Carbon dioxide makes up about 0.04% of our atmosphere. While that might not seem like a lot, it still comes out to more than three trillion tons of CO2 floating around in the air. Structurally, a CO2 molecule is pretty simple: one carbon atom with an oxygen atom on either side. It has two major environmental functions. First, plants and other photosynthesizing organisms pull it out of the air and combine it with water to create carbohydrates, which are the foundation of the food chain. Second, CO2 acts as a greenhouse gas, absorbing heat from the earth and preventing it from escaping out into space.
The Greenhouse Effect
Without its atmosphere, earth would be one giant snowball. Earth only gets enough energy from the sun to warm its surface to about 0°F—pretty warm compared to outer space, but still far too cold for life to exist. In reality, earth’s average surface temperature is a much more comfortable 58°F. Certain gasses in our atmosphere, such as CO2, methane, and water vapor, absorb and “recycle” some of the sun’s energy, keeping earth much warmer than the sun can by itself. This is the greenhouse effect. Without it, life on earth would not be possible. It occurs because the earth and the sun give off different kinds of electromagnetic energy, and because CO2 and other greenhouse gasses absorb one kind (from the earth) while allowing the other (from the sun) to pass through them.
Electromagnetic (EM) energy may sound exotic, but it’s actually very common. Light from the sun, microwave energy, radio waves, and even heat—all things we encounter on a daily basis—are types of electromagnetic energy. EM energy travels in the form of a wave—that is, it fluctuates between “peaks” where its electromagnetic field is strongest, and “troughs” where it is weakest. Different kinds of EM energy have different wavelengths, which can be visualized as ripples on the surface of a pond. The wavelength of the ripples is the distance from the top of one ripple to the top of the next ripple. Ripples that are very close together have a short wavelength, while more widely spaced ripples have a long wavelength. Wavelength is important because it determines how various kinds of EM energy interact with matter. As far as the greenhouse effect goes, the two kinds of EM energy that we are most interested in are visible light (sunlight), and infrared energy (a kind of heat). The key difference between the two is that visible light has a shorter wavelength than infrared energy.
Energy: Emission and Equilibrium
How hot an object is, and thus how much energy it emits, depends on how much energy it absorbs from its surroundings. The more energy it absorbs, the hotter it gets and the more energy it emits. If you sit in front of an empty fireplace, you’ll only be as warm as the room around you. But if you build a fire in that fireplace, the heat that it gives off will warm you and the rest of the room. By using the fire to add extra energy to the room, you are increasing its overall temperature. This is as true on a planetary scale as it is in your living room. If the earth suddenly started receiving more energy from the sun—or if less of the energy it did receive was able to escape back into space—its overall temperature would go up. All objects give off electromagnetic energy of some kind. The kind of energy they emit depends on how hot they are. Warmer objects give off energy with short wavelengths, while cooler objects give off energy with long wavelengths. For example, humans and other animals are only warm enough to give off infrared energy, which has a relatively long wavelength and is invisible to the naked eye. The filament in an incandescent light bulb, on the other hand, gets so hot when you turn it on that it glows with visible light, which has a shorter wavelength.
Which of these interactions occurs depends on three factors: the object’s elemental composition, its molecular structure, and the energy’s wavelength. Some objects will absorb one kind of energy while allowing other kinds to pass right through them. For example, the wavelength of visible light ranges from 390 to 700 nanometers. Within that range are bands that correspond to different colors of light (in order of decreasing wavelength: red, orange, yellow, green, blue, and violet). When we wear sunglasses, we see everything with a tint. This is because sunglass lenses do not transmit every wavelength of visible light. Amber-tinted sunglasses are transparent to red, orange, yellow, and green light, but absorb blue and violet light. We can only see the colors that reach our eyes. As it happens, when you can’t see blue and violet, the world appears amber-colored.Matter can interact with electromagnetic energy in several ways. Objects can absorb EM energy. Microwave ovens use this interaction to cook food: substances such as water and fat absorb microwave energy (which has a long wavelength) and heat up as a result. An object can also reflect EM energy, as when visible light bounces off a mirror. Transmission occurs when EM energy passes through an object without interacting with it at all. We can see through glass because it transmits visible light instead of absorbing or reflecting it.
Solar vs Terrestrial Energy Emissions
The sun is blindingly hot. Its surface is nearly 10,000°F, more than hot enough to give off visible light, and massive amounts of it. When the earth’s surface absorbs light from the sun, it heats up. As the earth’s surface gets hotter, it re-emits the energy that it absorbed back into space. However, since the earth absorbs a tiny fraction of the sun’s total energy, it only gets warm enough to emit infrared energy, not visible light (as you can probably imagine, a planet hot enough to emit visible light would not be a comfortable place to live). If you’ve ever stood on a blacktop on a hot, sunny day and felt the heat rising off the asphalt, you have some idea how this works: light goes in, infrared energy (heat) comes out.
The Greenhouse Gasses
It’s worth pointing out that the term “greenhouse effect” is actually somewhat misleading. A greenhouse stays warm because its glass windows let visible light in to warm its interior, but physically block warm air from leaving. Carbon dioxide and other greenhouse gasses do not trap warm air. They absorb outgoing electromagnetic energy and then reradiate some of it back toward the earth’s surface, supplementing the energy that earth gets from the sun. However, since there isn’t a better term to describe this phenomenon, we’ll just have to stick with “greenhouse effect.” Not all of the gasses that make up our atmosphere interact with energy in the same way. Nitrogen and oxygen, which make up the vast majority of our atmosphere (78% and 20%, respectively) let both visible light from the sun and infrared energy from the earth pass right through them. Other gasses in the atmosphere, including water vapor, carbon dioxide, and methane, are also transparent to visible light, but behave much differently when they come into contact with infrared energy emitted by the earth. Instead of letting infrared energy pass through them, these “greenhouse gasses” absorb it, which causes them to heat up. As they get hotter, they emit infrared energy of their own, some of which is directed back toward the earth’s surface, where it gets reabsorbed. As noted earlier, the earth only gets enough energy from the sun to warm it to 0°F. By recycling some of the earth’s outgoing energy, greenhouse gasses increase the overall amount of energy that earth receives. More incoming energy means warmer surface temperatures. It’s this extra energy that pushes the earth’s average surface temperature up to 58°F. Without them, earth would be too cold for life to exist.