heat than carbon dioxide, methane is made of one carbon and four hydrogen atoms. It is found in very small quantities in the atmosphere but is able to make a big impact on warming. Methane gas is also used as a fuel. When burned, it releases carbon dioxide greenhouse gas into the atmosphere.
Above: The Earth’s surface, warmed by the Sun, radiates heat into the atmosphere. Some heat is absorbed by greenhouse gases like carbon dioxide and then radiated to space (A). Some heat makes its way to space directly (B). Some heat is absorbed by greenhouse gases and then radiated back towards the Earth’s surface (C). With more carbon dioxide in the atmosphere later this Century, more heat will be stopped by greenhouse gases, warming the planet. (Image: Lisa Gardiner/Windows to the Universe)
More Greenhouse Gases = A Warmer Earth
Even though only a tiny amount of the gases in Earth’s atmosphere are greenhouse gases, they have a huge effect on climate. Sometime during this century, the amount of the greenhouse gas carbon dioxide in the atmosphere is expected to double. Other greenhouse gases like methane and nitrous oxide are increasing as well. The quantity of greenhouse gases is increasing as fossil fuels are burned, releasing the gases and other air pollutants into the atmosphere. Greenhouse gases also make their way to the atmosphere from other sources. Farm animals, for example, release methane gas as they digest food. As cement is made from limestone, it releases carbon dioxide.
With more greenhouse gases in the air, heat passing through on its way out of the atmosphere is more likely to be stopped. The added greenhouse gases absorb the heat. They then radiate this heat. Some of the heat will head away from the Earth, some of it will be absorbed by another greenhouse gas molecule, and some of it will wind up back at the planet’s surface again. With more greenhouse gases, heat will stick around, warming the planet.