The Greenhouse Effect

 There is a vast amount of disinformation out there about the greenhouse effect. This is mostly because it's a highly politicized bit of science. If not, there wouldn't be much information about it of any kind, like rates of stress-induced creep in iron-terbium alloys.

But instead, as Mark Twain said, it's not what you don't know that gets you, it's what you know that ain't so. And there is a YUGE amount of stuff that ain't so about the greenhouse effect on the internet. This ranges from the "we're all gonna die" idiocy of the far left to the "there's no such thing as the greenhouse effect" of the far right. (It was one of the latter that goaded me into writing this post.)

So what is the greenhouse effect? It's a term used by astronomers to refer to some of the major phenomena of a planetary atmosphere. The easiest way to see it is to compare two planets, one with and one without an atmosphere. Luckily, we happen to have two such planets handy: the Earth and the Moon. As an added bonus, they are the same distance from the Sun and thus get the same amount of sunlight.

So here's Buzz Aldrin standing on the Moon. Behind him you see the shadow of the Lunar Module. The temperature of the ground in shadow is 160 degrees below zero Fahrenheit. The temperature of the ground he's standing on is plus 180. He's wearing those thick-soled shoes for a reason.

Here's the greenhouse effect in one picture. In red we see the temperature on the Moon over the course of a year. It varies, top to bottom, by roughly 300C or 540F. 

The blue line is the temperature where I live, measured every 20 minutes at the local airport. The day/night variation is more like 20 degrees. (There is also a seasonal variation that the Moon doesn't have, due to the Earth's axial tilt.)

The difference between them is the greenhouse effect. Without the greenhouse effect, we would freeze to cryogenic temperatures every night, and literally boil every day. Without the greenhouse effect, we could not live and would never have evolved.

What is the major effect of the greenhouse effect? You may have heard that it is to raise temperatures. Wrong. Look at the graph again. The major, obvious, effect is that the temperatures on the Moon vary 25 times more than the ones on Earth. The greenhouse effect compresses temperatures. As a secondary effect, it compresses them toward a level that is about two thirds up the Moon's scale.

How can that be? You will have heard that the greenhouse effect is like a blanket around you on a cold night. This is a misleading metaphor. You generate heat internally, so insulating you from a cold environment will warm you up. The Earth generates heat as well, but the amount of it is minor compared to the heat it gets from the Sun. So the appropriate metaphor is to put a blanket around you while you are standing next to a big hot fire. Perhaps you have done this in real life; I have. The blanket warms your back but cools your front. And that is what having an atmosphere does to a planet.

So what about the part where sunlight comes in one atmospheric window, but goes out another, and we're closing the outgoing one? 

Here's a chart of how this works. Scattering (which makes the sky blue) and ozone protect us from hard UV (another of the many things that would kill you on the Moon). Only about 3/4 of the Sun's light gets to the surface. Most is visible light but there is a substantial amount of infrared as well. You can feel warmth from the Sun on your face and other bare skin. There are plenty of places on Earth where the heat goes back out through the near-IR bands as well; you'd feel the heat from the sands of the Sahara or central Australia too!

Even so, the majority goes out through the 10-micron atmospheric window as well. The three curves there represent the patterns of energy as a function of wavelength for -82F (black), +8F (blue), and 98F (purple) respectively. All of the Earth is somewhere in this range, so most of the thermal radiation it produces will go out this "atmospheric window."

The atmospheric window is the gap in the absorption spectrum of water as seen on the top "major components" line. We can put it in perspective with the other energy flows through the atmosphere with this chart (with its two authors):

The atmospheric window is shown in red over near the right side of the graphic.

As we noted, the atmospheric window is a function of the moisture content of the air. Chances are you have personal experience of this too. There are moist cloudy nights where the temperature hardly drops at all. There are clear, cold, nights where there is hardly any water in the air.

The atmospheric window is smallest when it is moist:

(generated by HITRAN, the standard program/dataset for gaseous absorption. The horizontal scale is frequency measured in kaysers.)

And slightly less moist. The blue is of course absorption by water. The green you see on the left side is CO2.

And finally, very dry air as over the Sahara or the Arctic. It should be clear that the CO2 is more important when the atmospheric window is bigger, i.e. when there is less of the major, water based, greenhouse effect.

But we have been dumping CO2 into the atmosphere at unprecedented rates. What has that done to the window?

Here is the absorption spectrum of CO2 in the region of interest:

It's centered at about 670 kaysers, which is why it is hiding off to the left in the above pictures of the atmospheric window, which is centered at 1000. Here it again, but I have added the amount of extra absorption corresponding to the extra CO2 that has been added since 1900 (in red):

Wow, is that all? In a word, yes. The response to CO2 is logarithmic, meaning that absorption is proportional to the logarithm of the amount, and not to the amount itself. The reason is, as you can see, the band that CO2 absorbs is already mostly saturated. The amount of carbon in the air over a square foot of earth's surface is roughly equivalent to a square foot of black wool cloth. If it were carbon soot, the sky would be completely solid black. But it isn't; it's not carbon but carbon dioxide, which is transparent at most frequencies except this one. 

Only at the frequencies near the edges, where CO2 is translucent, does adding any more of it make much difference.

So here is the atmospheric window, this time with the CO2 we've added in the past century:

Humid air:

Moist:

Fair:

Arid:

So, the bottom line for CO2 is that it only makes much of a difference when the atmospheric window is already at its biggest, and the actual greenhouse effect is almost entirely due to water. And given that three quarters of the Earth is ocean, there's not much we could do about that. 

Even if we wanted to.

We would expect the effect of CO2 to appear primarily where there is not much water in the air. In other words, deserts and places where the water freezes out. Have a look:

As a bottom line, let repeat: the greenhouse effect is very real. It is absolutely desperately important for any life on earth: without it, the very air would freeze every night. It is 99.9% natural. It's been here since the earth had an atmosphere and liquid water. 

And that's a wrap!