Greenhouse gases-the chosen ones

How exactly do greenhouse gases warm us up?

It is well-known that the excessive amount of Greenhouse Gas concentration in the current atmosphere aggravated the Greenhouse effect(the process by which Greenhouse Gases warm us up), which caused Global Warming. Before and after the sudden, abnormal addition of greenhouse gases to the atmosphere caused by us humans, the greenhouse effect has worked on Earth in two completely different modes: The Friendly Mode in before the 18th century(when the industrial revolution occurred) and the Destructive Mode in the current world.

Friendly Mode

The Friendly Mode is the original mode that the Greenhouse effect is designed to work in. When infrared radiation reaches the Earth's atmosphere from the sun(Yes, that is where Earth’s warmth comes from), a large portion of them that is not reflected back into space passes through the atmosphere and reaches the Earth's surface.

Infrared radiation is essentially an invisible wave of heat energy. When that wave of energy comes in contact with objects on the Earth’s surface—trees, houses, living organisms—they get absorbed and the objects heat up. However, some of this newly-absorbed energy is lost to the constant, outward emission of heat energy(in the form of infrared radiation too) from the object. This constant emitting of infrared radiation is just a natural thing that every object on Earth, and Earth itself, would do.

Tip: You might be wondering why objects can still stay warm even when constantly emitting the warmth they just obtained. But notice that as the objects lose heat energy from constant emission, they are also gaining energy from the sunshine that is always present and the infrared radiation emitted by the objects around.

Now, this re-emitted infrared radiation is sent in every direction. Some reach adjacent objects and get absorbed; others that are oriented upwards enter the atmosphere.

This is where the greenhouse effect and the greenhouse gases come into play. Greenhouse gases are different from other gases in the atmosphere—making them “the chosen ones”—in that they are able to absorb infrared radiation.

There are two requirements for a gas to be able to absorb infrared radiation: 1)its molecules have to be oscillating dipoles. 2) the frequency of vibration in its molecules needs to equal the frequency of the infrared radiation.

Tip: You might be wondering why we didn’t talk about these two requirements when the absorption of infrared radiation by “trees, houses, and living organisms” were discussed. That is because those objects on the earth’s surface are solids, and matter in different physical states(solids, liquids, and gases) obey different requirements in order to absorb infrared radiation. The two requirements written in the paragraph above are for gas to absorb infrared radiation, and don’t apply to the solids on Earth’s surface.

Greenhouse gases fulfill both of these requirements, thanks to the complicated molecular structure these gases are known for. Let’s compare oxygen(a normal gas that has a simple molecular structure), and carbon dioxide(a greenhouse gas that has a complicated molecular structure). When different gas molecules vibrate--a natural thing that every molecule on earth would do when they have energy, which they always have--they vibrate in different ways due to their different molecular structures. An oxygen molecule, with two oxygen atoms connected by a bond(as shown in the image below), can only do one type of vibration called the symmetric stretch due to its very simple structure. In such a vibration, no positive or negative charge is created at both ends of the oxygen molecule. A carbon dioxide molecule, with one oxygen atom at each end and one carbon atom in the middle, can vibrate in many other modes different from the symmetric stretch thanks to its much more complicated structure(three atoms!). One of the extra vibrations CO2 can conduct is the mode of bending.

Bending is a type of vibration that does create a positive charge at one end and a negative charge at another end of the gas molecule. As the bending mode occurs continuously, —the two “branches” of oxygen atoms move above and below the level of the carbon atom—the positive and negative charges at the two ends of the CO2 molecule are exchanged repetitively. This switching of charges between two ends of a gas molecule makes it an oscillating dipole—a molecule that has its two ends alternating between becoming positively and negatively charged. (See graph below)

Don’t worry about why the symmetric stretch mode does not create different charges at the two ends of the gas molecule but the bending mode does, this information shall be taught in future classes to keep this lesson from being too packed.

Some types of vibration allow the gas molecule to become an oscillating dipole, while some don’t. The number of vibrations and the type of vibrations the gas molecules of a certain type of gas can exhibit are decided by the structure of that gas molecule. Gas molecules that have more complicated structures are usually able to do a greater variety of, and more complicated types of vibrations. Greenhouse gases, such as carbon dioxide, are characterized by their complex molecular structures. Therefore, the fact that greenhouse gases just happen to perform the vibrations that would let the molecules become oscillating dipoles is no coincidence.

Now that we know how greenhouse gases fulfill the “oscillating dipole” requirement when most of the other gases cannot, we still need some explanation as to how Greenhouse gases fulfill the second requirement of having the same frequency as infrared radiation.

Now, you might be wondering why infrared radiation would have a frequency. Remember the statement mentioned above that infrared radiation is a wave of heat energy? Infrared radiation(infrared light) is a type of radiation(light), and radiations always travel in the form of waves. Light waves have peaks(the highest points) and troughs(the lowest points), just like the waves we see in oceans. (See diagram 1) A light wave’s frequency is defined by the number of times the light wave completes a full oscillation(reaches its peak and then its trough) in a second(see diagram 2). If that sounds too complicated, interpret light frequency this way—the more squeezed the graph of a light wave looks, the higher frequency the light wave possesses(see diagram 3). Lights waves are categorized according to their frequencies. A light wave with a 300GHz-400THz frequency is classified as infrared light.

Diagram 1

Diagram 2

Diagram 3

In the comparison of oxygen and carbon dioxide above, it is mentioned that oxygen, due to its simpler structure, can only conduct a vibration called the symmetric stretch. However, carbon dioxide’s much more complicated molecular structure allows its gas molecules to conduct three modes of vibrations. Different modes of vibration make gas molecules vibrate at different frequencies. The symmetric stretch exhibited by both O2 and CO2 molecules makes the molecules vibrate at a frequency different from the frequency of infrared light. However, the bending mode of vibration that is only exhibited by CO2 molecules makes the CO2 molecules vibrate in a range of frequency that overlaps with the range of 300GHz-400THz—the frequency range for infrared light. The more complex modes of vibrations that are only observed on greenhouse gas molecules tend to create frequencies that resemble the frequency of infrared lights. In contrast, simpler modes of vibration tend to make a gas molecule vibrate at frequencies that are different from the frequency of infrared lights. This means that gases that only exhibit one simple mode of vibration and no other modes of more complicated vibration can not absorb infrared light, while gases that can exhibit complex modes of vibration—such as greenhouse gases—are able to absorb infrared light.

Greenhouse gases absorb infrared light and then re-emit them(greenhouse gases, like all other objects on Earth, constantly emit infrared radiation) in all directions. Some of this infrared radiation gets absorbed by other greenhouse gases around; some of this infrared radiation may get emitted towards space; others are emitted towards the earth's surface again. This third group of infrared radiation(that is emitted towards the ground) travels down to warm the objects on Earth’s surface again. This exchange of infrared radiation—heat—between the atmosphere and Earth’s surface so that the infrared radiation repetitively warms up the two places without getting lost in space is called the greenhouse effect. The infrared radiation travels down to the Earth's surface, warms something there, then is emitted to the atmosphere, and after warming some gas there is emitted back down to the ground again to repeat this process.

You might be wondering if the infrared radiation from the sun(that is coming to the Earth for the first time) gets blocked or reflected into space given that greenhouse gases can absorb infrared radiation and then emit them in all directions. Yes, it turns out that 30% of the radiation(not just infrared radiation but all radiations) coming towards the Earth from the sun is reflected back into space. The remaining 70% of radiation most possibly got to the earth's surface after getting absorbed—then emitted—then absorbed again—then emitted again by the various gases that get in their way.

This is the friendly mode of the greenhouse effect, where this repetitive heating of infrared radiation keeps Earth warm. Without the greenhouse effect, the temperature of Earth will average at a freezing -18 degrees Celsius. The presence of greenhouse gases has allowed for the survival of humans on Earth by providing a temperate environment.

Destructive mode

In the post-industrial era, the atmospheric concentration of greenhouse gases soared due to a significant increase in greenhouse gas emissions at human civilizations. This increase in the atmospheric greenhouse gas concentration caused the portion of infrared radiation that escapes into space to reduce. After greenhouse gases absorbed the infrared radiation emitted from the earth's surface, a portion of infrared radiation will be emitted towards space, but there is a possibility that that radiation will be “intercepted” by other greenhouse gases in the atmosphere and instead get re-emitted towards some other direction. With an increase in the greenhouse gas concentration in the atmosphere, cases, where these infrared radiation oriented towards space, will get absorbed by additional greenhouse gas and instead get re-emitted back towards the Earth's surface will increase. In other words, it will be harder for infrared radiation to pass the atmosphere and escape into space now that the concentration of greenhouse gases in the atmosphere increased. Thus, more infrared radiation is kept between the atmosphere and the earth's surface, repetitively warming up the two, and causing Earth’s temperature to increase.

Greenhouse gases are special existences that greatly impact our lives. Having the special ability to be able to absorb infrared radiation, greenhouse gases can heat up the Earth increases its concentration, and freeze the Earth in its depletion. These complex gases important to the survival of all organisms on Earth are really the “chosen ones”. But essentially, greenhouse gases are the ones chosen to create a comfortable life for all organisms, including us. Perhaps it is time for us humans to investigate deep into potential methods that could return our greenhouse effect to its “friendly mode”, and let the greenhouse gases serve the purpose of protecting and not harming human civilization, again.