A Greenhouse Effect Analogy
ACS Climate Science Toolkit | Narratives
When you stand in sunlight, you feel warmer than when you are in shadow, so you can feel that light from the sun carries energy that can warm an object—you. You can feel the warming effect, even if the sunlight comes through a window before it hits you. In the same way, the soil, pots, plants, and so on inside a greenhouse, like the one pictured here, are warmed by the sunlight that shines through its transparent walls and roof. The air inside the greenhouse is then warmed by contact with the warm objects. If the door and ventilation windows are closed, the warm air can’t escape, so the temperature of everything in the greenhouse goes up.
Nowadays we hear a lot about the “greenhouse effect” because the Earth is warm and it is some gases in the air (greenhouse gases) that are responsible for keeping the Earth warm and suitable for life as we know it. Energy from sunlight is the source of the warmth in a greenhouse, because the sunlight can pass right through the transparent enclosure. Energy from sunlight is also the source of the warmth on Earth, because the atmosphere is pretty transparent to sunlight. However, the way the Earth and its atmosphere work to stay warm is different from the way a greenhouse works, so the term “greenhouse effect” is a bit misleading.
A different analogy is the atmosphere acting like a blanket. Think of yourself under a blanket in a cold room. You represent the Earth, a warm body giving off energy, what we usually call “heat”. The blanket represents the atmospheric layer of greenhouse gases. As the heat energy leaves your body it is absorbed by the innermost fibers of the blanket. As they give off some of that energy, they warm the next layer of fibers and so on and on until some energy leaves the outermost cold fiber layer and is lost to the room. Just as the sun continually warms the Earth, you continually produce energy through respiration. You will finally reach a balance where the energy leaving the blanket is equal to the energy you produce and your skin will remain at a constant temperature, just like the surface of the Earth.
Among the Earth’s blanket of greenhouse gases, carbon dioxide is the one you probably hear about most often, because it is increasing in the atmosphere as we burn a great deal of coal, oil, and gas for energy. Besides carbon dioxide, greenhouse gases include water vapor, methane (natural gas), nitrous oxide (from fertilizer use), and chlorine- and fluorine-containing gases used in air conditioning units and as solvents. Adding more greenhouse gas to the atmosphere increases the amount of absorption and release of heat energy required for the energy from the surface to reach the top of the atmosphere and go off into space.
In our blanket analogy, this is like putting on another blanket, so there are more layers of blanket fibers for the energy to pass through to reach the top. The old energy balance is upset and after a time a new one takes its place with you feeling warmer than before. This is what is occurring for the Earth as well. The increased amounts of greenhouse gases our activities are adding to the atmosphere have upset the balance that was in place since the end of the last ice age and the Earth is getting warmer than it was before we started burning large amounts of fossil fuels.