Heat is a form of energy, but it’s certainly not the only one; there are so many I won’t bother you with a list. In any complex system — an automobile engine, the human body, planet Earth — energy is constantly changing from one form into another, so if some object has energy (and it always does), sooner or later that energy will distribute itself among the various available forms. Since one of those is heat, in general terms when you gain energy you’ll end up with more heat, when you lose energy, there’s less of it to go around and that means less can be heat.
Another important form of energy is electromagnetic waves. It’s a fancy science term but in at least one form it’s something we’re all familiar with: light. More generically we can call it radiation.
As an object transforms its energy into various types some of it will become radiation, and in that form it just can’t stand still. Not only does it have to move, it moves fast — at the speed of light — and doesn’t need anything to keep going, it can move through empty space. Because it travels so far so fast and needs nothing to keep going, it makes a great escape artist.
That’s why, when something has heat (and it always has some heat) it radiates energy; radiation is the principal way objects get rid of energy, and losing energy is how things cool down. The flip side is that gaining energy is one of the main ways things heat up.
To warm the planet as a whole, we don’t want to get that energy from Earth; just moving it from one part of the world to another won’t do the job. Earth heats up when it gets energy we didn’t have before, energy that comes in from space. That too arrives as radiation, from the sun.
The sun radiates copious amounts of energy because it’s so hot and it’s so big. Earth too has heat, so it too radiates energy. What we get from the sun is what heats us up, what we radiate out to space is what cools us down, and the globe’s temperature is a never-ending contest between the two processes.
[Note: It can happen that an object heats up without receiving energy from an outside source, when energy goes through one of its transformations. For example, radioactive material can decay, and when it does its nuclear energy takes other forms, including both heat and radiation. Because there’s radioactive material inside the Earth, the heat and radiation of its decay makes Earth’s interior quite hot. But radioactive decay doesn’t produce enough energy to keep Earth’s surface warm enough to live on; for that we need energy from the sun, without which the world would be a very cold place indeed.]
Somewhere along its travels radiation might encounter some matter, and when it does, three things can happen to it. First, it might be transmitted, passing right through as though the matter wasn’t even there. We’d call such matter transparent to the radiation. Second, it might be scattered, sent in some different direction (if sent in a very specific direction we might call it reflection). Third, it can be absorbed, in which case it ceases to be radiation — when radiation is absorbed the energy takes some different form, possibly heat.
The energy we gain from the sun and lose to space is in the form of radiation: electromagnetic waves. We could just as well call it light, although not all of it is. The word “light” usually refers to visible light, but is also often used to describe ultraviolet and infrared, different forms of radiation. They’re different because electromagnetic waves, like all waves, have a wavelength. Different forms have different wavelengths.
Visible light is radiation our eyes can actually see. It has very short wavelengths, from 16 millionths of an inch to 28 millions of an inch. [Note: Scientists use the metric system, so the wavelength range of visible light is usually given as 400 nm to 700 nm, with “nm” meaning nano-meters, i.e. billionths of a meter.] Even visible light differs according to wavelength, corresponding to color. The shortest visible wavelength (16 millionths of an inch) is that of violet light, next shortest blue, etc., covering the entire rainbow of colors all the way to deep red, which has the longest visible wavelength (28 millionths of an inch).
But the wavelength of radiation isn’t limited to what’s visible. Some light has wavelength too short for our eyes to see; it’s called ultraviolet.[Note: Bees can see shorter wavelengths than we can, so they see some of the ultraviolet we can’t, but fail to see red like we do.] And yes, there’s light with wavelength too long to see, called infrared.
Whether coming or going, between Earth’s surface and outer space radiation is destined to encounter some matter: the atmosphere. Radiation doesn’t come with a label saying whether it’s from the sun or from the Earth. It’s electromagnetic waves.
But radiation does have a wavelength. One of the great discoveries of physics is that when an object radiates because it’s hot, i.e. when it emits thermal radiation, the wavelength which carries the most energy depends on the object’s temperature.
The hotter an object is, the shorter the wavelength carrying away the most energy. Not all of the energy is radiated away at shorter wavelengths, in fact all wavelengths are still “on the table,” but the hotter an object is, the shorter the wavelength carrying most of its radiation energy.
The sun is a lot hotter than the Earth. Earth’s overall average temperature (at the surface, at least), is about 288K (Kelvins, a temperature scale for which a temperature of zero means no heat energy at all), but the temperature of the sun’s surface is nearly 6,000K. Most of the sun’s radiation is therefore at short wavelengths, those which correspond to visible light. But most of Earth’s radiation is at longer wavelengths, far into the infrared.
When radiation passes through the atmosphere, a lot of things can happen to it. The dust in the air might absorb it, heating up. Other tiny particles called aerosols may absorb radiation if they’re dark or scatter it if they’re bright. In fact, aerosols from huge volcanic eruptions will scatter incoming sunlight back to space, blocking incoming energy and cooling Earth off, but only temporarily, until the aerosols settle out of the air (which, for very large volcanic eruptions, takes a couple of years).
Then there are all those gases in the atmosphere. Most of the air is nitrogen (about 78%) and oxygen (about 21%), but for the most part they’re transparent to radiation, letting it pass unhindered. But there are other gases, which we can call trace gases for no other reason than they’re uncommon, which can interact with radiation strongly.
The most important are water vapor (H2O), carbon dioxide (CO2), and methane (CH4). They’re transparent to visible light, so have very little impact on the incoming radiation from space because most of that is visible light. But they react strongly with infrared light, absorbing it and then re-radiating it, so they have a profound influence on Earth’s radiation because most of that is infrared.
Technically the best term to describe such is probably “infrared-active gas,” but a much more common term is “greenhouse gas.” Because they react strongly with infrared, they significantly obstruct Earth’s radiation to space. That makes it harder for Earth to cool off. And that makes Earth’s surface hotter.
A greenhouse tends to be considerably hotter than the surrounding environment, and that’s where the name comes from. But an actual greenhouse stays hot mainly by inhibiting convection, the flow of air, while greenhouse gases keep Earth hot by inhibiting radiation. So, the term “greenhouse gas” isn’t really very accurate technically. But, it’s in almost universal use so we’re stuck with it.
The greenhouse gases in Earth’s atmosphere really do affect temperature profoundly. Without them — if we had no water vapor or carbon dioxide or methane in the air — Earth’s surface would be about 33C colder than it is (that’s about 60F). Our planet would be frozen, and life would be very different.
But the greenhouse gases which are present naturally keep Earth comfortably habitable for life. What we’ve been doing, however, is increase the amount of greenhouse gas in the atmosphere. We’ve increase the amount of CO2 by about 40% above its level before the industrial revolution. We’ve increased the amount of CH4 to more than twice its level before civilization.
As we’ve done so, we’ve increased Earth’s temperature. Because of that, we’re also increasing the amount of water vapor in the air. From thermodynamics we know that hotter air holds more water vapor than colder air, according to a relationship known as the Clausius-Clapeyron equation. Adding CO2 warms Earth, which icreases water vapor in the air, and water vapor itself is a potent greenhouse gas, which further warms the planet. It’s one of those feedbacks which makes an increase in CO2 even more potent as a planet-warming influence.
And that’s what a greenhouse gas really is. It’s an infrared-active gas, one that inhibits Earth’s radiation (mostly infrared) from reaching space, and that leads to global warming. The most potent is water vapor, simply because it’s the most abundant. CO2 comes next, then methane, and there are others too. One thing is absolutely certain — but deniers still try to deny it — the reason greenhouse gases have increased so much lately is: us.
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