Why Greenhouses have nothing to do with the Greenhouse Effect, and more importantly, why CAN’T I microwave toast?
A greenhouse is a glass house sealed to keep air in but made of glass allow sunlight in. This sunlight contributes to the heat in the greenhouse by warming the ground or other material in the greenhouse, and of course the light energy is used by the plants. But the point of a greenhouse is to keep air that is warmed, by the sun and/or heaters that may be required in the greenhouse, from wafting away.
This is not how the so-called “greenhouse” effect works. There is no thing out there keeping warm air from wafting away from the planet. The air just stays there, greenhouse effect or not, moving around and doing the weather thing, and looking blue much of the time.
It is possible to find descriptions of the greenhouse effect (in the atmosphere) that make the analogy very directly, but this is incorrect. A gardener’s greenhouse works because it keeps air that has been warmed from leaving the vicinity at the same time it lets in light for plants to use, while the greenhouse effect in the atmosphere is different enough that the gardener’s greenhouse analogy is not as useful as it might seem. So let’s look at the Earth greenhouse effect, and in so doing, focus on what a “greenhouse gas” is and how it works.1
The sun is very hot. In part, this means that the matter that the sun is made of emits energy of some kind, and since it is VERY hot, this energy tends to be of very high frequency (short wavelength) … in what we call the “electromagnetic” range of frequency. The relationship between an object’s temperature and the frequency/wavelength of energy it emits is a matter of physics beyond our current scope, but you can think of it this way: A slowly moving object (say something vibrating at a few hundred up to several thousand times a second) will “hum” … it will emit sound. Each movement of the object “back and forth” makes a “wave” of sound, so the faster the movement, the higher pitch the sound.
Electromagnetic radiation … which sometimes goes by the name of “light” or “radio waves” and so on … is a kind of energy that can be stored in and sometimes comes out of atoms. It is a phenomenon happening at a much higher frequency than this sound wave analogy, and instead of being a series of sound waves (which involves the repeated compression of, for instance, air molecules) it is a series of waves and/or particles sometimes going by the name of photons. As you probably know already, this kind of phenomenon cannot be described as a stream of “things” (photons) in a way that explains all of its properties, and it cannot be described as a “wave” of energy in a way that explains all of it’s properties. If you really need to think about electromagnetic radiation in detail, you have to think of it as both/either/or particle and wave. Fortunately, you don’t need to think about it at this level to understand the greenhouse effect. What you do need to know is that this form of energy has a wide range of wavelengths, some of which we see (“light”); the frequency of the the energy determines much of its properties; and hotter things emit higher frequency wavelengths because the atoms in the hotter things are wiggling back and forth faster.
A quick digression on frequency and wavelength: Frequency is the rate at which something vibrates, or changes its state … like from negative to positive charge, etc. measured in units such as “billion cycles per second.” “Wavelength” is measuring the same exact thing, but instead of frequency per second, it is how much distance is traveled by this energy … typically moving at the speed of light … before it completes one full cycle from one state to the other. Think of it as the distance between the tips of waves on the sea. If the distance between the wave tips is shorter, there are more waves hitting the shore per minute, but if the distance is greater, fewer waves hit the shore per minute. Higher frequency (many waves) = shorter wave length, lower frequency (few waves) = longer wave length. (The analogy of waves on the sea will only get you so far, however.)
If you knew about a certain wavelength, and discovered an energy of shorter wave length, you might think of calling this “shortwave.” If you then discovered even shorter wavelength (higher frequency) energy, you might have to call this “microwave” (because you already used the word “short”) etc. Thus we have things we call shortwave radios and microwave ovens. These different machines use energies of different wavelengths.
Light (electromagnetic radiation that our eyes have evolved to convert to neural signals … i.e., energy we can see) has a range of wavelengths from about 700 to 400 nanometers (nanometers are very small … there are 1,000,000,000 of them in a meter). Energy that is of higher frequency is called ultra violet, because the highest frequency light we see is what we call “violet” in color, so higher frequency is “ultra” (ultra = extreme). Energy of lower frequency is called “infrared” because the lower end of the frequency range of visible light is called by us humans “red” … “Infra” means beneath, as in infrastructure (the roads, sewers, etc.) or “inferior.”
When you feel heat, you are actually perceiving energy that is down in this infrared range of wavelength. The next level down in frequency from infrared is called “Microwave.” The boundaries between these named ranges of wavelength/frequency are not always stark in terms of the effects of the energy. The higher frequency end of microwaves and the lower range of infrared will both cook your food. The higher or middle end of infrared happens to cook your food in a way that facilitates the famous “Maillard reaction” … a reaction between sugars and amino acids that makes your food taste good. This is why microwaves and “heat” both cook your food but the food comes out differently in taste and texture depending on method.
Yes, this IS related to global warming. The difference between microwaving vs. toasting a piece of bread has to do with the way in which specific, different, molecules react to specific, different, wavelengths of energy. A bunch of water molecules heated in a microwave or on a stove is the same … hot water. The various molecules in a slice of bread heated in a microwave vs. in a toaster react very differently, producing very different results (something inedible vs. toast).
A gas is a “greenhouse gas” because of the way it (its molecules!) reacts to a particular form of radiation (infrared).
Energy From the Sun
Most of the energy that reaches the surface of the earth is high frequency, including light. Light is only barely affected by the gases that make up our atmosphere. In other words, as the light wave/particles are moving through the atmosphere of the earth, most of them don’t get absorbed by the stuff the air is made out of.
(Now, this is not a coincidence. That which we call “light” moves around pretty freely in our planet’s atmosphere. We evolved on this planet. Our eyes can detect in fine detail this energy that moves around freely. Our eyes can’t detect the energy that is typically trapped by the magnetic field of the earth, because it is never around, so why would natural selection shape our eyes to be able to “see” this? If we evolved on a different kind of planet, physicists, would probably have a somewhat different set of instruments to detect and measure the energies they are so interested in. Perhaps an “optical” telescope would be used for a somewhat different (shifted one way or another, or narrower, or broader) range of “light.” OK, that was today’s shameless promotion of thinking-of- EVERYTHING -in-terms-of-evolution digression…)
When this high frequency (short wavelength) energy from the sun encounters the relatively solid matter that the earth’s surface is made of, including rocks, plants, liquid water, etc., it is absorbed by that matter. Not so much by the MOLECULES that matter is made of, but by the ATOMS that those molecules are made of. At this energy level (light, radiation, and such) the absorption is happening at the atomic level … this is an important fact.
You can think of it this way: Photons (light “particles”) have a very high probability of encountering an atom in, say, a rock. The atom “absorbs” the photon …. this means the photon essentially becomes part of the atom for the time being. An atom with this extra bit (the photon) changes. The way it changes is that one of the electrons (the outermost part of the atom is a cloudy space within which the electrons are flying around) stores this energy, what physicists refer to as “becoming excited.” This is probably why physicists do not get a lot of dates.
What has happened here is that high-frequency energy, the kind of energy that is emitted by a very hot object, has found its way to a cool object (earth surface temperatures are cool relative to the sun), and gotten stored there in the atoms that object is made of.
The Earth is a Big Space Heater
Now, this relatively cool matter can release the energy (depending on various laws of physics I won’t go into), but since the wavelength (frequency) of the energy released by an object is proportional to the temperature of the object (remember that from several paragraphs back?) this energy is of lower wavelength than sunlight. So, the relatively hard surface of the earth converts high frequency energy into lower frequency energy. This low frequency energy is what we think of as “heat.”
In this way, the surface of the earth is a simple machine that converts sunlight into heat. So, as long as the sun is shining on the earth, the surface of the earth is a big heater. Since the atmosphere is sitting right there on top of the surface, this big heater (the earth’s surface) heats up the atmosphere. Eventually, this heat … now in the atmosphere … makes its way to the outer limits of the atmosphere where it radiates off into space.
Neither rain, nor sleet, nor snow, nor gloom of night …
On its way towards outer space, this heat energy is absorbed by the molecules of the atmosphere itself, then re-released. Think of a bit of energy as a letter that you put in the mailbox. You know that when you put the letter in the mailbox, it does not simply disappear and rematerialize in the recipient’s mailbox. The letter changes hands many times, from the postal worker who picks it up, to other postal workers who sort the mail, move it from one place to the next, to the postal worker who eventually puts it in the recipient’s box. Hold this analogy in your mind for a moment…
If the atmosphere was made of a gas that is lousy at absorbing heat energy, the heat would radiate more or less directly into outer space, the total time required being a function of the total thickness of the atmosphere (and some other things). But if the atmosphere contains a certain number of molecules that are good at absorbing heat energy, that is like having a lot of extra postal workers … a certain unit of heat energy leaving the surface of the earth will be absorbed by a molecule, held for a while, then released, again and again. The greater the relative number of these heat-absorbing molecules in the atmosphere, the more this will happen, and the total amount of time this energy hangs around in the atmosphere will be greatly increased.
So the difference is … a postal worker picks up the letter directly from you and drives it directly to the recipient, vs. there is a system of many postal workers mucking around with your letter.
Imagine that a bit of gas absorbs a unit of heat. It then releases the heat. It will release the heat in all directions around itself. So a unit of heat that may have been moving “up” towards the outer edge of the atmosphere gets stopped and then released, and some of it continues on it’s way to the edge of the atmosphere, but some of it is released back towards the surface of the earth. So it isn’t just the number of postal workers (greenhouse gas molecules) that slows down the delivery (of your postcard, or of the heat to outer space) because there are more “handlers.” Imagine every postal worker has a 50-50 chance of sending your post card on in the right direction towards it’s destination, and a 50-50 chance of sending it back in the direction of the sender. That’s what the gas does. It randomizes the heat’s flow.
So a million letters mailed each day in an efficient postal system all move through the system quickly, so at any moment there is just over a million or so letters in the various bins and boxes in the postal system. But if we add just a few postal workers and program their behavior to be random, the letters will build up, and the bins, boxes, trucks, and mailbags will on average have more letters on a given day than otherwise.
Greenhouse gases are molecules that absorb and release heat passing through the system in random directions. The more greenhouse gases, the more the heat is passed around in random directions in the atmosphere, and the more the atmosphere “swells” with this heat.
Hot object (sun) irradiates cold object (the earth) with high frequency energy, cold object (earth’s surface) converts high frequency energy into low frequency energy (heat) which radiates away. Greenhouse gas molecules interfere with this process by randomizing the direction in which the heat goes.
But … what are greenhouse gases, already?
So why do some molecules absorb (and release) this heat while others don’t so much? It’s a matter of how the atoms that make up the molecule are bound together. The atoms in a molecule are held together by electromagnetic forces. The nature of this binding between atoms varies in different kinds of molecules. A molecule made of two identical atoms (which is how atmospheric nitrogen or oxygen usually occurs, two molecules each) are bound together with a kind of tightness and symmetry that they essentially act like they were a single atom, when it comes to low frequency radiation (like heat). Heat moves across a collection of gas molecules of this type a bit like waves in water … the molecules all sit there but the movement of the molecules (heat) passes across this matrix of molecules: Heat “arrives” by pushing on some molecules, then the molecules just push the next ones in line … and thus the heat passes along. (sort of) But if the molecule is made of different atoms, put together a certain way, then the relationship among the atoms in the molecule is in a sense flexible, so this heat energy (motion) can go from a wave of movement across a matrix to a bunch of movement WITHIN the molecule itself. Thus the energy is trapped for a while inside that molecule.
When the molecule then releases this energy, there is no “memory” of the direction in which it was moving … the energy now simply moves outward from the molecule. There may be a directionality to that … frankly I don’t know … there must be in some cases … but the direction of emission of this energy from a given molecule which is floating around in the air is not related to the direction from which the heat originally came, and there are a lot of molecules, so the effect is omni directional. The non-greenhouse gas molecules are like the hallways in the post office … they have nothing to do with stopping or redirecting the energy (letters). The greenhouse molecules are the perfect random postal workers. They stop the energy (letter), hold on for a while, and then send the energy (letters) off in a random direction.
Dry “atmosphere” is made of Nitrogen (78%), Oxygen (21%) and Argon (1%), and a tiny amount of other gases. The atmosphere can then include varying amounts of water vapor. Less than one percent of dry air is carbon dioxide and other greenhouse gases not counting water. At “100% humidity” something like 7% (depending on temperature) of the air is water vapor, and there can be as little as almost zero locally. Water vapor is a greenhouse gas.
Water vapor is both fairly fixed and highly variable. The total amount of free water on the earth does not really change, and how much is in the atmosphere at one point in time in a given spot varies a lot. The other atmospheric gases don’t change back and forth between gas and liquid (or solid) like water does, so they are more or less a constant (on a day to day basis) but since water converts back and forth between liquid or solid and gas form at typical Earth atmospheric temperatures (through evaporation and precipitation), it varies all the time. This, mainly, is what we know of as weather (along with a few details such as if the non-gas water is liquid or ice/snow!). The point is that humans do not directly change the water vapor system in any way that alters the greenhouse effect, but by adding (or removing) the other greenhouse gases, we can have large effect.
So how long it takes for your letter to get delivered on a given day may have to do with how many other people send mail that day (seasonally varying perhaps) and things like traffic, delays at airports, etc. but that all evens out over time so there is an average delivery rate. But if you go and hire twice as many inefficient postal workers, you slow down all delivery, on average, and over the long term.
Carbon Dioxide is the main greenhouse gas that humans add to the atmosphere. Prior to human effects it is estimated that the level of this gas in the atmosphere was about 260 – 280 parts per million for the previous thousands of years. The current level, elevated primarily because of human activities, is about 380 parts per million. That’s a lot of postal workers.
1In an earlier version of his post, a few commenters insisted that the analogy is the same because glass transmits light but not heat, so a gardener’s greenhouse is like the atmosphere. However, glass transmits heat pretty nicely. The reason a gardener’s greenhouse works is not because the glass does not transmit heat, but rather, because glass does not let the warmed air to escape. One can add to this, and this has been done only recently in actual greenhouses, double layer glass, which has air trapped between two layers of glass. The air does not transmit heat very well, so the greenhouse is reasonably well insulated.